Dinosaur-bird connection
Encyclopedia
The origin of birds is a contentious and central topic within evolutionary biology. A close relationship between birds and dinosaurs was first proposed in the nineteenth century after the discovery of the primitive bird Archaeopteryx
in Germany
. Most researchers now support the view that birds are a group of theropod
dinosaur
s that evolved
during the Mesozoic Era
.
Birds share a myriad of unique skeletal features with dinosaurs. Moreover, fossils of more than twenty species of dinosaur have been collected which preserve feathers. There are even very small dinosaurs, such as Microraptor
and Anchiornis
, which have long, vaned, arm and leg feathers forming wings. The Jurassic basal avialan Pedopenna
also shows these long foot feathers. Witmer (2009) has concluded that this evidence is sufficient to demonstrate that avian evolution went through a four-winged stage.
Fossil evidence also demonstrates that birds and dinosaurs shared features such as hollow, pneumatized bones, gastrolith
s in the digestive
system, nest-building and brooding behaviors
. The ground-breaking discovery of fossilized Tyrannosaurus rex
soft tissue allowed a molecular comparison of cellular
anatomy and protein sequencing
of collagen
tissue, both of which demonstrated that T. rex and birds are more closely related to each other than either is to Alligator. A second molecular study robustly supported the relationship of birds to dinosaurs, though it did not place birds within Theropoda, as expected. This study utilized eight additional collagen sequences extracted from a femur of Brachylophosaurus canadensis, a hadrosaur.
Only a few scientists still debate the dinosaurian origin of birds, suggesting descent from other types of archosaur
ian reptile
s. Among the consensus that supports dinosaurian ancestry, the exact sequence of evolutionary events that gave rise to the early birds within maniraptoran theropods is a hot topic. The origin of bird flight
is a separate but related question for which there are also several proposed answers.
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Scientific
investigation into the origin of birds began shortly after the 1859 publication of Charles Darwin
's On the Origin of Species, the ground-breaking book which described his theory of evolution by natural selection
. In 1860, a fossilized feather was discovered in Germany
's Late Jurassic
solnhofen limestone
. Christian Erich Hermann von Meyer
described this feather as Archaeopteryx lithographica the next year, and Richard Owen
described a nearly complete skeleton in 1863, recognizing it as a bird despite many features reminiscent of reptile
s, including clawed forelimbs and a long, bony tail.
Biologist
Thomas Henry Huxley, known as "Darwin's Bulldog" for his ferocious support of the new theory of evolution, almost immediately seized upon Archaeopteryx as a transitional fossil
between birds and reptiles. Starting in 1868, Huxley made detailed comparisons of Archaeopteryx with various prehistoric reptiles and found that it was most similar to dinosaurs like Hypsilophodon
and Compsognathus
. The discovery in the late 1870s of the iconic "Berlin specimen" of Archaeopteryx, complete with a set of reptilian teeth, provided further evidence. Huxley was the first to propose an evolutionary relationship between birds and dinosaurs, although he was opposed by the very influential Owen. Huxley's conclusions were accepted by many biologists, including Baron Franz Nopcsa
, while others, notably Harry Seeley
, argued that the similarities were due to convergent evolution
.
A turning point came in the early twentieth century with the writings of Gerhard Heilmann
of Denmark
. An artist by trade, Heilmann had a scholarly interest in birds and from 1913 to 1916 published the results of his research in several parts, dealing with the anatomy, embryology
, behavior, paleontology, and evolution of birds. His work, originally written in Danish
as Vor Nuvaerende Viden om Fuglenes Afstamning, was compiled, translated into English, and published in 1926 as The Origin of Birds
.
Like Huxley, Heilmann compared Archaeopteryx and other birds to an exhaustive list of prehistoric reptiles, and also came to the conclusion that theropod dinosaurs like Compsognathus were the most similar. However, Heilmann noted that birds possessed clavicle
s (collar bones) fused to form a bone called the furcula
("wishbone"), and while clavicles were known in more primitive reptiles, they had not yet been recognized in dinosaurs. Since he was a firm believer in Dollo's law
, which states that evolution is not reversible, Heilmann could not accept that clavicles were lost in dinosaurs and re-evolved in birds. He was therefore forced to rule out dinosaurs as bird ancestors and ascribe all of their similarities to convergent evolution
. Heilmann stated that bird ancestors would instead be found among the more primitive "thecodont
" grade of reptiles. Heilmann's extremely thorough approach ensured that his book became a classic in the field, and its conclusions on bird origins, as with most other topics, were accepted by nearly all evolutionary biologists for the next four decades.
Clavicles are relatively delicate bones and therefore in danger of being destroyed or at least damaged beyond recognition. Nevertheless clavicles had been found in theropod dinosaurs before Heilman wrote his book, but had gone unrecognized.
The absence of clavicles in dinosaurs became the orthodox view despite the discovery of clavicles in the primitive theropod Segisaurus
in 1936. The next report of clavicles in a dinosaur was in 1983, and that was in a Russian article published before the end of the Cold War
.
Contrary to what Heilman believed, paleontologists now accept that clavicles and in most cases furculae are a standard feature not just of theropods but of saurischia
n dinosaurs. Up to late 2007 ossified furculae (i.e. made of bone rather than cartilage
) have been found in nearly all types of theropods except the most basal ones, Eoraptor
and Herrerasaurus
. The original report of a furcula in the primitive theropod Segisaurus
(1936) has been confirmed by a re-examination in 2005. Joined, furcula-like clavicles have also been found in Massospondylus
, an Early Jurassic sauropodomorph
.
The tide began to turn against the 'thecodont' hypothesis after the 1964 discovery of a new theropod dinosaur in Montana
. In 1969, this dinosaur was described and named Deinonychus
by John Ostrom
of Yale University
. The next year, Ostrom redescribed a specimen of Pterodactylus
in the Dutch
Teyler Museum
as another skeleton of Archaeopteryx. The specimen consisted mainly of a single wing and its description made Ostrom aware of the similarities between the wrists of Archaeopteryx and Deinonychus.
In 1972, British
paleontologist Alick Walker
hypothesized that birds arose not from 'thecodonts' but from crocodile
ancestors like Sphenosuchus
. Ostrom's work with both theropods and early birds led him to respond with a series of publications in the mid-1970s in which he laid out the many similarities between birds and theropod dinosaurs, resurrecting the ideas first put forth by Huxley over a century before. Ostrom's recognition of the dinosaurian ancestry of birds, along with other new ideas about dinosaur metabolism, activity levels, and parental care, began what is known as the Dinosaur renaissance
, which began in the 1970s and continues to this day.
Ostrom's revelations also coincided with the increasing adoption of phylogenetic systematics (cladistics), which began in the 1960s with the work of Willi Hennig
. Cladistics is a method of arranging species based strictly on their evolutionary relationships, using a statistical analysis of their anatomical characteristics. In the 1980s, cladistic methodology was applied to dinosaur phylogeny for the first time by Jacques Gauthier
and others, showing unequivocally that birds were a derived group of theropod dinosaurs. Early analyses suggested that dromaeosaurid theropods like Deinonychus were particularly closely related to birds, a result which has been corroborated many times since.
The early 1990s saw the discovery of spectacularly preserved bird fossils in several Early Cretaceous
geological formations in the northeastern Chinese province of Liaoning. In 1996, Chinese paleontologists described Sinosauropteryx
as a new genus of bird from the Yixian Formation
, but this animal was quickly recognized as a theropod dinosaur closely related to Compsognathus. Surprisingly, its body was covered by long filamentous structures. These were dubbed 'protofeathers' and considered to be homologous
with the more advanced feathers of birds, although some scientists disagree with this assessment. Chinese and North America
n scientists described Caudipteryx
and Protarchaeopteryx
soon after. Based on skeletal features, these animals were non-avian dinosaurs, but their remains bore fully formed feathers closely resembling those of birds. "Archaeoraptor
", described without peer review in a 1999 issue of National Geographic
, turned out to be a smuggled forgery, but legitimate remains continue to pour out of the Yixian, both legally and illegally. Feathers or "protofeathers" have been found on a wide variety of theropods in the Yixian, and the discoveries of extremely bird-like dinosaurs, as well as dinosaur-like primitive birds, have almost entirely closed the morphological gap between theropods and birds.
A small minority, including ornithologists Alan Feduccia
and Larry Martin
, continues to assert that birds are instead the descendants of earlier archosaur
s, such as Longisquama
or Euparkeria
. Embryological
studies of bird developmental biology
have raised questions about digit homology in bird and dinosaur forelimbs. However, due to the cogent evidence provided by comparative anatomy and phylogenetics, as well as the dramatic feathered dinosaur fossils from China, the idea that birds are derived dinosaurs, first championed by Huxley and later by Nopcsa and Ostrom, enjoys near-unanimous support among today's paleontologists.
, rather than the evolution of flight, was the driving force for the emergence of this clade. Muscles became larger in prospectively endothermic saurians, according to this hypothesis, as a response to the loss of the vertebrate mitochondrial uncoupling protein UCP1. In mammal
s, UCP1 functions within brown adipose tissue
to protect newborns against hypothermia
. In modern birds and their ancestors, skeletal muscle serves a similar function. In this view, bipedality and other avian skeletal
alterations were side effects of muscle hyperplasia, with further evolutionary modifications of the forelimbs, including adaptations for flight or swimming, and vestigiality, being secondary consequences of two-leggedness.
, in keeping with tradition, almost always use Archaeopteryx as a specifier to help define Aves. Aves has more rarely been defined as a crown group
consisting only of modern birds. Nearly all palaeontologists regard birds as coelurosauria
n theropod
dinosaur
s. Within Coelurosauria, multiple cladistic
analyses have found support for a clade
named Maniraptora
, consisting of therizinosaur
oids, oviraptorosaur
s, troodontid
s, dromaeosaurid
s, and birds. Of these, dromaeosaurids and troodontids are usually united in the clade Deinonychosauria
, which is a sister group to birds (together forming the node-clade Eumaniraptora
) within the stem-clade Paraves
.
Other studies have proposed alternative phylogenies in which certain groups of dinosaurs that are usually considered non-avian are suggested to have evolved from avian ancestors. For example, a 2002 analysis found oviraptorosaurs to be basal avians. Alvarezsaurid
s, known from Asia
and the Americas
, have been variously classified as basal
maniraptorans, paravians, the sister taxon of ornithomimosaur
s, as well as specialized early birds. The genus Rahonavis
, originally described as an early bird, has been identified as a non-avian dromaeosaurid in several studies. Dromaeosaurids and troodontids themselves have also been suggested to lie within Aves rather than just outside it.
features are shared by birds and theropod dinosaurs. Some of the more interesting similarities are discussed here:
, the first good example of a "feathered dinosaur", was discovered in 1861. The initial specimen was found in the solnhofen limestone
in southern Germany, which is a lagerstätte
, a rare and remarkable geological formation known for its superbly detailed fossils. Archaeopteryx is a transitional fossil
, with features clearly intermediate between those of modern reptiles and birds. Discovered just two years after Darwin's seminal Origin of Species, its discovery spurred the nascent debate between proponents of evolutionary biology and creationism
. This early bird is so dinosaur-like that, without a clear impression of feathers in the surrounding rock, at least one specimen was mistaken for Compsognathus
.
Since the 1990s, a number of additional feathered dinosaurs
have been found, providing even stronger evidence of the close relationship between dinosaurs and modern birds. Most of these specimens were unearthed in Liaoning
province, northeastern China
, which was part of an island continent during the Cretaceous period. Though feathers have been found only in the lagerstätte
of the Yixian Formation
and a few other places, it is possible that non-avian dinosaurs elsewhere in the world were also feathered. The lack of widespread fossil evidence for feathered non-avian dinosaurs may be because delicate features like skin and feathers are not often preserved by fossil
ization and thus are absent from the fossil record.
A recent development in the debate centers around the discovery of impressions of "protofeathers" surrounding many dinosaur fossils. These protofeathers suggest that the tyrannosauroids
may have been feathered. However, others claim that these protofeathers are simply the result of the decomposition of collagenous fiber that underlaid the dinosaurs' integument.
The feathered dinosaurs discovered so far include Beipiaosaurus
, Caudipteryx
, Dilong
, Microraptor
, Protarchaeopteryx
, Shuvuuia
, Sinornithosaurus
, Sinosauropteryx
, and Jinfengopteryx
, along with dinosaur-like birds, such as Confuciusornis
, which are anatomically closer to modern avians. All of them have been found in the same area and formation, in northern China. The Dromaeosauridae
family, in particular, seems to have been heavily feathered and at least one dromaeosaurid, Cryptovolans, may have been capable of flight.
between birds and dinosaurs. However, the multiple skeletal features also shared by the two groups represent the more important link for paleontologist
s. Furthermore, it is increasingly clear that the relationship between birds and dinosaurs, and the evolution of flight, are more complex topics than previously realized. For example, while it was once believed that birds evolved from dinosaurs in one linear progression, some scientists, most notably Gregory S. Paul
, conclude that dinosaurs such as the dromaeosaur
s may have evolved from birds, losing the power of flight while keeping their feathers in a manner similar to the modern ostrich
and other ratite
s.
Comparisons of bird and dinosaur skeletons, as well as cladistic analysis
, strengthens the case for the link, particularly for a branch of theropods called maniraptor
s. Skeletal similarities include the neck, pubis
, wrist
(semi-lunate carpal
), arm and pectoral girdle, shoulder blade
, clavicle
, and breast bone
.
. The lungs of theropod dinosaurs (carnivores that walked on two legs and had birdlike feet) likely pumped air into hollow sacs in their skeleton
s, as is the case in birds. "What was once formally considered unique to birds was present in some form in the ancestors of birds", O'Connor said. The study was funded in part by the National Science Foundation
.
(CT) scans conducted in 2000 of the chest cavity of a specimen of the ornithopod
Thescelosaurus
found the apparent remnants of complex four-chambered hearts, much like those found in today's mammals and birds. The idea is controversial within the scientific community, coming under fire for bad anatomical science or simply wishful thinking.
A study published in 2011 applied multiple lines of inquiry to the question of the object's identity, including more advanced CT scanning, histology
, X-ray diffraction
, X-ray photoelectron spectroscopy
, and scanning electron microscopy. From these methods, the authors found that: the object's internal structure does not include chambers but is made up of three unconnected areas of lower density material, and is not comparable to the structure of an ostrich
's heart; the "walls" are composed of sedimentary minerals not known to be produced in biological systems, such as goethite, feldspar
minerals, quartz
, and gypsum
, as well as some plant fragments; carbon
, nitrogen
, and phosphorus
, chemical element
s important to life, were lacking in their samples; and cardiac cellular structures were absent. There was one possible patch with animal cellular structures. The authors found their data supported identification as a concretion of sand from the burial environment, not the heart, with the possibility that isolated areas of tissues were preserved.
The question of how this find reflects metabolic rate and dinosaur internal anatomy is moot, though, regardless of the object's identity. Both modern crocodilia
ns and bird
s, the closest living relatives of dinosaurs, have four-chambered hearts (albeit modified in crocodilians), so dinosaurs probably had them as well; the structure is not necessarily tied to metabolic rate.
Mei
and Sinornithoides
demonstrate that the dinosaurs slept like certain modern birds, with their heads tucked under their arms. This behavior, which may have helped to keep the head warm, is also characteristic of modern birds.
and ornithopod Tenontosaurus
. Because the line of dinosaurs that includes Allosaurus and Tyrannosaurus diverged from the line that led to Tenontosaurus very early in the evolution of dinosaurs, this suggests that dinosaurs in general produced medullary tissue.
Several Citipati
specimens have been found resting over the eggs in its nest in a position most reminiscent of brooding
.
Numerous dinosaur species, for example Maiasaura
, have been found in herds mixing both very young and adult individuals, suggesting rich interactions between them.
A dinosaur embryo was found without teeth, which suggests some parental care was required to feed the young dinosaur, possibly the adult dinosaur regurgitated food into the young dinosaur's mouth (see altricial
). This behaviour is seen in numerous bird species; parent birds regurgitate food into the hatchling's mouth.
stones. These stones are swallowed by animals to aid digestion and break down food and hard fibres once they enter the stomach. When found in association with fossil
s, gizzard stones are called gastrolith
s.
Gizzard stones are also found in some fish (mullets, mud shad, and the gilaroo, a type of trout) and in crocodiles.
One of the best examples of soft tissue impressions in a fossil dinosaur was discovered in Petraroia, Italy
. The discovery was reported in 1998, and described the specimen of a small, very young coelurosaur
, Scipionyx
samniticus. The fossil includes portions of the intestines, colon, liver, muscles, and windpipe of this immature dinosaur.
In the March 2005 issue of Science
, Dr. Mary Higby Schweitzer
and her team announced the discovery of flexible material resembling actual soft tissue inside a 68-million-year-old Tyrannosaurus rex
leg bone
from the Hell Creek Formation
in Montana
. After recovery, the tissue was rehydrated by the science team. The seven collagen
types obtained from the bone fragments, compared to collagen data from living birds (specifically, a chicken
), suggest that older theropods and birds are closely related.
When the fossilized bone was treated over several weeks to remove mineral content from the fossilized bone marrow cavity (a process called demineralization), Schweitzer found evidence of intact structures such as blood vessel
s, bone matrix, and connective tissue (bone fibers). Scrutiny under the microscope further revealed that the putative dinosaur soft tissue had retained fine structures (microstructures) even at the cellular level. The exact nature and composition of this material, and the implications of Dr. Schweitzer's discovery, are not yet clear; study and interpretation of the specimens is ongoing.
The successful extraction of ancient DNA from dinosaur fossils has been reported on two separate occasions, but upon further inspection and peer review
, neither of these reports could be confirmed. However, a functional visual peptide
of a theoretical dinosaur has been inferred using analytical phylogenetic reconstruction methods on gene sequences of related modern species such as reptiles and birds. In addition, several protein
s have putatively been detected in dinosaur fossils, including hemoglobin
.
s, which arose soon after the discovery of Archaeopteryx
in 1862. Two theories have dominated most of the discussion since then: the cursorial ("from the ground up") theory proposes that birds evolved from small, fast predators that ran on the ground; the arboreal ("from the trees down") theory proposes that powered flight evolved from unpowered gliding by arboreal (tree-climbing) animals. A more recent theory, "wing-assisted incline running" (WAIR), is a variant of the cursorial theory and proposes that wings developed their aerodynamic
functions as a result of the need to run quickly up very steep slopes, for example to escape from predators.
The cursorial theory of the origin of flight was first proposed by Samuel Wendell Williston
, and elaborated upon by Baron Nopcsa
. This hypothesis proposes that some fast-running animals with long tails used their arms to keep their balance while running. Modern versions of this theory differ in many details from the Williston-Nopcsa version, mainly as a result of discoveries since Nopcsa's time.
Nopcsa theorized that increasing the surface area of the outstretched arms could have helped small cursorial predators to keep their balance, and that the scales of the forearms became elongated, evolving
into feathers. The feathers could also have been used as a trap to catch insects or other prey. Progressively, the animals would have leapt for longer distances, helped by their evolving wings. Nopcsa also proposed that there were three main stages in the evolution of flight. First, passive flight was realized, in which the developed wing structures served as a sort of parachute
. Second, active flight was possible, in which the animal achieved flight by flapping its wings. He used Archaeopteryx as an example of this second stage. Finally, birds gained the ability to soar.
It is now thought that feathers did not evolve from scales, as feathers are made of different protein
s. More seriously, Nopcsa's theory assumes that feathers evolved as part of the evolution of flight, and recent discoveries prove that assumption is false.
Feather
s are very common in coelurosaurian dinosaurs
(including the early tyrannosauroid
Dilong
). Modern bird
s are classified as coelurosaurs by nearly all palaeontologists, though not by a few ornithologists
. The modern version of the "from the ground up" hypothesis argues that birds' ancestors were small, feathered, ground-running predatory dinosaurs (rather like roadrunners
in their hunting style) that used their forelimbs for balance while pursuing prey, and that the forelimbs and feathers later evolved in ways that provided gliding and then powered flight. The most widely suggested original functions of feathers include thermal insulation and competitive displays, as in modern birds.
All of the Archaeopteryx fossils come from marine sediments, and it has been suggested that wings may have helped the birds run over water in the manner of the Jesus Christ Lizard (Common basilisk
).
Most recent refutations of the "from the ground up" hypothesis attempt to refute the modern version's assumption that birds are modified coelurosaurian dinosaurs. The strongest attacks are based on embryological analyses
which conclude that birds' wings are formed from digits 2, 3, and 4 (corresponding to the index, middle, and ring fingers in humans; the first of a bird's three digits forms the alula, which they use to avoid stalling
in low-speed flight, for example when landing); but the hands of coelurosaurs are formed by digits 1, 2, and 3 (thumb and first two fingers in humans). However, these embryological analyses were immediately challenged on the embryological grounds that the "hand" often develops differently in clade
s that have lost some digits in the course of their evolution, and that birds' "hands" do develop from digits 1, 2, and 3. This debate is complex and not yet resolved - see "Digit homology" below.
chicks, and proposes that wings developed their aerodynamic functions as a result of the need to run quickly up very steep slopes such as tree trunks, for example to escape from predators. This makes it a specialized type of cursorial ("from the ground up") theory. Note that in this scenario birds need downforce to give their feet increased grip. But early birds, including Archaeopteryx
, lacked the shoulder mechanism by which modern birds' wings produce swift, powerful upstrokes; since the downforce on which WAIR depends is generated by upstrokes, it seems that early birds were incapable of WAIR. Because WAIR is a behavioural trait without osteological specializations, the phylogenetic placement of the flight stroke before the divergence of Neornithes makes it impossible to determine if WAIR is ancestral to the avian flight stroke or derived from it.
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Most versions of the arboreal hypothesis state that the ancestors of birds were very small dinosaurs that lived in trees, springing from branch to branch. This small dinosaur already had feathers, which were co-opted by evolution to produce longer, stiffer forms that were useful in aerodynamics, eventually producing wings. Wings would have then evolved and become increasingly refined as devices to give the leaper more control, to parachute, to glide, and to fly in stepwise fashion. The arboreal hypothesis also notes that, for arboreal animals, aerodynamics are far more energy efficient, since such animals simply fall in order to achieve minimum gliding speeds.
Several small dinosaurs from the Jurassic or Early Cretaceous, all with feathers, have been interpreted as possibly having arboreal and/or aerodynamic adaptations. These include Scansoriopteryx
, Epidexipteryx
, Microraptor
, Pedopenna
, and Anchiornis
. Anchiornis
is particularly important to this subject, as it lived at the beginning of the Late Jurassic, long before Archaeopteryx
.
Analysis of the proportions of the toe bones of the most primitive birds Archaeopteryx and Confuciusornis
, compared to those of living species, suggest that the early species may have lived both on the ground and in trees.
One study suggested that the earliest birds and their immediate ancestors did not climb trees. This study determined that the amount of toe claw curvature of early birds was more like that seen in modern ground-foraging birds than in perching birds.
Archaeopteryx
was the first and for a long time the only known feathered Mesozoic
animal (or dinosaur, if one accepts the majority view that birds are modified dinosaurs). As a result, discussion of the evolution of birds and of bird flight centered on Archaeopteryx at least until the mid-1990s.
There has been debate about whether Archaeopteryx
could really fly. It appears that Archaeopteryx had the brain structures and inner-ear balance sensors that birds use to control their flight. Archaeopteryx also had a wing feather arrangement like that of modern birds and similarly asymmetrical flight feathers on its wings and tail. But Archaeopteryx lacked the shoulder mechanism by which modern birds' wings produce swift, powerful upstrokes (see diagram above of supracoracoideus pulley); this may mean that it and other early birds were incapable of flapping flight and could only glide.
But the discovery since the early 1990s of many feathered dinosaurs
means that Archaeopteryx is no longer the key figure in the evolution of bird flight. Other small, feathered coelurosaurs from the Cretaceous
and Late Jurassic
show features that may be precursors of avian flight, for example: Rahonavis
, a ground-runner which had a Velociraptor
-like raised sickle claw on the second toe and which some paleontologists think was better adapted for flight than Archaeopteryx; Scansoriopteryx
, an arboreal dinosaur that may provide some support for the "from the trees down" theory; Microraptor
, an arboreal dinosaur that may have been capable of powered flight but, if so, more like a biplane
, as it had well-developed feathers on its legs. As early as 1915, some scientists had argued that the evolution of bird flight may have gone through a four-winged (or tetrapteryx) stage.
|2=Microraptor
(dromaeosaur)
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s
}}
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Simplified cladogram from Mayr et al. (2005)Archaeopteryx
Archaeopteryx , sometimes referred to by its German name Urvogel , is a genus of theropod dinosaur that is closely related to birds. The name derives from the Ancient Greek meaning "ancient", and , meaning "feather" or "wing"...
in Germany
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
. Most researchers now support the view that birds are a group of theropod
Theropoda
Theropoda is both a suborder of bipedal saurischian dinosaurs, and a clade consisting of that suborder and its descendants . Dinosaurs belonging to the suborder theropoda were primarily carnivorous, although a number of theropod groups evolved herbivory, omnivory, and insectivory...
dinosaur
Dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...
s that evolved
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
during the Mesozoic Era
Mesozoic
The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...
.
Birds share a myriad of unique skeletal features with dinosaurs. Moreover, fossils of more than twenty species of dinosaur have been collected which preserve feathers. There are even very small dinosaurs, such as Microraptor
Microraptor
Microraptor is a genus of small, four-winged dromaeosaurid dinosaurs. Numerous well-preserved fossil specimens have been recovered from Liaoning, China...
and Anchiornis
Anchiornis
Anchiornis is a genus of small, feathered, deinonychosaurian dinosaur. The genus Anchiornis contains the type species Anchiornis huxleyi, named in honor of Thomas Henry Huxley, an early proponent of biological evolution, and the first to propose a close evolutionary relationship between birds and...
, which have long, vaned, arm and leg feathers forming wings. The Jurassic basal avialan Pedopenna
Pedopenna
Pedopenna is a genus of small, feathered, maniraptoran dinosaur from the Daohugou Beds in China. It is possibly older than Archaeopteryx, though the age of the Daohugou Beds where it was found is debated...
also shows these long foot feathers. Witmer (2009) has concluded that this evidence is sufficient to demonstrate that avian evolution went through a four-winged stage.
Fossil evidence also demonstrates that birds and dinosaurs shared features such as hollow, pneumatized bones, gastrolith
Gastrolith
A gastrolith, also called a stomach stone or gizzard stones, is a rock held inside a gastrointestinal tract. Gastroliths are retained in the muscular gizzard and used to grind food in animals lacking suitable grinding teeth. The grain size depends upon the size of the animal and the gastrolith's...
s in the digestive
Gastrointestinal tract
The human gastrointestinal tract refers to the stomach and intestine, and sometimes to all the structures from the mouth to the anus. ....
system, nest-building and brooding behaviors
Avian incubation
Incubation refers to the process by which certain oviparous animals hatch their eggs, and to the development of the embryo within the egg. The most vital factor of incubation is the constant temperature required for its development over a specific period. Especially in domestic fowl, the act of...
. The ground-breaking discovery of fossilized Tyrannosaurus rex
Tyrannosaurus
Tyrannosaurus meaning "tyrant," and sauros meaning "lizard") is a genus of coelurosaurian theropod dinosaur. The species Tyrannosaurus rex , commonly abbreviated to T. rex, is a fixture in popular culture. It lived throughout what is now western North America, with a much wider range than other...
soft tissue allowed a molecular comparison of cellular
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
anatomy and protein sequencing
Protein sequencing
Protein sequencing is a technique to determine the amino acid sequence of a protein, as well as which conformation the protein adopts and the extent to which it is complexed with any non-peptide molecules...
of collagen
Collagen
Collagen is a group of naturally occurring proteins found in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content...
tissue, both of which demonstrated that T. rex and birds are more closely related to each other than either is to Alligator. A second molecular study robustly supported the relationship of birds to dinosaurs, though it did not place birds within Theropoda, as expected. This study utilized eight additional collagen sequences extracted from a femur of Brachylophosaurus canadensis, a hadrosaur.
Only a few scientists still debate the dinosaurian origin of birds, suggesting descent from other types of archosaur
Archosaur
Archosaurs are a group of diapsid amniotes whose living representatives consist of modern birds and crocodilians. This group also includes all extinct non-avian dinosaurs, many extinct crocodilian relatives, and pterosaurs. Archosauria, the archosaur clade, is a crown group that includes the most...
ian reptile
Reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...
s. Among the consensus that supports dinosaurian ancestry, the exact sequence of evolutionary events that gave rise to the early birds within maniraptoran theropods is a hot topic. The origin of bird flight
Bird flight
Flight is the main mode of locomotion used by most of the world's bird species. Flight assists birds while feeding, breeding and avoiding predators....
is a separate but related question for which there are also several proposed answers.
Huxley, Archaeopteryx and early research
Scientific method
Scientific method refers to a body of techniques for investigating phenomena, acquiring new knowledge, or correcting and integrating previous knowledge. To be termed scientific, a method of inquiry must be based on gathering empirical and measurable evidence subject to specific principles of...
investigation into the origin of birds began shortly after the 1859 publication of Charles Darwin
Charles Darwin
Charles Robert Darwin FRS was an English naturalist. He established that all species of life have descended over time from common ancestry, and proposed the scientific theory that this branching pattern of evolution resulted from a process that he called natural selection.He published his theory...
's On the Origin of Species, the ground-breaking book which described his theory of evolution by natural selection
Natural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....
. In 1860, a fossilized feather was discovered in Germany
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
's Late Jurassic
Late Jurassic
The Late Jurassic is the third epoch of the Jurassic Period, and it spans the geologic time from 161.2 ± 4.0 to 145.5 ± 4.0 million years ago , which is preserved in Upper Jurassic strata. In European lithostratigraphy, the name "Malm" indicates rocks of Late Jurassic age...
solnhofen limestone
Solnhofen limestone
The Solnhofen Plattenkalk is a Jurassic Konservat-Lagerstätte that preserves a rare assemblage of fossilized organisms, including highly detailed imprints of soft bodied organisms such as sea jellies...
. Christian Erich Hermann von Meyer
Christian Erich Hermann von Meyer
Christian Erich Hermann von Meyer was a German palaeontologist.He was born at Frankfurt am Main.In 1832 von Meyer issued a work entitled Palaeologica, and in course of time he published a series of memoirs on various fossil organic remains: molluscs, crustaceans, fishes and higher vertebrata,...
described this feather as Archaeopteryx lithographica the next year, and Richard Owen
Richard Owen
Sir Richard Owen, FRS KCB was an English biologist, comparative anatomist and palaeontologist.Owen is probably best remembered today for coining the word Dinosauria and for his outspoken opposition to Charles Darwin's theory of evolution by natural selection...
described a nearly complete skeleton in 1863, recognizing it as a bird despite many features reminiscent of reptile
Reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...
s, including clawed forelimbs and a long, bony tail.
Biologist
Biologist
A biologist is a scientist devoted to and producing results in biology through the study of life. Typically biologists study organisms and their relationship to their environment. Biologists involved in basic research attempt to discover underlying mechanisms that govern how organisms work...
Thomas Henry Huxley, known as "Darwin's Bulldog" for his ferocious support of the new theory of evolution, almost immediately seized upon Archaeopteryx as a transitional fossil
Transitional fossil
A transitional fossil is any fossilized remains of a lifeform that exhibits characteristics of two distinct taxonomic groups. A transitional fossil is the fossil of an organism near the branching point where major individual lineages diverge...
between birds and reptiles. Starting in 1868, Huxley made detailed comparisons of Archaeopteryx with various prehistoric reptiles and found that it was most similar to dinosaurs like Hypsilophodon
Hypsilophodon
Hypsilophodon is an ornithopod dinosaur genus from the Early Cretaceous period of Europe. It was a small bipedal animal with an herbivorous or possibly omnivorous diet...
and Compsognathus
Compsognathus
Compsognathus was a small, bipedal, carnivorous theropod dinosaur. The animal was the size of a turkey and lived around 150 million years ago, the early Tithonian stage of the late Jurassic Period, in what is now Europe. Paleontologists have found two well-preserved fossils, one in Germany...
. The discovery in the late 1870s of the iconic "Berlin specimen" of Archaeopteryx, complete with a set of reptilian teeth, provided further evidence. Huxley was the first to propose an evolutionary relationship between birds and dinosaurs, although he was opposed by the very influential Owen. Huxley's conclusions were accepted by many biologists, including Baron Franz Nopcsa
Franz Nopcsa von Felso-Szilvás
Baron Franz Nopcsa von Felső-Szilvás was a Hungarian-born aristocrat, adventurer, scholar, and paleontologist...
, while others, notably Harry Seeley
Harry Seeley
Harry Govier Seeley was a British paleontologist.-Career:Seeley was born in London, the son of Richard Hovill Seeley, goldsmith, and his second wife Mary Govier. He attended classes at the Royal School of Mines, Kensington before becoming an assistant to Adam Sedgwick at the Woodwardian Museum,...
, argued that the similarities were due to convergent evolution
Convergent evolution
Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...
.
Heilmann and the thecodont hypothesis
Gerhard Heilmann
Gerhard Heilmann was a Danish artist and paleontologist who created artistic depictions of Archeopteryx, Proavis and other early bird relatives apart from writing The Origin of Birds, a pioneering and influential account of bird evolution...
of Denmark
Denmark
Denmark is a Scandinavian country in Northern Europe. The countries of Denmark and Greenland, as well as the Faroe Islands, constitute the Kingdom of Denmark . It is the southernmost of the Nordic countries, southwest of Sweden and south of Norway, and bordered to the south by Germany. Denmark...
. An artist by trade, Heilmann had a scholarly interest in birds and from 1913 to 1916 published the results of his research in several parts, dealing with the anatomy, embryology
Embryology
Embryology is a science which is about the development of an embryo from the fertilization of the ovum to the fetus stage...
, behavior, paleontology, and evolution of birds. His work, originally written in Danish
Danish language
Danish is a North Germanic language spoken by around six million people, principally in the country of Denmark. It is also spoken by 50,000 Germans of Danish ethnicity in the northern parts of Schleswig-Holstein, Germany, where it holds the status of minority language...
as Vor Nuvaerende Viden om Fuglenes Afstamning, was compiled, translated into English, and published in 1926 as The Origin of Birds
The Origin of Birds (book)
The Origin of Birds is an early synopsis of bird evolution written in 1926 by Gerhard Heilmann, a Danish artist and amateur zoologist. The book was born from a series of articles published between 1913 and 1916 in Danish, and although republished as a book it received mainly criticism from...
.
Like Huxley, Heilmann compared Archaeopteryx and other birds to an exhaustive list of prehistoric reptiles, and also came to the conclusion that theropod dinosaurs like Compsognathus were the most similar. However, Heilmann noted that birds possessed clavicle
Clavicle
In human anatomy, the clavicle or collar bone is a long bone of short length that serves as a strut between the scapula and the sternum. It is the only long bone in body that lies horizontally...
s (collar bones) fused to form a bone called the furcula
Furcula
The ' is a forked bone found in birds, formed by the fusion of the two clavicles. In birds, its function is the strengthening of the thoracic skeleton to withstand the rigors of flight....
("wishbone"), and while clavicles were known in more primitive reptiles, they had not yet been recognized in dinosaurs. Since he was a firm believer in Dollo's law
Dollo's law
Dollo's law of irreversibility is a hypothesis proposed in 1893 by French-born Belgian paleontologist Louis Dollo which states that evolution is not reversible...
, which states that evolution is not reversible, Heilmann could not accept that clavicles were lost in dinosaurs and re-evolved in birds. He was therefore forced to rule out dinosaurs as bird ancestors and ascribe all of their similarities to convergent evolution
Convergent evolution
Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...
. Heilmann stated that bird ancestors would instead be found among the more primitive "thecodont
Thecodont
Thecodont , now considered an obsolete term, was formerly used to describe a diverse range of early archosaurs that first appeared in the Latest Permian and flourished until the end of the Triassic period...
" grade of reptiles. Heilmann's extremely thorough approach ensured that his book became a classic in the field, and its conclusions on bird origins, as with most other topics, were accepted by nearly all evolutionary biologists for the next four decades.
The absence of clavicles in dinosaurs became the orthodox view despite the discovery of clavicles in the primitive theropod Segisaurus
Segisaurus
Segisaurus is a genus of small coelophysoid theropod dinosaur, that measured approximately 1 metre in length. The only known specimen was discovered in 1933 in early Jurassic strata in Tsegi Canyon, Arizona, for which it was named. Segisaurus is the only dinosaur to have ever been excavated from...
in 1936. The next report of clavicles in a dinosaur was in 1983, and that was in a Russian article published before the end of the Cold War
Cold War
The Cold War was the continuing state from roughly 1946 to 1991 of political conflict, military tension, proxy wars, and economic competition between the Communist World—primarily the Soviet Union and its satellite states and allies—and the powers of the Western world, primarily the United States...
.
Contrary to what Heilman believed, paleontologists now accept that clavicles and in most cases furculae are a standard feature not just of theropods but of saurischia
Saurischia
Saurischia meaning 'lizard' and ischion meaning 'hip joint') is one of the two orders, or basic divisions, of dinosaurs. In 1888, Harry Seeley classified dinosaurs into two orders, based on their hip structure...
n dinosaurs. Up to late 2007 ossified furculae (i.e. made of bone rather than cartilage
Cartilage
Cartilage is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs...
) have been found in nearly all types of theropods except the most basal ones, Eoraptor
Eoraptor
Eoraptor was one of the world's earliest dinosaurs. It was a two-legged saurischian, close to the ancestry of theropods and sauropodomorphs. It lived ca. 231.4 million years ago, in what is now the northwestern region of Argentina...
and Herrerasaurus
Herrerasaurus
Herrerasaurus was one of the earliest dinosaurs. All known specimens of this carnivore have been discovered in rocks of late Ladinian age in northwestern Argentina...
. The original report of a furcula in the primitive theropod Segisaurus
Segisaurus
Segisaurus is a genus of small coelophysoid theropod dinosaur, that measured approximately 1 metre in length. The only known specimen was discovered in 1933 in early Jurassic strata in Tsegi Canyon, Arizona, for which it was named. Segisaurus is the only dinosaur to have ever been excavated from...
(1936) has been confirmed by a re-examination in 2005. Joined, furcula-like clavicles have also been found in Massospondylus
Massospondylus
Massospondylus and ) is a genus of prosauropod dinosaur from the early Jurassic Period . It was described by Sir Richard Owen in 1854 from remains found in South Africa, and is thus one of the first dinosaurs to have been named...
, an Early Jurassic sauropodomorph
Sauropodomorpha
Sauropodomorpha is an extinct clade of long-necked, herbivorous, saurischian dinosaurs which includes the sauropods and their ancestral relatives. Sauropods generally grew to very large sizes, had long necks and tails, were quadrupedal, and became the largest animals to ever walk the Earth. The...
.
Ostrom, Deinonychus and the Dinosaur Renaissance
Montana
Montana is a state in the Western United States. The western third of Montana contains numerous mountain ranges. Smaller, "island ranges" are found in the central third of the state, for a total of 77 named ranges of the Rocky Mountains. This geographical fact is reflected in the state's name,...
. In 1969, this dinosaur was described and named Deinonychus
Deinonychus
Deinonychus was a genus of carnivorous dromaeosaurid dinosaur. There is one described species, Deinonychus antirrhopus. This 3.4 meter long dinosaur lived during the early Cretaceous Period, about 115–108 million years ago . Fossils have been recovered from the U.S...
by John Ostrom
John Ostrom
John H. Ostrom was an American paleontologist who revolutionized modern understanding of dinosaurs in the 1960s, when he demonstrated that dinosaurs are more like big non-flying birds than they are like lizards , an idea first proposed by Thomas Henry Huxley in the 1860s, but which had garnered...
of Yale University
Yale University
Yale University is a private, Ivy League university located in New Haven, Connecticut, United States. Founded in 1701 in the Colony of Connecticut, the university is the third-oldest institution of higher education in the United States...
. The next year, Ostrom redescribed a specimen of Pterodactylus
Pterodactylus
Pterodactylus is a genus of pterosaurs, whose members are popularly known as pterodactyls. It was the first to be named and identified as a flying reptile...
in the Dutch
Netherlands
The Netherlands is a constituent country of the Kingdom of the Netherlands, located mainly in North-West Europe and with several islands in the Caribbean. Mainland Netherlands borders the North Sea to the north and west, Belgium to the south, and Germany to the east, and shares maritime borders...
Teyler Museum
Teylers Museum
Teyler's Museum , located in Haarlem, is the oldest museum in the Netherlands. The museum is in the former home of Pieter Teyler van der Hulst . He was a wealthy cloth merchant and Amsterdam banker of Scottish descent, who bequeathed his fortune for the advancement of religion, art and science...
as another skeleton of Archaeopteryx. The specimen consisted mainly of a single wing and its description made Ostrom aware of the similarities between the wrists of Archaeopteryx and Deinonychus.
In 1972, British
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
paleontologist Alick Walker
Alick Walker
Alick Donald Walker was a British palaeontologist, after whom the Alwalkeria genus of dinosaur is named....
hypothesized that birds arose not from 'thecodonts' but from crocodile
Crocodile
A crocodile is any species belonging to the family Crocodylidae . The term can also be used more loosely to include all extant members of the order Crocodilia: i.e...
ancestors like Sphenosuchus
Sphenosuchus
Sphenosuchus is an extinct genus of sphenosuchian from the Elliot Formation of South Africa, discovered and described early in the 20th century. The skull is excellently preserved but other than elements of the forelimb and isolated parts of the hind limb, the Sphenosuchus material is incomplete...
. Ostrom's work with both theropods and early birds led him to respond with a series of publications in the mid-1970s in which he laid out the many similarities between birds and theropod dinosaurs, resurrecting the ideas first put forth by Huxley over a century before. Ostrom's recognition of the dinosaurian ancestry of birds, along with other new ideas about dinosaur metabolism, activity levels, and parental care, began what is known as the Dinosaur renaissance
Dinosaur renaissance
The dinosaur renaissance was a small-scale scientific revolution that started in the late 1960s, and led to renewed academic and popular interest in dinosaurs...
, which began in the 1970s and continues to this day.
Ostrom's revelations also coincided with the increasing adoption of phylogenetic systematics (cladistics), which began in the 1960s with the work of Willi Hennig
Willi Hennig
Emil Hans Willi Hennig was a German biologist who is considered the founder of phylogenetic systematics, also known as cladistics. With his works on evolution and systematics he revolutionised the view of the natural order of beings...
. Cladistics is a method of arranging species based strictly on their evolutionary relationships, using a statistical analysis of their anatomical characteristics. In the 1980s, cladistic methodology was applied to dinosaur phylogeny for the first time by Jacques Gauthier
Jacques Gauthier
Jacques Armand Gauthier is a vertebrate paleontologist, comparative morphologist, and systematist, and one of the founders of the use of cladistics in biology....
and others, showing unequivocally that birds were a derived group of theropod dinosaurs. Early analyses suggested that dromaeosaurid theropods like Deinonychus were particularly closely related to birds, a result which has been corroborated many times since.
Modern research and feathered dinosaurs in China
Early Cretaceous
The Early Cretaceous or the Lower Cretaceous , is the earlier or lower of the two major divisions of the Cretaceous...
geological formations in the northeastern Chinese province of Liaoning. In 1996, Chinese paleontologists described Sinosauropteryx
Sinosauropteryx
Sinosauropteryx was the first genus of dinosaur outside of Avialae to be found with evidence of feathers. They were covered with "furry" coats of very simple filament-like feathers...
as a new genus of bird from the Yixian Formation
Yixian Formation
The Yixian Formation is a geological formation in Jinzhou, Liaoning, People's Republic of China, that spans 11 million years during the early Cretaceous period...
, but this animal was quickly recognized as a theropod dinosaur closely related to Compsognathus. Surprisingly, its body was covered by long filamentous structures. These were dubbed 'protofeathers' and considered to be homologous
Homology (biology)
Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...
with the more advanced feathers of birds, although some scientists disagree with this assessment. Chinese and North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
n scientists described Caudipteryx
Caudipteryx
Caudipteryx is a genus of peacock-sized theropod dinosaurs that lived in the Aptian age of the early Cretaceous Period . They were feathered and remarkably birdlike in their overall appearance....
and Protarchaeopteryx
Protarchaeopteryx
Protarchaeopteryx is a genus of turkey-sized feathered theropod dinosaur from China. Known from the Jianshangou bed of the Yixian Formation, it lived during the early Aptian age of the Early Cretaceous, approximately 124.6 million years ago...
soon after. Based on skeletal features, these animals were non-avian dinosaurs, but their remains bore fully formed feathers closely resembling those of birds. "Archaeoraptor
Archaeoraptor
"Archaeoraptor" is the generic name informally assigned in 1999 to a fossil from China in an article published in National Geographic magazine. The magazine claimed that the fossil was a "missing link" between birds and terrestrial theropod dinosaurs. Even prior to this publication there had been...
", described without peer review in a 1999 issue of National Geographic
National Geographic Magazine
National Geographic, formerly the National Geographic Magazine, is the official journal of the National Geographic Society. It published its first issue in 1888, just nine months after the Society itself was founded...
, turned out to be a smuggled forgery, but legitimate remains continue to pour out of the Yixian, both legally and illegally. Feathers or "protofeathers" have been found on a wide variety of theropods in the Yixian, and the discoveries of extremely bird-like dinosaurs, as well as dinosaur-like primitive birds, have almost entirely closed the morphological gap between theropods and birds.
A small minority, including ornithologists Alan Feduccia
Alan Feduccia
Alan Feduccia is a paleornithologist, specializing in the origins and phylogeny of birds. He is now Professor Emeritus at the University of North Carolina. Feduccia's principal authored works include two books, The Age of Birds and The Origin and Evolution of Birds, and numerous papers in various...
and Larry Martin
Larry Martin
Larry Martin is an American vertebrate paleontologist and curator of the Natural History Museum and Biodiversity Research Center at the University of Kansas. Among Martin's work is research on the Triassic reptile Longisquama and theropod dinosaur Caudipteryx...
, continues to assert that birds are instead the descendants of earlier archosaur
Archosaur
Archosaurs are a group of diapsid amniotes whose living representatives consist of modern birds and crocodilians. This group also includes all extinct non-avian dinosaurs, many extinct crocodilian relatives, and pterosaurs. Archosauria, the archosaur clade, is a crown group that includes the most...
s, such as Longisquama
Longisquama
Longisquama insignis is an extinct lizard-like reptile known only from one poorly preserved and incomplete fossil. It lived during the middle or late Triassic Period, 230-225 million years ago, in what is now Kyrgyzstan...
or Euparkeria
Euparkeria
Euparkeria was a small African reptile of the early Triassic period between 248-245 million years ago, close to the ancestry of the archosaurs.- Palaeobiology :...
. Embryological
Embryology
Embryology is a science which is about the development of an embryo from the fertilization of the ovum to the fetus stage...
studies of bird developmental biology
Developmental biology
Developmental biology is the study of the process by which organisms grow and develop. Modern developmental biology studies the genetic control of cell growth, differentiation and "morphogenesis", which is the process that gives rise to tissues, organs and anatomy.- Related fields of study...
have raised questions about digit homology in bird and dinosaur forelimbs. However, due to the cogent evidence provided by comparative anatomy and phylogenetics, as well as the dramatic feathered dinosaur fossils from China, the idea that birds are derived dinosaurs, first championed by Huxley and later by Nopcsa and Ostrom, enjoys near-unanimous support among today's paleontologists.
Thermogenic muscle hypothesis
A new theory of bird origins suggests that selection for the expansion of skeletal muscleSkeletal muscle
Skeletal muscle is a form of striated muscle tissue existing under control of the somatic nervous system- i.e. it is voluntarily controlled. It is one of three major muscle types, the others being cardiac and smooth muscle...
, rather than the evolution of flight, was the driving force for the emergence of this clade. Muscles became larger in prospectively endothermic saurians, according to this hypothesis, as a response to the loss of the vertebrate mitochondrial uncoupling protein UCP1. In mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s, UCP1 functions within brown adipose tissue
Brown adipose tissue
Brown adipose tissue or brown fat is one of two types of fat or adipose tissue found in mammals....
to protect newborns against hypothermia
Hypothermia
Hypothermia is a condition in which core temperature drops below the required temperature for normal metabolism and body functions which is defined as . Body temperature is usually maintained near a constant level of through biologic homeostasis or thermoregulation...
. In modern birds and their ancestors, skeletal muscle serves a similar function. In this view, bipedality and other avian skeletal
Skeleton
The skeleton is the body part that forms the supporting structure of an organism. There are two different skeletal types: the exoskeleton, which is the stable outer shell of an organism, and the endoskeleton, which forms the support structure inside the body.In a figurative sense, skeleton can...
alterations were side effects of muscle hyperplasia, with further evolutionary modifications of the forelimbs, including adaptations for flight or swimming, and vestigiality, being secondary consequences of two-leggedness.
Phylogeny
Archaeopteryx has historically been considered the first bird, or Urvogel. Although newer fossil discoveries eliminated the gap between theropods and Archaeopteryx, as well as the gap between Archaeopteryx and modern birds, phylogenetic taxonomistsPhylogenetic nomenclature
Phylogenetic nomenclature or phylogenetic taxonomy is an alternative to rank-based nomenclature, applying definitions from cladistics . Its two defining features are the use of phylogenetic definitions of biological taxon names, and the lack of obligatory ranks...
, in keeping with tradition, almost always use Archaeopteryx as a specifier to help define Aves. Aves has more rarely been defined as a crown group
Crown group
A crown group is a group consisting of living representatives, their ancestors back to the most recent common ancestor of that group, and all of that ancestor's descendants. The name was given by Willi Hennig, the formulator of phylogenetic systematics, as a way of classifying living organisms...
consisting only of modern birds. Nearly all palaeontologists regard birds as coelurosauria
Coelurosauria
Coelurosauria is the clade containing all theropod dinosaurs more closely related to birds than to carnosaurs. In the past, it was used to refer to all small theropods, although this classification has been abolished...
n theropod
Theropoda
Theropoda is both a suborder of bipedal saurischian dinosaurs, and a clade consisting of that suborder and its descendants . Dinosaurs belonging to the suborder theropoda were primarily carnivorous, although a number of theropod groups evolved herbivory, omnivory, and insectivory...
dinosaur
Dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...
s. Within Coelurosauria, multiple cladistic
Cladistics
Cladistics is a method of classifying species of organisms into groups called clades, which consist of an ancestor organism and all its descendants . For example, birds, dinosaurs, crocodiles, and all descendants of their most recent common ancestor form a clade...
analyses have found support for a clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
named Maniraptora
Maniraptora
Maniraptora is a clade of coelurosaurian dinosaurs which includes the birds and the dinosaurs that were more closely related to them than to Ornithomimus velox. It contains the major subgroups Avialae, Deinonychosauria, Oviraptorosauria and Therizinosauria. Ornitholestes and the Alvarezsauroidea...
, consisting of therizinosaur
Therizinosaur
Therizinosaurs are theropod dinosaurs belonging to the clade Therizinosauria. Therizinosaur fossils have been found in Early through Late Cretaceous deposits in Mongolia, the People's Republic of China and Western North America...
oids, oviraptorosaur
Oviraptorosauria
Oviraptorosaurs are a group of feathered maniraptoran dinosaurs from the Cretaceous Period of what are now Asia and North America. They are distinct for their characteristically short, beaked, parrot - like skulls, with or without bony crests atop the head...
s, troodontid
Troodontidae
Troodontidae is a family of bird-like theropod dinosaurs. During most of the 20th century, troodontid fossils were few and scrappy and they have therefore been allied, at various times, with many dinosaurian lineages...
s, dromaeosaurid
Dromaeosauridae
Dromaeosauridae is a family of bird-like theropod dinosaurs. They were small- to medium-sized feathered carnivores that flourished in the Cretaceous Period. The name Dromaeosauridae means 'running lizards', from Greek dromeus meaning 'runner' and sauros meaning 'lizard'...
s, and birds. Of these, dromaeosaurids and troodontids are usually united in the clade Deinonychosauria
Deinonychosauria
The Deinonychosauria were a clade of coelurosaurian theropod dinosaurs from the Late Jurassic and Cretaceous periods. These omnivores and carnivores are known for their switchblade-like second toe claws and for displaying numerous bird-like characteristics. The clade has been divided into two...
, which is a sister group to birds (together forming the node-clade Eumaniraptora
Paraves
Paraves is a branch-based clade containing birds and other closely related dinosaurs. The paravians include the Avialae, including Archaeopteryx and modern birds, and the Deinonychosauria, which includes the dromaeosaurids and troodontids....
) within the stem-clade Paraves
Paraves
Paraves is a branch-based clade containing birds and other closely related dinosaurs. The paravians include the Avialae, including Archaeopteryx and modern birds, and the Deinonychosauria, which includes the dromaeosaurids and troodontids....
.
Other studies have proposed alternative phylogenies in which certain groups of dinosaurs that are usually considered non-avian are suggested to have evolved from avian ancestors. For example, a 2002 analysis found oviraptorosaurs to be basal avians. Alvarezsaurid
Alvarezsauridae
Alvarezsauridae is an enigmatic family of small, long-legged running dinosaurs. Although originally thought to represent the earliest known flightless birds, a consensus of recent work suggests that they are primitive members of the Maniraptora. Other work found them to be the sister group to the...
s, known from Asia
Asia
Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...
and the Americas
Americas
The Americas, or America , are lands in the Western hemisphere, also known as the New World. In English, the plural form the Americas is often used to refer to the landmasses of North America and South America with their associated islands and regions, while the singular form America is primarily...
, have been variously classified as basal
Basal (phylogenetics)
In phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram.A basal group forms an outgroup to the rest of the clade, such as in the following example:...
maniraptorans, paravians, the sister taxon of ornithomimosaur
Ornithomimosauria
The Ornithomimosauria, ornithomimosaurs or ostrich dinosaurs were theropod dinosaurs which bore a superficial resemblance to modern ostriches. They were fast, omnivorous or herbivorous dinosaurs from the Cretaceous Period of Laurasia...
s, as well as specialized early birds. The genus Rahonavis
Rahonavis
Rahonavis is a genus of bird-like dinosaur from the Late Cretaceous of what is now northwestern Madagascar. It is known from a partial skeleton found in Maevarano Formation rocks at a quarry near Berivotra, Mahajanga Province...
, originally described as an early bird, has been identified as a non-avian dromaeosaurid in several studies. Dromaeosaurids and troodontids themselves have also been suggested to lie within Aves rather than just outside it.
Features linking birds and dinosaurs
Many distinct anatomicalAnatomy
Anatomy is a branch of biology and medicine that is the consideration of the structure of living things. It is a general term that includes human anatomy, animal anatomy , and plant anatomy...
features are shared by birds and theropod dinosaurs. Some of the more interesting similarities are discussed here:
Feathers
ArchaeopteryxArchaeopteryx
Archaeopteryx , sometimes referred to by its German name Urvogel , is a genus of theropod dinosaur that is closely related to birds. The name derives from the Ancient Greek meaning "ancient", and , meaning "feather" or "wing"...
, the first good example of a "feathered dinosaur", was discovered in 1861. The initial specimen was found in the solnhofen limestone
Solnhofen limestone
The Solnhofen Plattenkalk is a Jurassic Konservat-Lagerstätte that preserves a rare assemblage of fossilized organisms, including highly detailed imprints of soft bodied organisms such as sea jellies...
in southern Germany, which is a lagerstätte
Lagerstätte
A Lagerstätte is a sedimentary deposit that exhibits extraordinary fossil richness or completeness.Palaeontologists distinguish two kinds....
, a rare and remarkable geological formation known for its superbly detailed fossils. Archaeopteryx is a transitional fossil
Transitional fossil
A transitional fossil is any fossilized remains of a lifeform that exhibits characteristics of two distinct taxonomic groups. A transitional fossil is the fossil of an organism near the branching point where major individual lineages diverge...
, with features clearly intermediate between those of modern reptiles and birds. Discovered just two years after Darwin's seminal Origin of Species, its discovery spurred the nascent debate between proponents of evolutionary biology and creationism
Creationism
Creationism is the religious beliefthat humanity, life, the Earth, and the universe are the creation of a supernatural being, most often referring to the Abrahamic god. As science developed from the 18th century onwards, various views developed which aimed to reconcile science with the Genesis...
. This early bird is so dinosaur-like that, without a clear impression of feathers in the surrounding rock, at least one specimen was mistaken for Compsognathus
Compsognathus
Compsognathus was a small, bipedal, carnivorous theropod dinosaur. The animal was the size of a turkey and lived around 150 million years ago, the early Tithonian stage of the late Jurassic Period, in what is now Europe. Paleontologists have found two well-preserved fossils, one in Germany...
.
Since the 1990s, a number of additional feathered dinosaurs
Feathered dinosaurs
The realization that dinosaurs are closely related to birds raised the obvious possibility of feathered dinosaurs. Fossils of Archaeopteryx include well-preserved feathers, but it was not until the early 1990s that clearly non-avialan dinosaur fossils were discovered with preserved feathers...
have been found, providing even stronger evidence of the close relationship between dinosaurs and modern birds. Most of these specimens were unearthed in Liaoning
Liaoning
' is a province of the People's Republic of China, located in the northeast of the country. Its one-character abbreviation is "辽" , a name taken from the Liao River that flows through the province. "Níng" means "peace"...
province, northeastern China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...
, which was part of an island continent during the Cretaceous period. Though feathers have been found only in the lagerstätte
Lagerstätte
A Lagerstätte is a sedimentary deposit that exhibits extraordinary fossil richness or completeness.Palaeontologists distinguish two kinds....
of the Yixian Formation
Yixian Formation
The Yixian Formation is a geological formation in Jinzhou, Liaoning, People's Republic of China, that spans 11 million years during the early Cretaceous period...
and a few other places, it is possible that non-avian dinosaurs elsewhere in the world were also feathered. The lack of widespread fossil evidence for feathered non-avian dinosaurs may be because delicate features like skin and feathers are not often preserved by fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
ization and thus are absent from the fossil record.
A recent development in the debate centers around the discovery of impressions of "protofeathers" surrounding many dinosaur fossils. These protofeathers suggest that the tyrannosauroids
Tyrannosauridae
Tyrannosauridae is a family of coelurosaurian theropod dinosaurs which comprises two subfamilies containing up to six genera, including the eponymous Tyrannosaurus. The exact number of genera is controversial, with some experts recognizing as few as three...
may have been feathered. However, others claim that these protofeathers are simply the result of the decomposition of collagenous fiber that underlaid the dinosaurs' integument.
Beipiaosaurus
Beipiaosaurus is a genus of therizinosauroid theropod dinosaur from the Early Cretaceous of China.The exact classification of therizinosaurs had in the past been hotly debated, since their prosauropod-like teeth and body structure indicate that they were generally herbivorous, unlike typical...
, Caudipteryx
Caudipteryx
Caudipteryx is a genus of peacock-sized theropod dinosaurs that lived in the Aptian age of the early Cretaceous Period . They were feathered and remarkably birdlike in their overall appearance....
, Dilong
Dilong (dinosaur)
Dilong is a genus of small tyrannosauroid dinosaur. The only species is Dilong paradoxus. It is from the Lower Cretaceous Yixian Formation near Lujiatun, Beipiao, in the western Liaoning province of China. It lived about 130 million years ago...
, Microraptor
Microraptor
Microraptor is a genus of small, four-winged dromaeosaurid dinosaurs. Numerous well-preserved fossil specimens have been recovered from Liaoning, China...
, Protarchaeopteryx
Protarchaeopteryx
Protarchaeopteryx is a genus of turkey-sized feathered theropod dinosaur from China. Known from the Jianshangou bed of the Yixian Formation, it lived during the early Aptian age of the Early Cretaceous, approximately 124.6 million years ago...
, Shuvuuia
Shuvuuia
Shuvuuia is a genus of bird-like theropod dinosaur from the late Cretaceous period of Mongolia. It is a member of the family Alvarezsauridae, small coelurosaurian dinosaurs which are characterized by short but powerful forelimbs specialized for digging. The type species is Shuvuuia deserti, or...
, Sinornithosaurus
Sinornithosaurus
Sinornithosaurus is a genus of feathered dromaeosaurid dinosaur from the early Cretaceous Period of the Yixian Formation in what is now China. It was the fifth non–avian feathered dinosaur genus discovered by 1999...
, Sinosauropteryx
Sinosauropteryx
Sinosauropteryx was the first genus of dinosaur outside of Avialae to be found with evidence of feathers. They were covered with "furry" coats of very simple filament-like feathers...
, and Jinfengopteryx
Jinfengopteryx
Jinfengopteryx is a genus of maniraptoran dinosaur. It was found in the Qiaotou member of the Huajiying Formation of Hebei Province, China, and is therefore of uncertain age...
, along with dinosaur-like birds, such as Confuciusornis
Confuciusornis
Confuciusornis is a genus of primitive crow-sized birds from the Early Cretaceous Yixian and Jiufotang Formations of China, dating from 125 to 120 million years ago...
, which are anatomically closer to modern avians. All of them have been found in the same area and formation, in northern China. The Dromaeosauridae
Dromaeosauridae
Dromaeosauridae is a family of bird-like theropod dinosaurs. They were small- to medium-sized feathered carnivores that flourished in the Cretaceous Period. The name Dromaeosauridae means 'running lizards', from Greek dromeus meaning 'runner' and sauros meaning 'lizard'...
family, in particular, seems to have been heavily feathered and at least one dromaeosaurid, Cryptovolans, may have been capable of flight.
Skeleton
Because feathers are often associated with birds, feathered dinosaurs are often touted as the missing linkTransitional fossil
A transitional fossil is any fossilized remains of a lifeform that exhibits characteristics of two distinct taxonomic groups. A transitional fossil is the fossil of an organism near the branching point where major individual lineages diverge...
between birds and dinosaurs. However, the multiple skeletal features also shared by the two groups represent the more important link for paleontologist
Paleontology
Paleontology "old, ancient", ὄν, ὀντ- "being, creature", and λόγος "speech, thought") is the study of prehistoric life. It includes the study of fossils to determine organisms' evolution and interactions with each other and their environments...
s. Furthermore, it is increasingly clear that the relationship between birds and dinosaurs, and the evolution of flight, are more complex topics than previously realized. For example, while it was once believed that birds evolved from dinosaurs in one linear progression, some scientists, most notably Gregory S. Paul
Gregory S. Paul
Gregory Scott Paul is a freelance researcher, author and illustrator who works in paleontology, and more recently has examined sociology and theology. He is best known for his work and research on theropod dinosaurs and his detailed illustrations, both live and skeletal...
, conclude that dinosaurs such as the dromaeosaur
Dromaeosauridae
Dromaeosauridae is a family of bird-like theropod dinosaurs. They were small- to medium-sized feathered carnivores that flourished in the Cretaceous Period. The name Dromaeosauridae means 'running lizards', from Greek dromeus meaning 'runner' and sauros meaning 'lizard'...
s may have evolved from birds, losing the power of flight while keeping their feathers in a manner similar to the modern ostrich
Ostrich
The Ostrich is one or two species of large flightless birds native to Africa, the only living member of the genus Struthio. Some analyses indicate that the Somali Ostrich may be better considered a full species apart from the Common Ostrich, but most taxonomists consider it to be a...
and other ratite
Ratite
A ratite is any of a diverse group of large, flightless birds of Gondwanan origin, most of them now extinct. Unlike other flightless birds, the ratites have no keel on their sternum—hence the name from the Latin ratis...
s.
Comparisons of bird and dinosaur skeletons, as well as cladistic analysis
Cladistics
Cladistics is a method of classifying species of organisms into groups called clades, which consist of an ancestor organism and all its descendants . For example, birds, dinosaurs, crocodiles, and all descendants of their most recent common ancestor form a clade...
, strengthens the case for the link, particularly for a branch of theropods called maniraptor
Maniraptora
Maniraptora is a clade of coelurosaurian dinosaurs which includes the birds and the dinosaurs that were more closely related to them than to Ornithomimus velox. It contains the major subgroups Avialae, Deinonychosauria, Oviraptorosauria and Therizinosauria. Ornitholestes and the Alvarezsauroidea...
s. Skeletal similarities include the neck, pubis
Pubis (bone)
In vertebrates, the pubic bone is the ventral and anterior of the three principal bones composing either half of the pelvis.It is covered by a layer of fat, which is covered by the mons pubis....
, wrist
Wrist
In human anatomy, the wrist is variously defined as 1) the carpus or carpal bones, the complex of eight bones forming the proximal skeletal segment of the hand;...
(semi-lunate carpal
Carpus
In tetrapods, the carpus is the sole cluster of bones in the wrist between the radius and ulna and the metacarpus. The bones of the carpus do not belong to individual fingers , whereas those of the metacarpus do. The corresponding part of the foot is the tarsus...
), arm and pectoral girdle, shoulder blade
Scapula
In anatomy, the scapula , omo, or shoulder blade, is the bone that connects the humerus with the clavicle ....
, clavicle
Furcula
The ' is a forked bone found in birds, formed by the fusion of the two clavicles. In birds, its function is the strengthening of the thoracic skeleton to withstand the rigors of flight....
, and breast bone
Keel (bird)
A keel or carina in bird anatomy is an extension of the sternum which runs axially along the midline of the sternum and extends outward, perpendicular to the plane of the ribs. The keel provides an anchor to which a bird's wing muscles attach, thereby providing adequate leverage for flight...
.
Lungs
Large meat-eating dinosaurs had a complex system of air sacs similar to those found in modern birds, according to an investigation which was led by Patrick M. O'Connor of Ohio UniversityOhio University
Ohio University is a public university located in the Midwestern United States in Athens, Ohio, situated on an campus...
. The lungs of theropod dinosaurs (carnivores that walked on two legs and had birdlike feet) likely pumped air into hollow sacs in their skeleton
Skeleton
The skeleton is the body part that forms the supporting structure of an organism. There are two different skeletal types: the exoskeleton, which is the stable outer shell of an organism, and the endoskeleton, which forms the support structure inside the body.In a figurative sense, skeleton can...
s, as is the case in birds. "What was once formally considered unique to birds was present in some form in the ancestors of birds", O'Connor said. The study was funded in part by the National Science Foundation
National Science Foundation
The National Science Foundation is a United States government agency that supports fundamental research and education in all the non-medical fields of science and engineering. Its medical counterpart is the National Institutes of Health...
.
Heart
Computed tomographyComputed tomography
X-ray computed tomography or Computer tomography , is a medical imaging method employing tomography created by computer processing...
(CT) scans conducted in 2000 of the chest cavity of a specimen of the ornithopod
Ornithopod
Ornithopods or members of the clade Ornithopoda are a group of ornithischian dinosaurs that started out as small, bipedal running grazers, and grew in size and numbers until they became one of the most successful groups of herbivores in the Cretaceous world, and dominated the North American...
Thescelosaurus
Thescelosaurus
Thescelosaurus was a genus of small ornithopod dinosaur that appeared at the very end of the Late Cretaceous period in North America. It was a member of the last dinosaurian fauna before the Cretaceous-Tertiary extinction event around 65.5 million years ago...
found the apparent remnants of complex four-chambered hearts, much like those found in today's mammals and birds. The idea is controversial within the scientific community, coming under fire for bad anatomical science or simply wishful thinking.
A study published in 2011 applied multiple lines of inquiry to the question of the object's identity, including more advanced CT scanning, histology
Histology
Histology is the study of the microscopic anatomy of cells and tissues of plants and animals. It is performed by examining cells and tissues commonly by sectioning and staining; followed by examination under a light microscope or electron microscope...
, X-ray diffraction
X-ray scattering techniques
X-ray scattering techniques are a family of non-destructive analytical techniques which reveal information about the crystallographic structure, chemical composition, and physical properties of materials and thin films...
, X-ray photoelectron spectroscopy
X-ray photoelectron spectroscopy
X-ray photoelectron spectroscopy is a quantitative spectroscopic technique that measures the elemental composition, empirical formula, chemical state and electronic state of the elements that exist within a material...
, and scanning electron microscopy. From these methods, the authors found that: the object's internal structure does not include chambers but is made up of three unconnected areas of lower density material, and is not comparable to the structure of an ostrich
Ostrich
The Ostrich is one or two species of large flightless birds native to Africa, the only living member of the genus Struthio. Some analyses indicate that the Somali Ostrich may be better considered a full species apart from the Common Ostrich, but most taxonomists consider it to be a...
's heart; the "walls" are composed of sedimentary minerals not known to be produced in biological systems, such as goethite, feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
minerals, quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
, and gypsum
Gypsum
Gypsum is a very soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O. It is found in alabaster, a decorative stone used in Ancient Egypt. It is the second softest mineral on the Mohs Hardness Scale...
, as well as some plant fragments; carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
, nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
, and phosphorus
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
, chemical element
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
s important to life, were lacking in their samples; and cardiac cellular structures were absent. There was one possible patch with animal cellular structures. The authors found their data supported identification as a concretion of sand from the burial environment, not the heart, with the possibility that isolated areas of tissues were preserved.
The question of how this find reflects metabolic rate and dinosaur internal anatomy is moot, though, regardless of the object's identity. Both modern crocodilia
Crocodilia
Crocodilia is an order of large reptiles that appeared about 84 million years ago in the late Cretaceous Period . They are the closest living relatives of birds, as the two groups are the only known survivors of the Archosauria...
ns and bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...
s, the closest living relatives of dinosaurs, have four-chambered hearts (albeit modified in crocodilians), so dinosaurs probably had them as well; the structure is not necessarily tied to metabolic rate.
Sleeping posture
Fossils of the troodontsTroodontidae
Troodontidae is a family of bird-like theropod dinosaurs. During most of the 20th century, troodontid fossils were few and scrappy and they have therefore been allied, at various times, with many dinosaurian lineages...
Mei
Mei (dinosaur)
Mei is a genus of duck-sized troodontid dinosaur first unearthed by paleontologists in Liaoning, China in 2004. Mei lived during the Early Cretaceous Period. Its binomial name, Mei long means sleeping dragon...
and Sinornithoides
Sinornithoides
Sinornithoides is a genus of troodontid theropod dinosaur. It lived during the Early Cretaceous . It measured approximately 1 meter long...
demonstrate that the dinosaurs slept like certain modern birds, with their heads tucked under their arms. This behavior, which may have helped to keep the head warm, is also characteristic of modern birds.
Reproductive biology
When laying eggs, female birds grow a special type of bone in their limbs. This medullary bone, which is rich in calcium, forms a layer inside the hard outer bone that is used to make eggshells. The presence of endosteally derived bone tissues lining the interior marrow cavities of portions of a Tyrannosaurus rex specimen's hind limb suggested that T. rex used similar reproductive strategies, and revealed the specimen to be female. Further research has found medullary bone in the theropod AllosaurusAllosaurus
Allosaurus is a genus of large theropod dinosaur that lived 155 to 150 million years ago during the late Jurassic period . The name Allosaurus means "different lizard". It is derived from the Greek /allos and /sauros...
and ornithopod Tenontosaurus
Tenontosaurus
Tenontosaurus is a genus of medium- to large-sized ornithopod dinosaur. The genus is known from the late Aptian to Albian ages of the middle Cretaceous period sediments of western North America, dating between 115 to 108 million years ago...
. Because the line of dinosaurs that includes Allosaurus and Tyrannosaurus diverged from the line that led to Tenontosaurus very early in the evolution of dinosaurs, this suggests that dinosaurs in general produced medullary tissue.
Brooding and care of young
Citipati
Citipati is a genus of oviraptorid theropod dinosaur from the Late Cretaceous Period of what is now Mongolia . It is one of the best-known oviraptorids, thanks to a number of well-preserved skeletons, including several specimens found in brooding positions atop nests of eggs...
specimens have been found resting over the eggs in its nest in a position most reminiscent of brooding
Avian incubation
Incubation refers to the process by which certain oviparous animals hatch their eggs, and to the development of the embryo within the egg. The most vital factor of incubation is the constant temperature required for its development over a specific period. Especially in domestic fowl, the act of...
.
Numerous dinosaur species, for example Maiasaura
Maiasaura
Maiasaura is a large duck-billed dinosaur genus that lived in the area currently covered by the state of Montana in the Upper Cretaceous Period , about 74 million years ago....
, have been found in herds mixing both very young and adult individuals, suggesting rich interactions between them.
A dinosaur embryo was found without teeth, which suggests some parental care was required to feed the young dinosaur, possibly the adult dinosaur regurgitated food into the young dinosaur's mouth (see altricial
Altricial
Altricial, meaning "requiring nourishment", refers to a pattern of growth and development in organisms which are incapable of moving around on their own soon after hatching or being born...
). This behaviour is seen in numerous bird species; parent birds regurgitate food into the hatchling's mouth.
Gizzard stones
Both birds and dinosaurs use gizzardGizzard
The gizzard, also referred to as the ventriculus, gastric mill, and gigerium, is an organ found in the digestive tract of some animals, including birds, reptiles, earthworms and some fish. This specialized stomach constructed of thick, muscular walls is used for grinding up food; often rocks are...
stones. These stones are swallowed by animals to aid digestion and break down food and hard fibres once they enter the stomach. When found in association with fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
s, gizzard stones are called gastrolith
Gastrolith
A gastrolith, also called a stomach stone or gizzard stones, is a rock held inside a gastrointestinal tract. Gastroliths are retained in the muscular gizzard and used to grind food in animals lacking suitable grinding teeth. The grain size depends upon the size of the animal and the gastrolith's...
s.
Gizzard stones are also found in some fish (mullets, mud shad, and the gilaroo, a type of trout) and in crocodiles.
Molecular evidence and soft tissue
Italy
Italy , officially the Italian Republic languages]] under the European Charter for Regional or Minority Languages. In each of these, Italy's official name is as follows:;;;;;;;;), is a unitary parliamentary republic in South-Central Europe. To the north it borders France, Switzerland, Austria and...
. The discovery was reported in 1998, and described the specimen of a small, very young coelurosaur
Coelurosauria
Coelurosauria is the clade containing all theropod dinosaurs more closely related to birds than to carnosaurs. In the past, it was used to refer to all small theropods, although this classification has been abolished...
, Scipionyx
Scipionyx
Scipionyx is a genus of theropod dinosaur from the Early Cretaceous of Italy, around 113 million years ago. There has been only one skeleton discovered, which is notable for the preservation of soft tissue and internal organs. It is the fossil of a juvenile only half a metre long...
samniticus. The fossil includes portions of the intestines, colon, liver, muscles, and windpipe of this immature dinosaur.
In the March 2005 issue of Science
Science (journal)
Science is the academic journal of the American Association for the Advancement of Science and is one of the world's top scientific journals....
, Dr. Mary Higby Schweitzer
Mary Higby Schweitzer
Mary Higby Schweitzer is a paleontologist at North Carolina State University known for leading the groups which discovered the remains of blood cells in dinosaur fossils and later discovered soft tissue remains in the Tyrannosaurus rex specimen MOR 1125, as well as evidence that the specimen was a...
and her team announced the discovery of flexible material resembling actual soft tissue inside a 68-million-year-old Tyrannosaurus rex
Tyrannosaurus
Tyrannosaurus meaning "tyrant," and sauros meaning "lizard") is a genus of coelurosaurian theropod dinosaur. The species Tyrannosaurus rex , commonly abbreviated to T. rex, is a fixture in popular culture. It lived throughout what is now western North America, with a much wider range than other...
leg bone
Bone
Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...
from the Hell Creek Formation
Hell Creek Formation
The Hell Creek Formation is an intensely-studied division of Upper Cretaceous to lower Paleocene rocks in North America, named for exposures studied along Hell Creek, near Jordan, Montana...
in Montana
Montana
Montana is a state in the Western United States. The western third of Montana contains numerous mountain ranges. Smaller, "island ranges" are found in the central third of the state, for a total of 77 named ranges of the Rocky Mountains. This geographical fact is reflected in the state's name,...
. After recovery, the tissue was rehydrated by the science team. The seven collagen
Collagen
Collagen is a group of naturally occurring proteins found in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content...
types obtained from the bone fragments, compared to collagen data from living birds (specifically, a chicken
Chicken
The chicken is a domesticated fowl, a subspecies of the Red Junglefowl. As one of the most common and widespread domestic animals, and with a population of more than 24 billion in 2003, there are more chickens in the world than any other species of bird...
), suggest that older theropods and birds are closely related.
When the fossilized bone was treated over several weeks to remove mineral content from the fossilized bone marrow cavity (a process called demineralization), Schweitzer found evidence of intact structures such as blood vessel
Blood vessel
The blood vessels are the part of the circulatory system that transports blood throughout the body. There are three major types of blood vessels: the arteries, which carry the blood away from the heart; the capillaries, which enable the actual exchange of water and chemicals between the blood and...
s, bone matrix, and connective tissue (bone fibers). Scrutiny under the microscope further revealed that the putative dinosaur soft tissue had retained fine structures (microstructures) even at the cellular level. The exact nature and composition of this material, and the implications of Dr. Schweitzer's discovery, are not yet clear; study and interpretation of the specimens is ongoing.
The successful extraction of ancient DNA from dinosaur fossils has been reported on two separate occasions, but upon further inspection and peer review
Peer review
Peer review is a process of self-regulation by a profession or a process of evaluation involving qualified individuals within the relevant field. Peer review methods are employed to maintain standards, improve performance and provide credibility...
, neither of these reports could be confirmed. However, a functional visual peptide
Peptide
Peptides are short polymers of amino acid monomers linked by peptide bonds. They are distinguished from proteins on the basis of size, typically containing less than 50 monomer units. The shortest peptides are dipeptides, consisting of two amino acids joined by a single peptide bond...
of a theoretical dinosaur has been inferred using analytical phylogenetic reconstruction methods on gene sequences of related modern species such as reptiles and birds. In addition, several protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s have putatively been detected in dinosaur fossils, including hemoglobin
Hemoglobin
Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates, with the exception of the fish family Channichthyidae, as well as the tissues of some invertebrates...
.
Origin of bird flight
Debates about the origin of bird flight are almost as old as the idea that birds evolved from dinosaurDinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...
s, which arose soon after the discovery of Archaeopteryx
Archaeopteryx
Archaeopteryx , sometimes referred to by its German name Urvogel , is a genus of theropod dinosaur that is closely related to birds. The name derives from the Ancient Greek meaning "ancient", and , meaning "feather" or "wing"...
in 1862. Two theories have dominated most of the discussion since then: the cursorial ("from the ground up") theory proposes that birds evolved from small, fast predators that ran on the ground; the arboreal ("from the trees down") theory proposes that powered flight evolved from unpowered gliding by arboreal (tree-climbing) animals. A more recent theory, "wing-assisted incline running" (WAIR), is a variant of the cursorial theory and proposes that wings developed their aerodynamic
Aerodynamics
Aerodynamics is a branch of dynamics concerned with studying the motion of air, particularly when it interacts with a moving object. Aerodynamics is a subfield of fluid dynamics and gas dynamics, with much theory shared between them. Aerodynamics is often used synonymously with gas dynamics, with...
functions as a result of the need to run quickly up very steep slopes, for example to escape from predators.
Cursorial ("from the ground up") theory
Samuel Wendell Williston
Samuel Wendell Williston was an American educator and paleontologist who was the first to propose that birds developed flight cursorially , rather than arboreally . He was also an entomologist, specialising in Diptera.-Early life:Williston was born in Boston, Massachusetts to Samuel Williston and...
, and elaborated upon by Baron Nopcsa
Franz Nopcsa von Felso-Szilvás
Baron Franz Nopcsa von Felső-Szilvás was a Hungarian-born aristocrat, adventurer, scholar, and paleontologist...
. This hypothesis proposes that some fast-running animals with long tails used their arms to keep their balance while running. Modern versions of this theory differ in many details from the Williston-Nopcsa version, mainly as a result of discoveries since Nopcsa's time.
Nopcsa theorized that increasing the surface area of the outstretched arms could have helped small cursorial predators to keep their balance, and that the scales of the forearms became elongated, evolving
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
into feathers. The feathers could also have been used as a trap to catch insects or other prey. Progressively, the animals would have leapt for longer distances, helped by their evolving wings. Nopcsa also proposed that there were three main stages in the evolution of flight. First, passive flight was realized, in which the developed wing structures served as a sort of parachute
Parachute
A parachute is a device used to slow the motion of an object through an atmosphere by creating drag, or in the case of ram-air parachutes, aerodynamic lift. Parachutes are usually made out of light, strong cloth, originally silk, now most commonly nylon...
. Second, active flight was possible, in which the animal achieved flight by flapping its wings. He used Archaeopteryx as an example of this second stage. Finally, birds gained the ability to soar.
It is now thought that feathers did not evolve from scales, as feathers are made of different protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s. More seriously, Nopcsa's theory assumes that feathers evolved as part of the evolution of flight, and recent discoveries prove that assumption is false.
Feather
Feather
Feathers are one of the epidermal growths that form the distinctive outer covering, or plumage, on birds and some non-avian theropod dinosaurs. They are considered the most complex integumentary structures found in vertebrates, and indeed a premier example of a complex evolutionary novelty. They...
s are very common in coelurosaurian dinosaurs
Coelurosauria
Coelurosauria is the clade containing all theropod dinosaurs more closely related to birds than to carnosaurs. In the past, it was used to refer to all small theropods, although this classification has been abolished...
(including the early tyrannosauroid
Tyrannosauridae
Tyrannosauridae is a family of coelurosaurian theropod dinosaurs which comprises two subfamilies containing up to six genera, including the eponymous Tyrannosaurus. The exact number of genera is controversial, with some experts recognizing as few as three...
Dilong
Dilong (dinosaur)
Dilong is a genus of small tyrannosauroid dinosaur. The only species is Dilong paradoxus. It is from the Lower Cretaceous Yixian Formation near Lujiatun, Beipiao, in the western Liaoning province of China. It lived about 130 million years ago...
). Modern bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...
s are classified as coelurosaurs by nearly all palaeontologists, though not by a few ornithologists
Ornithology
Ornithology is a branch of zoology that concerns the study of birds. Several aspects of ornithology differ from related disciplines, due partly to the high visibility and the aesthetic appeal of birds...
. The modern version of the "from the ground up" hypothesis argues that birds' ancestors were small, feathered, ground-running predatory dinosaurs (rather like roadrunners
Geococcyx
The roadrunners are two species of bird in the genus Geococcyx of the cuckoo family, Cuculidae, native to North and Central America...
in their hunting style) that used their forelimbs for balance while pursuing prey, and that the forelimbs and feathers later evolved in ways that provided gliding and then powered flight. The most widely suggested original functions of feathers include thermal insulation and competitive displays, as in modern birds.
All of the Archaeopteryx fossils come from marine sediments, and it has been suggested that wings may have helped the birds run over water in the manner of the Jesus Christ Lizard (Common basilisk
Common Basilisk
The Common Basilisk is a lizard found in Central and South American rainforests near rivers and streams.The basilisk is part of the corytophanid family...
).
Most recent refutations of the "from the ground up" hypothesis attempt to refute the modern version's assumption that birds are modified coelurosaurian dinosaurs. The strongest attacks are based on embryological analyses
Embryology
Embryology is a science which is about the development of an embryo from the fertilization of the ovum to the fetus stage...
which conclude that birds' wings are formed from digits 2, 3, and 4 (corresponding to the index, middle, and ring fingers in humans; the first of a bird's three digits forms the alula, which they use to avoid stalling
Stall (flight)
In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as angle of attack increases. This occurs when the critical angle of attack of the foil is exceeded...
in low-speed flight, for example when landing); but the hands of coelurosaurs are formed by digits 1, 2, and 3 (thumb and first two fingers in humans). However, these embryological analyses were immediately challenged on the embryological grounds that the "hand" often develops differently in clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
s that have lost some digits in the course of their evolution, and that birds' "hands" do develop from digits 1, 2, and 3. This debate is complex and not yet resolved - see "Digit homology" below.
Wing-assisted incline running
The WAIR hypothesis was prompted by observation of young chukarChukar
The Chukar Partridge or Chukar is a Eurasian upland gamebird in the pheasant family Phasianidae. It has been considered to form a superspecies complex along with the Rock Partridge, Philby's Partridge and Przevalski's Partridge and treated in the past as conspecific particularly with the first...
chicks, and proposes that wings developed their aerodynamic functions as a result of the need to run quickly up very steep slopes such as tree trunks, for example to escape from predators. This makes it a specialized type of cursorial ("from the ground up") theory. Note that in this scenario birds need downforce to give their feet increased grip. But early birds, including Archaeopteryx
Archaeopteryx
Archaeopteryx , sometimes referred to by its German name Urvogel , is a genus of theropod dinosaur that is closely related to birds. The name derives from the Ancient Greek meaning "ancient", and , meaning "feather" or "wing"...
, lacked the shoulder mechanism by which modern birds' wings produce swift, powerful upstrokes; since the downforce on which WAIR depends is generated by upstrokes, it seems that early birds were incapable of WAIR. Because WAIR is a behavioural trait without osteological specializations, the phylogenetic placement of the flight stroke before the divergence of Neornithes makes it impossible to determine if WAIR is ancestral to the avian flight stroke or derived from it.
Arboreal ("from the trees down") theory
Several small dinosaurs from the Jurassic or Early Cretaceous, all with feathers, have been interpreted as possibly having arboreal and/or aerodynamic adaptations. These include Scansoriopteryx
Scansoriopteryx
Scansoriopteryx is a genus of avialan dinosaur. Described from only a single juvenile fossil specimen found in Liaoning, China, Scansoriopteryx is a sparrow-sized animal that shows adaptations in the foot indicating an arboreal lifestyle. It possessed an unusual, elongated third finger...
, Epidexipteryx
Epidexipteryx
Epidexipteryx is a genus of small avialan dinosaur, known from one fossil specimen in the collection of the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing. Epidexipteryx represents the earliest known example of ornamental feathers in the fossil record. The type specimen is...
, Microraptor
Microraptor
Microraptor is a genus of small, four-winged dromaeosaurid dinosaurs. Numerous well-preserved fossil specimens have been recovered from Liaoning, China...
, Pedopenna
Pedopenna
Pedopenna is a genus of small, feathered, maniraptoran dinosaur from the Daohugou Beds in China. It is possibly older than Archaeopteryx, though the age of the Daohugou Beds where it was found is debated...
, and Anchiornis
Anchiornis
Anchiornis is a genus of small, feathered, deinonychosaurian dinosaur. The genus Anchiornis contains the type species Anchiornis huxleyi, named in honor of Thomas Henry Huxley, an early proponent of biological evolution, and the first to propose a close evolutionary relationship between birds and...
. Anchiornis
Anchiornis
Anchiornis is a genus of small, feathered, deinonychosaurian dinosaur. The genus Anchiornis contains the type species Anchiornis huxleyi, named in honor of Thomas Henry Huxley, an early proponent of biological evolution, and the first to propose a close evolutionary relationship between birds and...
is particularly important to this subject, as it lived at the beginning of the Late Jurassic, long before Archaeopteryx
Archaeopteryx
Archaeopteryx , sometimes referred to by its German name Urvogel , is a genus of theropod dinosaur that is closely related to birds. The name derives from the Ancient Greek meaning "ancient", and , meaning "feather" or "wing"...
.
Analysis of the proportions of the toe bones of the most primitive birds Archaeopteryx and Confuciusornis
Confuciusornis
Confuciusornis is a genus of primitive crow-sized birds from the Early Cretaceous Yixian and Jiufotang Formations of China, dating from 125 to 120 million years ago...
, compared to those of living species, suggest that the early species may have lived both on the ground and in trees.
One study suggested that the earliest birds and their immediate ancestors did not climb trees. This study determined that the amount of toe claw curvature of early birds was more like that seen in modern ground-foraging birds than in perching birds.
Diminished significance of Archaeopteryx
Archaeopteryx
Archaeopteryx , sometimes referred to by its German name Urvogel , is a genus of theropod dinosaur that is closely related to birds. The name derives from the Ancient Greek meaning "ancient", and , meaning "feather" or "wing"...
was the first and for a long time the only known feathered Mesozoic
Mesozoic
The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...
animal (or dinosaur, if one accepts the majority view that birds are modified dinosaurs). As a result, discussion of the evolution of birds and of bird flight centered on Archaeopteryx at least until the mid-1990s.
There has been debate about whether Archaeopteryx
Archaeopteryx
Archaeopteryx , sometimes referred to by its German name Urvogel , is a genus of theropod dinosaur that is closely related to birds. The name derives from the Ancient Greek meaning "ancient", and , meaning "feather" or "wing"...
could really fly. It appears that Archaeopteryx had the brain structures and inner-ear balance sensors that birds use to control their flight. Archaeopteryx also had a wing feather arrangement like that of modern birds and similarly asymmetrical flight feathers on its wings and tail. But Archaeopteryx lacked the shoulder mechanism by which modern birds' wings produce swift, powerful upstrokes (see diagram above of supracoracoideus pulley); this may mean that it and other early birds were incapable of flapping flight and could only glide.
But the discovery since the early 1990s of many feathered dinosaurs
Feathered dinosaurs
The realization that dinosaurs are closely related to birds raised the obvious possibility of feathered dinosaurs. Fossils of Archaeopteryx include well-preserved feathers, but it was not until the early 1990s that clearly non-avialan dinosaur fossils were discovered with preserved feathers...
means that Archaeopteryx is no longer the key figure in the evolution of bird flight. Other small, feathered coelurosaurs from the Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
and Late Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
show features that may be precursors of avian flight, for example: Rahonavis
Rahonavis
Rahonavis is a genus of bird-like dinosaur from the Late Cretaceous of what is now northwestern Madagascar. It is known from a partial skeleton found in Maevarano Formation rocks at a quarry near Berivotra, Mahajanga Province...
, a ground-runner which had a Velociraptor
Velociraptor
Velociraptor is a genus of dromaeosaurid theropod dinosaur that existed approximately 75 to 71 million years ago during the later part of the Cretaceous Period. Two species are currently recognized, although others have been assigned in the past. The type species is V. mongoliensis; fossils...
-like raised sickle claw on the second toe and which some paleontologists think was better adapted for flight than Archaeopteryx; Scansoriopteryx
Scansoriopteryx
Scansoriopteryx is a genus of avialan dinosaur. Described from only a single juvenile fossil specimen found in Liaoning, China, Scansoriopteryx is a sparrow-sized animal that shows adaptations in the foot indicating an arboreal lifestyle. It possessed an unusual, elongated third finger...
, an arboreal dinosaur that may provide some support for the "from the trees down" theory; Microraptor
Microraptor
Microraptor is a genus of small, four-winged dromaeosaurid dinosaurs. Numerous well-preserved fossil specimens have been recovered from Liaoning, China...
, an arboreal dinosaur that may have been capable of powered flight but, if so, more like a biplane
Biplane
A biplane is a fixed-wing aircraft with two superimposed main wings. The Wright brothers' Wright Flyer used a biplane design, as did most aircraft in the early years of aviation. While a biplane wing structure has a structural advantage, it produces more drag than a similar monoplane wing...
, as it had well-developed feathers on its legs. As early as 1915, some scientists had argued that the evolution of bird flight may have gone through a four-winged (or tetrapteryx) stage.
Secondary flightlessness in dinosaurs
(bird)|2=Microraptor
Microraptor
Microraptor is a genus of small, four-winged dromaeosaurid dinosaurs. Numerous well-preserved fossil specimens have been recovered from Liaoning, China...
(dromaeosaur)
}}
|2=Dromaeosaur
Dromaeosauridae
Dromaeosauridae is a family of bird-like theropod dinosaurs. They were small- to medium-sized feathered carnivores that flourished in the Cretaceous Period. The name Dromaeosauridae means 'running lizards', from Greek dromeus meaning 'runner' and sauros meaning 'lizard'...
s
}}
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Groups usually regarded as birds are in bold type.
A hypothesis, credited to Gregory Paul and propounded in his books Predatory Dinosaurs of the World (1988) and Dinosaurs of the Air (2002), suggests that some groups of non-flying carnivorous dinosaurs, especially deinonychosaurs but perhaps others such as oviraptorosaur
Oviraptorosauria
Oviraptorosaurs are a group of feathered maniraptoran dinosaurs from the Cretaceous Period of what are now Asia and North America. They are distinct for their characteristically short, beaked, parrot - like skulls, with or without bony crests atop the head...
s, therizinosaur
Therizinosaur
Therizinosaurs are theropod dinosaurs belonging to the clade Therizinosauria. Therizinosaur fossils have been found in Early through Late Cretaceous deposits in Mongolia, the People's Republic of China and Western North America...
s, alvarezsaurids
Alvarezsauridae
Alvarezsauridae is an enigmatic family of small, long-legged running dinosaurs. Although originally thought to represent the earliest known flightless birds, a consensus of recent work suggests that they are primitive members of the Maniraptora. Other work found them to be the sister group to the...
and ornithomimosaur
Ornithomimosauria
The Ornithomimosauria, ornithomimosaurs or ostrich dinosaurs were theropod dinosaurs which bore a superficial resemblance to modern ostriches. They were fast, omnivorous or herbivorous dinosaurs from the Cretaceous Period of Laurasia...
s, are actually descended from birds. Paul also proposed that the bird ancestor of these groups was more advanced in its flight adaptations than Archaeopteryx
Archaeopteryx
Archaeopteryx , sometimes referred to by its German name Urvogel , is a genus of theropod dinosaur that is closely related to birds. The name derives from the Ancient Greek meaning "ancient", and , meaning "feather" or "wing"...
. This would mean that Archaeopteryx
Archaeopteryx
Archaeopteryx , sometimes referred to by its German name Urvogel , is a genus of theropod dinosaur that is closely related to birds. The name derives from the Ancient Greek meaning "ancient", and , meaning "feather" or "wing"...
is thus less closely related to extant birds than these dinosaurs are.
Paul's hypothesis received additional support when Mayr et al. (2005) analyzed a new, tenth specimen of Archaeopteryx
Archaeopteryx
Archaeopteryx , sometimes referred to by its German name Urvogel , is a genus of theropod dinosaur that is closely related to birds. The name derives from the Ancient Greek meaning "ancient", and , meaning "feather" or "wing"...
, and concluded that Archaeopteryx was the sister clade to the Deinonychosauria, but that the more advanced bird Confuciusornis
Confuciusornis
Confuciusornis is a genus of primitive crow-sized birds from the Early Cretaceous Yixian and Jiufotang Formations of China, dating from 125 to 120 million years ago...
was within the Dromaeosauridae. This result supports Paul's hypothesis, suggesting that the Deinonychosauria and the Troodontidae are part of Aves, the bird lineage proper, and secondarily flightless. This paper, however, excluded all other birds and thus did not sample their character distributions. The paper was criticized by Corfe and Butler (2006) who found the authors could not support their conclusions statistically. Mayr et al. agreed that the statistical support was weak, but added that it is also weak for the alternative scenarios.
Paul's hypothesis about the position of Archaeopteryx is not supported by current cladistic analyses which generally find that Archaeopteryx is closer to birds, within the clade Avialae
Avialae
Avialae is a clade of dinosaurs containing their only living representatives, birds , and the most immediate extinct relatives of birds.-Competing definitions:...
, than it is to deinonychosaurs or oviraptorosaurs. However, the version of this theory stating that some non-flying carnivorous dinosaurs may have had flying ancestors is supported by some fossils. Especially, Microraptor
Microraptor
Microraptor is a genus of small, four-winged dromaeosaurid dinosaurs. Numerous well-preserved fossil specimens have been recovered from Liaoning, China...
, Pedopenna
Pedopenna
Pedopenna is a genus of small, feathered, maniraptoran dinosaur from the Daohugou Beds in China. It is possibly older than Archaeopteryx, though the age of the Daohugou Beds where it was found is debated...
, and Anchiornis
Anchiornis
Anchiornis is a genus of small, feathered, deinonychosaurian dinosaur. The genus Anchiornis contains the type species Anchiornis huxleyi, named in honor of Thomas Henry Huxley, an early proponent of biological evolution, and the first to propose a close evolutionary relationship between birds and...
all have winged feet, share many features, and lie close to the base of the clade Paraves
Paraves
Paraves is a branch-based clade containing birds and other closely related dinosaurs. The paravians include the Avialae, including Archaeopteryx and modern birds, and the Deinonychosauria, which includes the dromaeosaurids and troodontids....
. This suggests that the ancestral paravian was a four-winged glider, and that larger Deinonychosaurs secondarily lost the ability to glide, while the bird lineage increased in aerodynamic ability as it progressed.
Digit homology
There is a debate between embryologistEmbryology
Embryology is a science which is about the development of an embryo from the fertilization of the ovum to the fetus stage...
s and paleontologist
Paleontology
Paleontology "old, ancient", ὄν, ὀντ- "being, creature", and λόγος "speech, thought") is the study of prehistoric life. It includes the study of fossils to determine organisms' evolution and interactions with each other and their environments...
s whether the hands of theropod
Theropoda
Theropoda is both a suborder of bipedal saurischian dinosaurs, and a clade consisting of that suborder and its descendants . Dinosaurs belonging to the suborder theropoda were primarily carnivorous, although a number of theropod groups evolved herbivory, omnivory, and insectivory...
dinosaurs and birds are essentially different, based on phalangeal
Phalanges of the hand
The phalanges of the hand are commonly known as the finger bones. They are fourteen in number, three for each finger, and two for the thumb.Each consists of a body and two extremities....
counts, a count of the number of phalanges (fingers) in the hand. This is an important and fiercely debated area of research because its results may challenge the consensus that birds are descendants of dinosaurs.
Embryologists and some paleontologists who oppose the bird-dinosaur link, have long numbered the digits of birds II-III-IV on the basis of multiple studies of the development in the egg.
This is based on the fact that in most amniote
Amniote
The amniotes are a group of tetrapods that have a terrestrially adapted egg. They include synapsids and sauropsids , as well as their fossil ancestors. Amniote embryos, whether laid as eggs or carried by the female, are protected and aided by several extensive membranes...
s, the first digit to form in a 5-fingered hand is digit IV, which develops a primary axis. Therefore, embryologists have identified the primary axis in birds as digit IV, and the surviving digits as II-III-IV. The fossils of advanced theropod
Theropoda
Theropoda is both a suborder of bipedal saurischian dinosaurs, and a clade consisting of that suborder and its descendants . Dinosaurs belonging to the suborder theropoda were primarily carnivorous, although a number of theropod groups evolved herbivory, omnivory, and insectivory...
(Tetanurae
Tetanurae
Tetanurae, or "stiff tails", is a clade that includes most theropod dinosaurs, as well as birds. Tetanurans first appear during the early or middle Jurassic Period.-Definition:...
) hands appear to have the digits I-II-III (some genera within Avetheropoda also have a reduced digit IV). If this is true, then the II-III-IV development of digits in birds is an indication against theropod (dinosaur) ancestry. However, with no ontogenical
Ontogeny
Ontogeny is the origin and the development of an organism – for example: from the fertilized egg to mature form. It covers in essence, the study of an organism's lifespan...
(developmental) basis to definitively state which digits are which on a theropod hand (because no non-avian theropods can be observed growing and developing today), the labelling of the theropod hand is not absolutely conclusive.
Paleontologists have traditionally identified avian digits as I-II-III. They argue that the digits of birds number I-II-III, just as those of theropod dinosaurs do, by the conserved phalangeal formula. The phalangeal count for archosaur
Archosaur
Archosaurs are a group of diapsid amniotes whose living representatives consist of modern birds and crocodilians. This group also includes all extinct non-avian dinosaurs, many extinct crocodilian relatives, and pterosaurs. Archosauria, the archosaur clade, is a crown group that includes the most...
s is 2-3-4-5-3; many archosaur
Archosaur
Archosaurs are a group of diapsid amniotes whose living representatives consist of modern birds and crocodilians. This group also includes all extinct non-avian dinosaurs, many extinct crocodilian relatives, and pterosaurs. Archosauria, the archosaur clade, is a crown group that includes the most...
lineages have a reduced number of digits, but have the same phalangeal formula
Phalanx bones
In anatomy, phalanx bones are those that form the fingers and toes. In primates such as humans and monkeys, the thumb and big toe have two phalanges, while the other fingers and toes consist of three. Phalanges are classified as long bones.The phalanges do not have individual names...
in the digits that remain. In other words, paleontologists assert that archosaurs of different lineages tend to lose the same digits when digit loss occurs, from the outside to the inside. The three digits of dromaeosaur
Dromaeosauridae
Dromaeosauridae is a family of bird-like theropod dinosaurs. They were small- to medium-sized feathered carnivores that flourished in the Cretaceous Period. The name Dromaeosauridae means 'running lizards', from Greek dromeus meaning 'runner' and sauros meaning 'lizard'...
s, and Archaeopteryx
Archaeopteryx
Archaeopteryx , sometimes referred to by its German name Urvogel , is a genus of theropod dinosaur that is closely related to birds. The name derives from the Ancient Greek meaning "ancient", and , meaning "feather" or "wing"...
have the same phalangeal formula of I-II-III as digits I-II-III of basal
Basal (phylogenetics)
In phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram.A basal group forms an outgroup to the rest of the clade, such as in the following example:...
archosaurs. Therefore, the lost digits would be V and IV. If this is true, then modern birds would also possess digits I-II-III. Also, one research team has proposed a frame-shift in the digits of the theropod line leading to birds (thus making digit I into digit II, II to III, and so forth).
However, such frame shifts are rare in amniotes and would have had to occur solely in the forelimbs and not the hindlimbs (a condition presently unknown in any animal) in the bird-theropod lineage in order to be consistent with the theropod origin of birds.
This is called Lateral Digit Reduction (LDR) versus Bilateral Digit Reduction (BDR) (see also Limusaurus
Limusaurus
Limusaurus is a genus of toothless herbivorous theropod dinosaur from the Jurassic Upper Shishugou Formation in the Junggar Basin of western China. Limusaurus is also the first definitely known ceratosaur from Eastern Asia, including China...
)
See also
- Bird ichnologyBird ichnologyBird ichnology is the study of avian life traces in ornithology and paleontology. Such life traces can include footprints, nests, feces and coproliths. Scientists gain insight about the behavior and diversity of birds by studying such evidence....
- List of extinct birds
- Feathered dinosaursFeathered dinosaursThe realization that dinosaurs are closely related to birds raised the obvious possibility of feathered dinosaurs. Fossils of Archaeopteryx include well-preserved feathers, but it was not until the early 1990s that clearly non-avialan dinosaur fossils were discovered with preserved feathers...
- Flightless birdFlightless birdFlightless birds are birds which lack the ability to fly, relying instead on their ability to run or swim. They are thought to have evolved from flying ancestors. There are about forty species in existence today, the best known being the ostrich, emu, cassowary, rhea, kiwi, and penguin...
s - Origin of avian flightOrigin of avian flightAround 350 BCE, Aristotle and other philosophers of the time were attempting to explain the aerodynamics of avian flight. Even after the discovery of the ancestral bird Archaeopteryx, over 150 years ago, debates still persist regarding the evolution of flight...
- Temporal paradox (paleontology)Temporal paradox (paleontology)The temporal paradox, or time problem is a controversial issue in the evolutionary relationships of birds. It was described by paleornithologist Alan Feduccia.-Objection to consensus:The concept of a "temporal paradox" is based on the following facts...
External links
- DinoBuzz A popular-level discussion of the dinosaur-bird hypothesis
- Archaeopteryx - FAQs - from the Usenet newsgroup talk.origins.