The Origin of Birds (book)
Encyclopedia
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 established scientists and got little attention within Denmark. The English edition of 1926, however, became highly influential at the time due to the breadth of evidence synthesized as well as the artwork used to support arguments. It was considered the last word on the subject of bird evolution for several decades after its publication.
Through the course of the research represented in the book, Heilmann considers and eventually rejects the possibility of all living and several extinct groups of reptiles as potential ancestors for modern birds, including crocodilians, pterosaurs and several groups of dinosaurs. Despite his acknowledgment that some of the smaller Jurassic
theropods had many similarities to Archaeopteryx
and modern birds, he determined that they were unlikely to be direct bird ancestors and that they were instead closely–related offshoots, and concluded that the similarities were a result of convergent evolution
rather than direct ancestry. Based essentially on a process of elimination, Heilmann arrives at the conclusion that birds must be descended from thecodonts, a group of archosaur
s which lived during the Permian
and Triassic
periods. Although this conclusion was later shown to be inaccurate, The Origin of Birds was regarded as a masterful piece of scholarship at the time and set the international agenda for research in bird evolution for nearly half a century, and much of its research remains of interest.
near Eichstätt
, Germany
. The three fossils consisted of two nearly complete skeletons found in 1861 and 1877 and a single feather from 1860. They had been discovered just a few decades after the discovery of the dinosaurs, and as some dinosaurs appeared somewhat birdlike, Archaeopteryx was regarded as a possible "missing link
" between reptiles and birds by many paleontologists at the time.
The similarities between Archaeopteryx, known dinosaurs and extant birds were examined and emphasized, with Thomas Huxley
championing the idea that Archaeopteryx as well as modern birds had more in common with theropod dinosaurs than any other group of animals. This was at the time in opposition to the view of anatomist Sir Richard Owen
of the British Museum
, who viewed Archaeopteryx as no different taxonomically from modern birds. Huxley's work was controversial, and this climate of uncertainty and contention about bird origins persisted well into the beginning of the 20th century.
While the dinosaur-bird connection (or lack thereof) was being pursued in paleontology, the problem of the evolution of flight
was under scrutiny as well. It was observed that a number of animals with moderate flying or gliding ability, such as bat
s, flying lizards and flying squirrel
s have arboreal
lifestyles. This led to the idea that the ancestors of birds must have gradually acquired the ability to fly from leaping among branches in the tops of trees. The Hungarian paleontologist Franz Nopcsa proposed an alternate hypothesis in 1907, arguing that the ancestors of birds were fast-running, bipedal animals related to theropod dinosaurs. When Heilmann came onto the paleontology scene, these two sets of conflicting theories provided the framework for his research and eventual conclusions.
, Heilmann succeeded in amassing his research with the help of several others, including the expert in prehistoric animals at the Zoological Museum in Copenhagen
, Adolf Herluf Winge, and the biologist D'Arcy Wentworth Thompson
of the University of Dundee
. Winge, though initially showing interest in Heilmann's work, later proved to be a source of frustration by refusing to engage Heilmann in depth on various scientific queries. Heilmann eventually broke off contact, expressing some bitterness at the apparent change of heart, which Heilmann later ascribed to Winge's belief in Lamarckism
.
Heilmann's original articles were publicly ignored by Danish zoologists, but caused considerable unrest behind the scenes. Danish zoologist R.H. Stamm, for example, mocked Heilmann in private letters to Helms shortly after the publication of his first article. Other Danish zoologists openly expressed their disdain for Heilmann's work as well, including the professors of zoology in Copenhagen University J.E.V. Boas
and Hector Jungersen. Despite this, Helms continued to steadfastly support Heilmann and his ideas, which eventually began to gain attention from abroad. In April 1913, the American expert in fossil birds R. W. Shufeldt came across Heilmann's first article by chance. Shufeldt, who was married to a Norwegian and therefore knew some Scandinavian, was able to interpret Heilmann's work and initiated contact with him shortly thereafter. This opened up the opportunity for international correspondence with distinguished paleontologists for Heilmann, which was instrumental to the recognition of his work outside of Denmark.
Heilmann's articles were later collected and published as a book in Danish in 1916 with the title as in his series, Vor Nuvaerende Viden om Fuglenes Afstamming ("our present knowledge about the origin of birds"). This met the same lack of interest and is thought to have been a result of publishing in Danish, as this made it inaccessible to many scientists working in the U.S. and much of Europe. The field of paleontology at the time was dominated by American and English scientists, and the discipline in continental Europe was dominated by Germany and to a lesser degree France.
Consequently, and at the urging of Shufeldt, Heilmann devoted much of the next few years attempting to find an English–language publisher willing to undertake a translation of his work. None of the larger publishers he approached were willing to do so unless Heilmann himself was prepared to finance it, which he was unable to do. In the meantime, Heilmann took the opportunity to revise and improve his manuscript, which included information he acquired from examining the Berlin specimen of Archaeopteryx for the first time in 1923 at the invitation of Josef Felix Pompeckj
, a professor at the Natural History Museum in Berlin
. Examining this important specimen in person allowed Heilmann to add some additional details and revisions to his understanding of the hip, the skull, and the flight feather
s. With the help of the English paleontologist Arthur Smith Woodward
of the British Museum, he finally met success in finding a small London publisher willing to produce an English version of his manuscript in 1926.
in 1926 by H. F. & G. Witherby. It was published in the United States the following year by D. Appleton & Company. The book was reprinted in 1972 by Dover Publications, Inc.
, with the only change being the grayscale reproduction of several illustrations originally published in color.
The English and Danish editions differed in several significant ways. The English edition was somewhat shorter and more concise, and included newer information that Heilmann had acquired by studying the actual fossils of Archaeopteryx in Berlin, as well as from foreign scientists who sent him photographs and plaster casts. It also contained considerably less harsh language towards Boas and others with whom he disagreed. Curiously, the English edition did not contain the transformational sequences inspired by D'Arcy Thompson, though Heilmann still briefly acknowledged the use of his methods in reconstructing his Proavis.
, Ichthyornis
, Archaeopteryx
, and "Archaeornis", the name used commonly at the time for the Berlin specimen, which was then thought to represent a separate genus. Throughout this section are many finely detailed and labeled renderings of different parts of the skeletal anatomy of these birds, as well as other groups of extinct reptiles and some modern birds. Heilmann's attention to detail in his artwork is inspired at least in part by his dissatisfaction with anatomical renderings of these animals in scholarly works of the time, which he deemed as "unsatisfactory" and "containing misleading errors."
Early in this section Heilmann embarks on a thorough description of the Berlin Archaeopteryx specimen, which includes detailed comparisons to specific aspects of modern birds. Following a comparison of its skull to that of Aetosaurus
, Euparkeria
, and a modern pigeon, Heilmann states that he disagrees with the scientific consensus of the time that the skull of Archaeopteryx is that of a true bird. He writes that the reptilian features of the skull are much more pronounced, citing features of the teeth, fenestrae
, and jaw structure as being undeniably un-birdlike. Heilmann found that much of Archaeopteryxs anatomy, in fact, was decidedly reptilian and generally opposed to that of modern birds. This included the pelvis
, which lacks a pectineal process and has a very different os sacrum
from modern birds, as well as the carpus
, which Heilmann wrote exhibited the same phenomena found in the wrists of modern reptiles. Its tail was also noted by Heilmann as being extremely reptilian and resembling nothing known of modern birds.
He goes on to state that other features of Archaeopteryx, however, are remarkably birdlike and bear little resemblance to their reptilian analogues. The hand of Archaeopteryx is observed as being one of its most remarkable features, having what Heilmann calls a "reptilian basis" which has come to support primary feathers. He compares this to a primitive five-digit reptile hand, noting the obvious differences, before outlining the striking similarity of the hand to that of the theropod Ornitholestes
. Here Heilmann goes into considerable detail about the wing arrangement of Archaeopteryx, drawing from his observation of the Berlin specimen. The latter part of this section deals with analyzing the skeletal anatomy of the fossil birds Hesperornis and Ichthyornis, but Heilmann ultimately decides that they are of no importance to his investigations. He concludes the section by stating that Archaeopteryx "may be characterized as a reptile in the disguise of a bird", and states that his studies must turn from the skeleton to the soft tissue in order to reach a final conclusion.
, cell division
, ontogeny
and comparative embryology
about the probable ancestry of birds. A fair amount of detail is devoted early in the section to comparative studies between the germ cells of many different species of extant bird and reptile (and several mammal
s), including some comments on the corkscrew locomotion observed in the spermatozoa cells of several bird and reptile species, but no mammals. He then goes on to offer a similar comparison between the egg cells of birds and reptiles, and finds considerably more similarity there than either has to the egg cell of a mammal. Following an analysis of the germ cells, he moves onward through the developmental cycle by next examining the process of fertilization and subsequent cleavage
of the zygote
. He presents here several figures and illustrations of the cleavage of the blastoderm
in reptiles and birds.
He examines in detail the expression of evolutionary stages in the development of embryos
, tracing from the process of cell division to the development of specific anatomical features. He finds a striking resemblance between the embryonic development of reptiles and birds, including details of the skeletal anatomy (with special attention to the hands and feet) and various organs. He notes that bird and reptile embryos develop visceral arches, hinting at their aquatic ancestry. Of more interest to his goal, Heilmann writes in a similar vein that the embryos of certain birds clearly show a three-clawed finger structure, at least one of which (the hoatzin
) retains actual claws after hatching. He mentions other anatomical features of bird embryos that hint at their reptilian ancestry as well, such as the embryonic splitting of the pygostyle
into distinct separate vertebrae.
He begins the section with an analysis of the temporal opening found in the skull of many extant birds. After a thorough comparison, he rejects the notion, which was common at the time, that this temporal opening was homologous with the supratemporal fenestra in reptiles. Instead, he concludes that it is a recent feature. Next he makes some observations on the wing structure of modern nestling birds. He finds that some species of extant birds have claws on their first and second fingers when very young, and some, like the hoatzin, will even use these temporary claws to climb about, including in the branches of trees. He also discovers that far more nestlings have a nonfunctional claw on the first digit, and some adult birds do as well.
Organs are next examined in detail, with various comparisons drawn between reptiles and birds. He starts with the brain, analyzing in detail the cerebral and cerebellar
structure of several animals including birds, crocodilians, and mammals. Though noticeably more developed, Heilmann finds that the general structure of the brain is very similar in birds and reptiles, and he describes the brain of modern birds as "further evolution of the peculiar characters already found in the reptile." He also finds the eye
s of birds and reptiles to be remarkably similar, especially the development of the lens
, as well as the ear
, which reveals a much wider gap between the Sauropsida
and mammals than with birds. He describes the sexual organs of birds and reptiles to likewise be structurally similar, and finds that while most species of male bird have lost the penis to reduce weight, those that retain it bear remarkable similarities to extant reptiles. He writes that secondary sexual characteristics are also similar between birds and reptiles, with both groups frequently utilizing bright colors and structures for display. He concludes the section by offering a few more comparisons of structures and organs, including the lungs, ambiens muscle, and beak and scale sheaths. Taken together, Heilmann interprets these many similarities as further evidence for the close kinship of birds and reptiles.
as the primary reason why he believes birds cannot be descended from theropod dinosaurs, despite their many morphological similarities. Dollo's Law states that a feature or organ once lost by evolution cannot be regained. One consistently confounding issue Heilmann had encountered in his research into the bird-reptile link was that modern birds possess a wishbone
and theropod dinosaurs, by his observations, did not. Since ancient reptilian fossils that predated dinosaurs clearly possessed a different sort of wishbone, Heilmann concluded that this feature could not have been lost and regained again over the course of evolution. Based on this law, he therefore rejected the possibility of a direct theropod ancestor of birds, though he acknowledged that theropods and birds must have shared a close relationship.
Throughout this section, Heilmann examines several groups of possible ancestors in addition to coelurosaurs, including pterosaur
s, predentate
s, and pseudosuchians. Based on his rejection of theropods due to the wishbone issue as well as what Heilmann saw as striking morphological similarities between the skull of Archaeopteryx, Aetosaurus and Euparkeria, Heilmann concludes that a pseudosuchian origin of birds is the most probable. The final part of this section addresses the issue of the Proavian, which Heilmann illustrated speculatively both in skeleton in a natural setting. He constructed a hypothetical skull for this animal based on a mathematical combination of the skulls of Archaeopteryx, Euparkeria, Aetosaurus, and Ornithosuchus
. He constructed its skeleton in a similar manner. In this section he also compares his own Proavis to a similar sort of Proavis constructed by the American naturalist William Beebe
. Heilmann examines and rejects Beebe's own Proavis (named "Tetrapteryx" by Beebe) based on his analyzation of Beebe's documentation of pelvic wings in bird embryos, which Heilmann found little evidence for.
He concludes this final section by fleshing out his Proavis and summarizing his view of bird origins, in which birds would have departed from reptiles at the pseudosuchians. From this branching point, birds and dinosaurs would have evolved along parallel evolutionary tracks for millions of years, as cousins rather than ancestors. He imagines that these reptiles would have gradually assumed a bipedal gait, and transformed eventually from terrestrial runners to arboreal climbers, developing leaping capabilities ever increasing in length. Along the way, the ancestral reptilian scales would have become "frayed" and gradually developed into feathers, beginning along the forearm and tail and gradually spreading to the entire body. The need for this animal to be an adept climber would have catalyzed the lengthening of its phalanges, which would eventually become long and strong enough to support a wing. Powerful muscles would have developed to anchor these limbs, which would have reacted upon the breastbone. All of this together would have facilitated the origin of an accelerated metabolic rate, resulting in the warm-blooded state known of modern birds. This development would have occurred alongside of the enlargement of the brain, needed to coordinate and supervise these refined features. It is in this way, Heilmann concludes, the reptile has been changed into a bird.
– remained respectable and widespread in the paleontological community until the publication of The Origin of Birds. Heilmann more than anyone else was responsible for a widespread rejection of the dinosaur-bird link. His conclusions involved more than the mere rejection of a dinosaurian ancestry for birds, however: for instance, he favored the arboreal origin of avian flight, he allied birds firmly with reptiles instead of mammals, and he was responsible for finally putting an end to the idea that birds descended from pterosaurs. Several aspects of his research have continued to be influential long after its publication.
, meaning that it has no unique, diagnostic characteristics (and is largely considered an obsolete grouping today). But the principal reason why Heilmann's hypothesis was proven incorrect lies in the issue of the clavicle. Heilmann came extremely close to linking theropods and birds to one another, even going so far as to write that "it would seem a rather obvious conclusion that it is amongst the Coelurosaurs that we are to look for the bird ancestor." However, he was prevented from doing so by the apparent lack of the clavicle in the predatory dinosaurs, which reflected his strict adherence to Dollo's Law: reptilian ancestors had possessed a clavicle, but had lost it at some point during their evolution to the dinosaurs. Therefore, in order for Heilmann to believe that bird ancestry lie in the dinosaurs, there would need to be proof of clavicles in dinosaurian clades.
Most coincidentally, a small theropod dinosaur named Oviraptor philoceratops
had been discovered while Heilmann was working on his book, and was described by Henry Fairfield Osborn
in 1924. Figure 8 of Osborn's paper, redrawn from the fossil, shows what was later proven to be an oviraptorid wishbone between the animal's arms. Unfortunately, this important structure was misidentified at the time. If Heilmann had examined this paper as closely as he had much of his source material – or had traveled to New York to see the specimens in person – he may have reversed his conclusions entirely.
As it was, Heilmann's conclusion was so persuasive that it forestalled further debate on the subject even in the face of additional conflicting evidence. In 1936, paleontologist Charles Lewis Camp
described a new theropod from the Jurassic of North America, Segisaurus
. Like Oviraptor, Segisaurus had an unmistakable clavicle, but unlike Oviraptor, it was also plainly identified as being such in the paper that described it. Despite this, the implication of a wishbone-bearing dinosaur was blatantly ignored until much later, and for many years the state of dinosaur research stagnated, possibly due to the effects of the Depression
and World War II
.
The reinvigoration of interest in the dinosaur–bird link was largely due to the discoveries and research of paleontologist John Ostrom
in the 1960s. In particular was his discovery and description of the well-preserved dromaeosaur dinosaur Deinonychus
. Ostrom described Deinonychus as being extremely birdlike, with avian features such as a furcula, large sternal plates, horizontal posture, a birdlike spine, and ossified sternal ribs and uncinate process
es. Ostrom's study of this animal had the effect of revolutionizing the way people thought about dinosaurs: as metabolically energetic, active predators. Some years later, Ostrom also reanalyzed the fossils of Archaeopteryx, concluding that the animal was more reptilian than originally described by Heilmann, noting in particular the similarity of its feet to those of Deinonychus, as well as a host of other features. In fact, so strong were the similarities that the hand of Archaeopteryx was later described as "a miniature version of Deinonychuss." These discoveries provided the basis for the revival of the dinosaur–ancestry hypothesis of bird origins.
in 1880. This general theory about the mode of the evolution of flight in birds has persisted to modern times, especially (but not exclusively) among opponents of a theropod origin of birds. This includes paleontologists such as Alan Feduccia
, who essentially agree with Heilmann's assertion that the ancestors of birds must have been arboreal.
The origin of bird flight itself is still hotly debated. It is clear that feathers must have been a prerequisite for flight in birds (though flight may have not been a prerequisite for feathers). There has yet to be a consensus on whether flight involved from the ground up or the trees down, and Heilmann was largely responsible for popularizing the trees-down idea early on. The modern idea of the arboreal (or trees-down) hypothesis has changed little since Heilmann's time, and it states that bird flight would have originated by climbing birds gliding down from the tops of trees, making gliding a precursor to flapping or powered flight
. As these early birds became more efficient gliders, they would have begun to extend their range and capability by developing more powerful flight. In this proposed mode of bird evolution, Heilmann's "Proavis" is strongly implicated, which would have probably been a climbing, perching, early-stage gliding quadruped. This hypothesis for the origin of bird flight has had many adherents, including Walter J. Bock and Alan Feduccia.
Competing with the arboreal hypothesis is the idea that birds evolved from running bird ancestors, known as the cursorial (or ground up) hypothesis. This scenario may have involved ancient birds jumping or running along the ground and briefly becoming airborne, perhaps to avoid obstacles or catch insects. As these animals strove to overcome the force of gravity, powered flight may have appeared early on. They may also have used their ground speed to run up trees or other steep slopes, developing increasingly sophisticated flapping mechanism to assist with this. This model requires a highly cursorial and feathered ancestor. Proponents of the cursorial hypothesis cite the legs, feet and hands of Archaeopteryx as inheritance from a cursorial maniraptoran ancestor. This model has also had many adherents over the years, including John Ostrom and Jacques Gauthier
.
The arboreal hypothesis was popular in Heilmann's day, even prior to his research, as it had been advanced by Marsh. It fell out of favor following Ostrom's research in the 1960s and 1970s, which suggested that the ancestors of birds were fast–running bipedal animals, lending credence to the cursorial model. The focus shifted back to the arboreal model, when several Chinese non–avian theropods from the Early Cretaceous were found in the early 2000s. These new finds, represented chiefly by Epidendrosaurus and Microraptor
, have been described as possessing features that indicate an arboreal lifestyle; Microraptor even has flight feathers on its legs, which suggest it was a glider. While the arboreal hypothesis is still popular in modern times, there are several proponents of the cursorial model and no consensus has been established. Adherence to the arboreal model is shared both by paleontologists who accept the dinosaurian ancestry of birds, and by the minority who still believe birds to have evolved from a non-dinosaurian group of reptiles. Although the arboreal model was somewhat popular before Heilmann's research, his writings helped to advance and popularize it and the idea continues to have a hold.
in 1915. This hypothesis was based on observations of bird embryos and hatchlings, which Beebe found to possess a presumably atavistic
fringe of flight feathers on their hindlimbs. His main evidence came from examination of incipient quill feathers on the thigh of a four-day old white-winged dove
. He theorized based on this embryological fringe and the recapitulation theory
that birds had once passed through a "Tetrapteryx" stage in their distant evolution, which he represented as a hypothetical four-winged gliding animal.
Heilmann, though excited about Beebe's idea, found little evidence for these leg-wings when studying the nestlings in the Zoological Museum collection in Copenhagen
. He also examined the nestlings of more basal bird species, such as the ostrich and the emu, searching for a trace of leg-wings there, again without success. Even after studying the nestlings of birds closely related to Beebe's doves, including pigeons, he still found no trace of leg-wings. Instead he found on the nestlings' thighs "a series of permanent feathers, and no atavism. If it were a genuine relic from such a very remote past, it would make its appearance, like a glimpse, in the embryo or squab, quickly to vanish again." Heilmann went on to outline the morphological difficulties involved in such a leg-wing, indicating that it could hinder survival. In effect, Beebe's Tetrapteryx theory was completely disregarded by Heilmann, and this remained the consensus in ornithological literature until much later. Despite this, Heilmann's Proavis illustrations sport a short border of feathers behind the thigh, which some authors have suggested was inspired by Beebe's Tetrapteryx idea.
Beebe was relatively undeterred by the scientific community's acceptance of Heilmann's rejection of his theory, as is evidenced by the fact that he was still writing about his Tetrapteryx hypothesis well into the 1940s. His adherence to his theory was well-placed, as in 2003 a revolutionary discovery was made in the early Cretaceous Jiufotang Formation
of Liaoning
, China: Microraptor gui, the small, four-winged dromaeosaur that had led to renewed credence of the arboreal model for the origin of bird flight. This animal's most remarkable feature was the existence of long, pennaceous feathers on both its arms and legs, forming a set of four aerodynamic wings that its discoverers theorized were used for gliding. This discovery had the immediate effect of resurrecting the idea that leg feathers may have had some bearing on the origin of flight in birds, building on the idea originally proposed by Beebe and rejected by Heilmann.
Today, Beebe's 1915 description of his hypothetical four-winged bird ancestor is regarded as prescient, and there is no doubt that Microraptor looks startlingly similar to the almost-century old Tetrapteryx illustrations. After its discovery, Microraptor had the effect of both reinvigorating the arboreal hypothesis as well as finally putting an end to the widespread acceptance of Heilmann's disregard for the Tetrapteryx theory.
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...
, a Danish
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...
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 established scientists and got little attention within Denmark. The English edition of 1926, however, became highly influential at the time due to the breadth of evidence synthesized as well as the artwork used to support arguments. It was considered the last word on the subject of bird evolution for several decades after its publication.
Through the course of the research represented in the book, Heilmann considers and eventually rejects the possibility of all living and several extinct groups of reptiles as potential ancestors for modern birds, including crocodilians, pterosaurs and several groups of dinosaurs. Despite his acknowledgment that some of the smaller 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...
theropods had many similarities to 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 modern birds, he determined that they were unlikely to be direct bird ancestors and that they were instead closely–related offshoots, and concluded that the similarities were a result of 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...
rather than direct ancestry. Based essentially on a process of elimination, Heilmann arrives at the conclusion that birds must be descended from thecodonts, a group 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...
s which lived during the Permian
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...
and Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
periods. Although this conclusion was later shown to be inaccurate, The Origin of Birds was regarded as a masterful piece of scholarship at the time and set the international agenda for research in bird evolution for nearly half a century, and much of its research remains of interest.
Background
When Heilmann began his research in the early 1900s, the early bird Archaeopteryx was only known from three fossils found in the limestone quarries of SolnhofenSolnhofen
Solnhofen is a municipality in the district of Weißenburg-Gunzenhausen in the region of Franconia in the Land of Bavaria in Germany. It lies within the Altmühl valley....
near Eichstätt
Eichstätt
Eichstätt is a town in the federal state of Bavaria, Germany, and capital of the District of Eichstätt. It is located along the Altmühl River, at , and had a population of 13,078 in 2002. It is home to the Katholische Universität Eichstätt-Ingolstadt, the lone Catholic university in Germany. The...
, 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...
. The three fossils consisted of two nearly complete skeletons found in 1861 and 1877 and a single feather from 1860. They had been discovered just a few decades after the discovery of the dinosaurs, and as some dinosaurs appeared somewhat birdlike, Archaeopteryx was regarded as a possible "missing link
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 reptiles and birds by many paleontologists at the time.
The similarities between Archaeopteryx, known dinosaurs and extant birds were examined and emphasized, with Thomas Huxley
Thomas Huxley
Thomas Henry Huxley PC FRS was an English biologist, known as "Darwin's Bulldog" for his advocacy of Charles Darwin's theory of evolution....
championing the idea that Archaeopteryx as well as modern birds had more in common with theropod dinosaurs than any other group of animals. This was at the time in opposition to the view of anatomist Sir 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...
of the British Museum
British Museum
The British Museum is a museum of human history and culture in London. Its collections, which number more than seven million objects, are amongst the largest and most comprehensive in the world and originate from all continents, illustrating and documenting the story of human culture from its...
, who viewed Archaeopteryx as no different taxonomically from modern birds. Huxley's work was controversial, and this climate of uncertainty and contention about bird origins persisted well into the beginning of the 20th century.
While the dinosaur-bird connection (or lack thereof) was being pursued in paleontology, the problem of the evolution of flight
Origin of avian flight
Around 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...
was under scrutiny as well. It was observed that a number of animals with moderate flying or gliding ability, such as bat
Bat
Bats are mammals of the order Chiroptera "hand" and pteron "wing") whose forelimbs form webbed wings, making them the only mammals naturally capable of true and sustained flight. By contrast, other mammals said to fly, such as flying squirrels, gliding possums, and colugos, glide rather than fly,...
s, flying lizards and flying squirrel
Flying squirrel
Flying squirrels, scientifically known as Pteromyini or Petauristini, are a tribe of 44 species of squirrels .- Description :...
s have arboreal
Arboreal locomotion
Arboreal locomotion is the locomotion of animals in trees. In every habitat in which trees are present, animals have evolved to move in them. Some animals may only scale trees occasionally, while others are exclusively arboreal. These habitats pose numerous mechanical challenges to animals...
lifestyles. This led to the idea that the ancestors of birds must have gradually acquired the ability to fly from leaping among branches in the tops of trees. The Hungarian paleontologist Franz Nopcsa proposed an alternate hypothesis in 1907, arguing that the ancestors of birds were fast-running, bipedal animals related to theropod dinosaurs. When Heilmann came onto the paleontology scene, these two sets of conflicting theories provided the framework for his research and eventual conclusions.
Publication
Between 1913 and 1916, Gerhard Heilmann published a series of articles in the journal of the Danish Ornithological Society, all heavily illustrated and dealing with the question of the origin of birds. He first proposed the idea of a popular treatise on bird evolution in 1912 to Otto Helms, the editor of the journal of the Danish Ornithological Society. Helms supported the idea but recommended that Heilmann first seek professional advice on the topic. Despite the lack of formal training in zoologyZoology
Zoology |zoölogy]]), is the branch of biology that relates to the animal kingdom, including the structure, embryology, evolution, classification, habits, and distribution of all animals, both living and extinct...
, Heilmann succeeded in amassing his research with the help of several others, including the expert in prehistoric animals at the Zoological Museum in Copenhagen
University of Copenhagen Zoological Museum
The Copenhagen Zoological Museum is a part of the Natural History Museum of Denmark which consist of four natural science museums.The permanent exhibition 'From pole to pole' show animals from around the world in big displays...
, Adolf Herluf Winge, and the biologist D'Arcy Wentworth Thompson
D'Arcy Wentworth Thompson
Sir D'Arcy Wentworth Thompson CB FRS FRSE was a Scottish biologist, mathematician, and classics scholar. A pioneering mathematical biologist, he is mainly remembered as the author of the 1917 book On Growth and Form, written largely in Dundee in 1915...
of the University of Dundee
University of Dundee
The University of Dundee is a university based in the city and Royal burgh of Dundee on eastern coast of the central Lowlands of Scotland and with a small number of institutions elsewhere....
. Winge, though initially showing interest in Heilmann's work, later proved to be a source of frustration by refusing to engage Heilmann in depth on various scientific queries. Heilmann eventually broke off contact, expressing some bitterness at the apparent change of heart, which Heilmann later ascribed to Winge's belief in Lamarckism
Lamarckism
Lamarckism is the idea that an organism can pass on characteristics that it acquired during its lifetime to its offspring . It is named after the French biologist Jean-Baptiste Lamarck , who incorporated the action of soft inheritance into his evolutionary theories...
.
Heilmann's original articles were publicly ignored by Danish zoologists, but caused considerable unrest behind the scenes. Danish zoologist R.H. Stamm, for example, mocked Heilmann in private letters to Helms shortly after the publication of his first article. Other Danish zoologists openly expressed their disdain for Heilmann's work as well, including the professors of zoology in Copenhagen University J.E.V. Boas
Johan Erik Vesti Boas
Johan Erik Vesti Boas , also J.E.V. Boas, was a Danish zoologist and a disciple of Carl Gegenbaur and Steenstrup. During the beginning and end of his career, Johan Erik Vesti Boas worked at the Zoological Museum of Copenhagen...
and Hector Jungersen. Despite this, Helms continued to steadfastly support Heilmann and his ideas, which eventually began to gain attention from abroad. In April 1913, the American expert in fossil birds R. W. Shufeldt came across Heilmann's first article by chance. Shufeldt, who was married to a Norwegian and therefore knew some Scandinavian, was able to interpret Heilmann's work and initiated contact with him shortly thereafter. This opened up the opportunity for international correspondence with distinguished paleontologists for Heilmann, which was instrumental to the recognition of his work outside of Denmark.
Heilmann's articles were later collected and published as a book in Danish in 1916 with the title as in his series, Vor Nuvaerende Viden om Fuglenes Afstamming ("our present knowledge about the origin of birds"). This met the same lack of interest and is thought to have been a result of publishing in Danish, as this made it inaccessible to many scientists working in the U.S. and much of Europe. The field of paleontology at the time was dominated by American and English scientists, and the discipline in continental Europe was dominated by Germany and to a lesser degree France.
Consequently, and at the urging of Shufeldt, Heilmann devoted much of the next few years attempting to find an English–language publisher willing to undertake a translation of his work. None of the larger publishers he approached were willing to do so unless Heilmann himself was prepared to finance it, which he was unable to do. In the meantime, Heilmann took the opportunity to revise and improve his manuscript, which included information he acquired from examining the Berlin specimen of Archaeopteryx for the first time in 1923 at the invitation of Josef Felix Pompeckj
Josef Felix Pompeckj
Josef Felix Pompeckj was a German paleontologist and geologist. He was born in Groß-Köllen . He studied geology and paleontology and received his doctorate in 1890. He worked in Tübingen, Munich and Hohenheim. In 1893 he became an extraordinary professor and in 1897 the keeper in Monachium Museum...
, a professor at the Natural History Museum in Berlin
Berlin
Berlin is the capital city of Germany and is one of the 16 states of Germany. With a population of 3.45 million people, Berlin is Germany's largest city. It is the second most populous city proper and the seventh most populous urban area in the European Union...
. Examining this important specimen in person allowed Heilmann to add some additional details and revisions to his understanding of the hip, the skull, and the flight feather
Flight feather
Flight feathers are the long, stiff, asymmetrically shaped, but symmetrically paired feathers on the wings or tail of a bird; those on the wings are called remiges while those on the tail are called rectrices . Their primary function is to aid in the generation of both thrust and lift, thereby...
s. With the help of the English paleontologist Arthur Smith Woodward
Arthur Smith Woodward
Sir Arthur Smith Woodward was an English palaeontologist.-Biography:Woodward was born in Macclesfield, Cheshire, England and was educated there and at Owens College, Manchester. He joined the staff of the Department of Geology at the Natural History Museum in 1882. He became assistant Keeper of...
of the British Museum, he finally met success in finding a small London publisher willing to produce an English version of his manuscript in 1926.
Editions
While most of the original material was published in the journal of the Danish Ornithological Society between the years of 1913 and 1916, the first English version of the book was published in LondonLondon
London is the capital city of :England and the :United Kingdom, the largest metropolitan area in the United Kingdom, and the largest urban zone in the European Union by most measures. Located on the River Thames, London has been a major settlement for two millennia, its history going back to its...
in 1926 by H. F. & G. Witherby. It was published in the United States the following year by D. Appleton & Company. The book was reprinted in 1972 by Dover Publications, Inc.
Dover Publications
Dover Publications is an American book publisher founded in 1941 by Hayward Cirker and his wife, Blanche. It publishes primarily reissues, books no longer published by their original publishers. These are often, but not always, books in the public domain. The original published editions may be...
, with the only change being the grayscale reproduction of several illustrations originally published in color.
The English and Danish editions differed in several significant ways. The English edition was somewhat shorter and more concise, and included newer information that Heilmann had acquired by studying the actual fossils of Archaeopteryx in Berlin, as well as from foreign scientists who sent him photographs and plaster casts. It also contained considerably less harsh language towards Boas and others with whom he disagreed. Curiously, the English edition did not contain the transformational sequences inspired by D'Arcy Thompson, though Heilmann still briefly acknowledged the use of his methods in reconstructing his Proavis.
Book outline
Heilmann's book was divided into four main parts. The first three draw evidence for bird evolution from the fossil record, from the embryos of birds and other animals, and from living birds, respectively. The fourth and most groundbreaking section examines several groups of extinct animals in order to determine the probable root of modern birds.Part I: Some Fossil Birds
In the first section, Heilmann examines in exquisite detail the fossil remains of several extinct birds, including HesperornisHesperornis
Hesperornis is a genus of flightless aquatic birds that spanned the first half of the Campanian age of the Late Cretaceous period . One of the lesser-known discoveries of the paleontologist O. C. Marsh in the late 19th century Bone Wars, it was an important early find in the history of avian...
, Ichthyornis
Ichthyornis
Ichthyornis is a genus of toothed seabirds from the Late Cretaceous of North America. Its fossil remains are known from the chalks of Alberta, Alabama, Kansas, New Mexico, Saskatchewan, and Texas, in strata that were laid down in the Western Interior Seaway during the Turonian-Campanian ages,...
, 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 "Archaeornis", the name used commonly at the time for the Berlin specimen, which was then thought to represent a separate genus. Throughout this section are many finely detailed and labeled renderings of different parts of the skeletal anatomy of these birds, as well as other groups of extinct reptiles and some modern birds. Heilmann's attention to detail in his artwork is inspired at least in part by his dissatisfaction with anatomical renderings of these animals in scholarly works of the time, which he deemed as "unsatisfactory" and "containing misleading errors."
Early in this section Heilmann embarks on a thorough description of the Berlin Archaeopteryx specimen, which includes detailed comparisons to specific aspects of modern birds. Following a comparison of its skull to that of Aetosaurus
Aetosaurus
Aetosaurus is an extinct genus of archosaur reptile belonging to the order Aetosauria. It is generally considered to be the most primitive aetosaur. Three species are currently recognized: A. ferratus, the type species from Germany and Italy; A. crassicauda from Germany; and A. arcuatus from...
, 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 :...
, and a modern pigeon, Heilmann states that he disagrees with the scientific consensus of the time that the skull of Archaeopteryx is that of a true bird. He writes that the reptilian features of the skull are much more pronounced, citing features of the teeth, fenestrae
Fenestrae
Fenestræ is a Latin word that means "window".* In histology, fenestræ are small pores in endothelial cells that allow for rapid exchange of molecules between sinusoid blood vessels and surrounding tissue...
, and jaw structure as being undeniably un-birdlike. Heilmann found that much of Archaeopteryxs anatomy, in fact, was decidedly reptilian and generally opposed to that of modern birds. This included the pelvis
Pelvis
In human anatomy, the pelvis is the lower part of the trunk, between the abdomen and the lower limbs .The pelvis includes several structures:...
, which lacks a pectineal process and has a very different os sacrum
Sacrum
In vertebrate anatomy the sacrum is a large, triangular bone at the base of the spine and at the upper and back part of the pelvic cavity, where it is inserted like a wedge between the two hip bones. Its upper part connects with the last lumbar vertebra, and bottom part with the coccyx...
from modern birds, as well as the carpus
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...
, which Heilmann wrote exhibited the same phenomena found in the wrists of modern reptiles. Its tail was also noted by Heilmann as being extremely reptilian and resembling nothing known of modern birds.
He goes on to state that other features of Archaeopteryx, however, are remarkably birdlike and bear little resemblance to their reptilian analogues. The hand of Archaeopteryx is observed as being one of its most remarkable features, having what Heilmann calls a "reptilian basis" which has come to support primary feathers. He compares this to a primitive five-digit reptile hand, noting the obvious differences, before outlining the striking similarity of the hand to that of the theropod Ornitholestes
Ornitholestes
Ornitholestes was a small theropod dinosaur of the late Jurassic of Western Laurasia . To date, it is known only from a single partial skeleton, and badly crushed skull found at the Bone Cabin Quarry near Medicine Bow, Wyoming, in 1900...
. Here Heilmann goes into considerable detail about the wing arrangement of Archaeopteryx, drawing from his observation of the Berlin specimen. The latter part of this section deals with analyzing the skeletal anatomy of the fossil birds Hesperornis and Ichthyornis, but Heilmann ultimately decides that they are of no importance to his investigations. He concludes the section by stating that Archaeopteryx "may be characterized as a reptile in the disguise of a bird", and states that his studies must turn from the skeleton to the soft tissue in order to reach a final conclusion.
Part II: Embryonic Stages of Reptiles and Birds
In this section, Heilmann draws evidence from his observations of germ cells, impregnationFertilisation
Fertilisation is the fusion of gametes to produce a new organism. In animals, the process involves the fusion of an ovum with a sperm, which eventually leads to the development of an embryo...
, cell division
Cell division
Cell division is the process by which a parent cell divides into two or more daughter cells . Cell division is usually a small segment of a larger cell cycle. This type of cell division in eukaryotes is known as mitosis, and leaves the daughter cell capable of dividing again. The corresponding sort...
, ontogeny
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...
and comparative embryology
Embryology
Embryology is a science which is about the development of an embryo from the fertilization of the ovum to the fetus stage...
about the probable ancestry of birds. A fair amount of detail is devoted early in the section to comparative studies between the germ cells of many different species of extant bird and reptile (and several 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), including some comments on the corkscrew locomotion observed in the spermatozoa cells of several bird and reptile species, but no mammals. He then goes on to offer a similar comparison between the egg cells of birds and reptiles, and finds considerably more similarity there than either has to the egg cell of a mammal. Following an analysis of the germ cells, he moves onward through the developmental cycle by next examining the process of fertilization and subsequent cleavage
Cleavage (embryo)
In embryology, cleavage is the division of cells in the early embryo. The zygotes of many species undergo rapid cell cycles with no significant growth, producing a cluster of cells the same size as the original zygote. The different cells derived from cleavage are called blastomeres and form a...
of the zygote
Zygote
A zygote , or zygocyte, is the initial cell formed when two gamete cells are joined by means of sexual reproduction. In multicellular organisms, it is the earliest developmental stage of the embryo...
. He presents here several figures and illustrations of the cleavage of the blastoderm
Blastoderm
-In amniotes:A blastoderm is the layer of cells formed at one pole of macrolecithal eggs such as the yolky egg of birds. The yolk prevents the division from taking place through the egg, resulting in meroblastic cleavage during the many cleavage divisions...
in reptiles and birds.
He examines in detail the expression of evolutionary stages in the development of embryos
Recapitulation theory
The theory of recapitulation, also called the biogenetic law or embryological parallelism—and often expressed as "ontogeny recapitulates phylogeny"—is a disproven hypothesis that in developing from embryo to adult, animals go through stages resembling or representing successive stages...
, tracing from the process of cell division to the development of specific anatomical features. He finds a striking resemblance between the embryonic development of reptiles and birds, including details of the skeletal anatomy (with special attention to the hands and feet) and various organs. He notes that bird and reptile embryos develop visceral arches, hinting at their aquatic ancestry. Of more interest to his goal, Heilmann writes in a similar vein that the embryos of certain birds clearly show a three-clawed finger structure, at least one of which (the hoatzin
Hoatzin
The Hoatzin , also known as the Hoactzin, Stinkbird, or Canje Pheasant, is a species of tropical bird found in swamps, riverine forest and mangrove of the Amazon and the Orinoco delta in South America...
) retains actual claws after hatching. He mentions other anatomical features of bird embryos that hint at their reptilian ancestry as well, such as the embryonic splitting of the pygostyle
Pygostyle
Pygostyle refers to a number of the final few caudal vertebrae fused into a single ossification, supporting the tail feathers and musculature. In modern birds, the rectrices attach to these....
into distinct separate vertebrae.
Part III: Some Anatomical and Biological Data
Part III deals with anatomical comparisons between extant birds and reptiles, wherein Heilmann finds traces of the relationship between them in examples of fenestrae, claws, the brain, sense organs, sexual organs, and other features. He concludes that many of these features are "nearly identical" between reptiles and birds. He cites other features as being clearly derived from one another, such as the avian feather essentially being a cylindrical, fringed scale.He begins the section with an analysis of the temporal opening found in the skull of many extant birds. After a thorough comparison, he rejects the notion, which was common at the time, that this temporal opening was homologous with the supratemporal fenestra in reptiles. Instead, he concludes that it is a recent feature. Next he makes some observations on the wing structure of modern nestling birds. He finds that some species of extant birds have claws on their first and second fingers when very young, and some, like the hoatzin, will even use these temporary claws to climb about, including in the branches of trees. He also discovers that far more nestlings have a nonfunctional claw on the first digit, and some adult birds do as well.
Organs are next examined in detail, with various comparisons drawn between reptiles and birds. He starts with the brain, analyzing in detail the cerebral and cerebellar
Cerebellum
The cerebellum is a region of the brain that plays an important role in motor control. It may also be involved in some cognitive functions such as attention and language, and in regulating fear and pleasure responses, but its movement-related functions are the most solidly established...
structure of several animals including birds, crocodilians, and mammals. Though noticeably more developed, Heilmann finds that the general structure of the brain is very similar in birds and reptiles, and he describes the brain of modern birds as "further evolution of the peculiar characters already found in the reptile." He also finds the eye
Eye
Eyes are organs that detect light and convert it into electro-chemical impulses in neurons. The simplest photoreceptors in conscious vision connect light to movement...
s of birds and reptiles to be remarkably similar, especially the development of the lens
Lens (anatomy)
The crystalline lens is a transparent, biconvex structure in the eye that, along with the cornea, helps to refract light to be focused on the retina. The lens, by changing shape, functions to change the focal distance of the eye so that it can focus on objects at various distances, thus allowing a...
, as well as the ear
Ear
The ear is the organ that detects sound. It not only receives sound, but also aids in balance and body position. The ear is part of the auditory system....
, which reveals a much wider gap between the Sauropsida
Sauropsida
Sauropsida is a group of amniotes that includes all existing reptiles and birds and their fossil ancestors, including the dinosaurs, the immediate ancestors of birds...
and mammals than with birds. He describes the sexual organs of birds and reptiles to likewise be structurally similar, and finds that while most species of male bird have lost the penis to reduce weight, those that retain it bear remarkable similarities to extant reptiles. He writes that secondary sexual characteristics are also similar between birds and reptiles, with both groups frequently utilizing bright colors and structures for display. He concludes the section by offering a few more comparisons of structures and organs, including the lungs, ambiens muscle, and beak and scale sheaths. Taken together, Heilmann interprets these many similarities as further evidence for the close kinship of birds and reptiles.
Part IV: The Proavian
In Heilmann's final section of The Origin of Birds, he seeks to synthesize the information he has documented in the previous three sections to uncover the probable origins in a particular group of ancestors. In doing so he discusses the specific morphology of a hypothetical creature that must have existed between modern birds and their reptilian ancestors, which he refers to as "the proavian." It is in this section that Heilmann, after making a bold and confident assertion that birds are absolutely descended from reptiles, cites Dollo's LawDollo'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...
as the primary reason why he believes birds cannot be descended from theropod dinosaurs, despite their many morphological similarities. Dollo's Law states that a feature or organ once lost by evolution cannot be regained. One consistently confounding issue Heilmann had encountered in his research into the bird-reptile link was that modern birds possess a wishbone
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...
and theropod dinosaurs, by his observations, did not. Since ancient reptilian fossils that predated dinosaurs clearly possessed a different sort of wishbone, Heilmann concluded that this feature could not have been lost and regained again over the course of evolution. Based on this law, he therefore rejected the possibility of a direct theropod ancestor of birds, though he acknowledged that theropods and birds must have shared a close relationship.
Throughout this section, Heilmann examines several groups of possible ancestors in addition to coelurosaurs, including pterosaur
Pterosaur
Pterosaurs were flying reptiles of the clade or order Pterosauria. They existed from the late Triassic to the end of the Cretaceous Period . Pterosaurs are the earliest vertebrates known to have evolved powered flight...
s, predentate
Ornithischia
Ornithischia or Predentata is an extinct order of beaked, herbivorous dinosaurs. The name ornithischia is derived from the Greek ornitheos meaning 'of a bird' and ischion meaning 'hip joint'...
s, and pseudosuchians. Based on his rejection of theropods due to the wishbone issue as well as what Heilmann saw as striking morphological similarities between the skull of Archaeopteryx, Aetosaurus and Euparkeria, Heilmann concludes that a pseudosuchian origin of birds is the most probable. The final part of this section addresses the issue of the Proavian, which Heilmann illustrated speculatively both in skeleton in a natural setting. He constructed a hypothetical skull for this animal based on a mathematical combination of the skulls of Archaeopteryx, Euparkeria, Aetosaurus, and Ornithosuchus
Ornithosuchus
Ornithosuchus is an extinct genus of crurotarsan from the Late Triassic Lossiemouth Sandstone of Scotland...
. He constructed its skeleton in a similar manner. In this section he also compares his own Proavis to a similar sort of Proavis constructed by the American naturalist William Beebe
William Beebe
William Beebe, born Charles William Beebe was an American naturalist, ornithologist, marine biologist, entomologist, explorer, and author...
. Heilmann examines and rejects Beebe's own Proavis (named "Tetrapteryx" by Beebe) based on his analyzation of Beebe's documentation of pelvic wings in bird embryos, which Heilmann found little evidence for.
He concludes this final section by fleshing out his Proavis and summarizing his view of bird origins, in which birds would have departed from reptiles at the pseudosuchians. From this branching point, birds and dinosaurs would have evolved along parallel evolutionary tracks for millions of years, as cousins rather than ancestors. He imagines that these reptiles would have gradually assumed a bipedal gait, and transformed eventually from terrestrial runners to arboreal climbers, developing leaping capabilities ever increasing in length. Along the way, the ancestral reptilian scales would have become "frayed" and gradually developed into feathers, beginning along the forearm and tail and gradually spreading to the entire body. The need for this animal to be an adept climber would have catalyzed the lengthening of its phalanges, which would eventually become long and strong enough to support a wing. Powerful muscles would have developed to anchor these limbs, which would have reacted upon the breastbone. All of this together would have facilitated the origin of an accelerated metabolic rate, resulting in the warm-blooded state known of modern birds. This development would have occurred alongside of the enlargement of the brain, needed to coordinate and supervise these refined features. It is in this way, Heilmann concludes, the reptile has been changed into a bird.
Legacy
In 1868, Thomas Huxley published On the animals which are most nearly intermediate between birds and reptiles, making a strong case for the bird-dinosaur ancestral link. Huxley's proposal that birds arose from dinosaurs – based primarily on his observation of the similarities between Archaeopteryx and CompsognathusCompsognathus
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...
– remained respectable and widespread in the paleontological community until the publication of The Origin of Birds. Heilmann more than anyone else was responsible for a widespread rejection of the dinosaur-bird link. His conclusions involved more than the mere rejection of a dinosaurian ancestry for birds, however: for instance, he favored the arboreal origin of avian flight, he allied birds firmly with reptiles instead of mammals, and he was responsible for finally putting an end to the idea that birds descended from pterosaurs. Several aspects of his research have continued to be influential long after its publication.
Wishbones and the dinosaur-bird link
The proposed thecodontian ancestry eventually fell out of favor, in part because the clade "thecodontia" is not monophyleticMonophyly
In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...
, meaning that it has no unique, diagnostic characteristics (and is largely considered an obsolete grouping today). But the principal reason why Heilmann's hypothesis was proven incorrect lies in the issue of the clavicle. Heilmann came extremely close to linking theropods and birds to one another, even going so far as to write that "it would seem a rather obvious conclusion that it is amongst the Coelurosaurs that we are to look for the bird ancestor." However, he was prevented from doing so by the apparent lack of the clavicle in the predatory dinosaurs, which reflected his strict adherence to Dollo's Law: reptilian ancestors had possessed a clavicle, but had lost it at some point during their evolution to the dinosaurs. Therefore, in order for Heilmann to believe that bird ancestry lie in the dinosaurs, there would need to be proof of clavicles in dinosaurian clades.
Most coincidentally, a small theropod dinosaur named Oviraptor philoceratops
Oviraptor
Oviraptor is a genus of small Mongolian theropod dinosaur, first discovered by the paleontologist Roy Chapman Andrews, and first described by Henry Fairfield Osborn, in 1924...
had been discovered while Heilmann was working on his book, and was described by Henry Fairfield Osborn
Henry Fairfield Osborn
Henry Fairfield Osborn, Sr. ForMemRS was an American geologist, paleontologist, and eugenicist.-Early life and career:...
in 1924. Figure 8 of Osborn's paper, redrawn from the fossil, shows what was later proven to be an oviraptorid wishbone between the animal's arms. Unfortunately, this important structure was misidentified at the time. If Heilmann had examined this paper as closely as he had much of his source material – or had traveled to New York to see the specimens in person – he may have reversed his conclusions entirely.
As it was, Heilmann's conclusion was so persuasive that it forestalled further debate on the subject even in the face of additional conflicting evidence. In 1936, paleontologist Charles Lewis Camp
Charles Lewis Camp
Charles Lewis Camp was a notable palaeontologist and zoologist, working from the University of California, Berkeley...
described a new theropod from the Jurassic of North America, 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...
. Like Oviraptor, Segisaurus had an unmistakable clavicle, but unlike Oviraptor, it was also plainly identified as being such in the paper that described it. Despite this, the implication of a wishbone-bearing dinosaur was blatantly ignored until much later, and for many years the state of dinosaur research stagnated, possibly due to the effects of the Depression
Great Depression
The Great Depression was a severe worldwide economic depression in the decade preceding World War II. The timing of the Great Depression varied across nations, but in most countries it started in about 1929 and lasted until the late 1930s or early 1940s...
and World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
.
The reinvigoration of interest in the dinosaur–bird link was largely due to the discoveries and research of paleontologist 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...
in the 1960s. In particular was his discovery and description of the well-preserved dromaeosaur dinosaur 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...
. Ostrom described Deinonychus as being extremely birdlike, with avian features such as a furcula, large sternal plates, horizontal posture, a birdlike spine, and ossified sternal ribs and uncinate process
Uncinate processes of ribs
The uncinate processes of the ribs are extensions of bone that in birds project caudally from the vertical segment of each rib. These processes help to strengthen the rib cage of birds by overlapping with the rib behind them...
es. Ostrom's study of this animal had the effect of revolutionizing the way people thought about dinosaurs: as metabolically energetic, active predators. Some years later, Ostrom also reanalyzed the fossils of Archaeopteryx, concluding that the animal was more reptilian than originally described by Heilmann, noting in particular the similarity of its feet to those of Deinonychus, as well as a host of other features. In fact, so strong were the similarities that the hand of Archaeopteryx was later described as "a miniature version of Deinonychuss." These discoveries provided the basis for the revival of the dinosaur–ancestry hypothesis of bird origins.
Models of flight evolution
Heilmann envisioned that birds evolved from ground–dwelling animals that became arboreal and capable of jumping between branches over time. Their descendants would eventually be able to glide as the length of leaps increased, leading to greater specialization and eventual flapping capabilities. This "from the trees down" hypothesis was originally proposed by Othniel C. MarshOthniel Charles Marsh
Othniel Charles Marsh was an American paleontologist. Marsh was one of the preeminent scientists in the field; the discovery or description of dozens of news species and theories on the origins of birds are among his legacies.Born into a modest family, Marsh was able to afford higher education...
in 1880. This general theory about the mode of the evolution of flight in birds has persisted to modern times, especially (but not exclusively) among opponents of a theropod origin of birds. This includes paleontologists such as 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...
, who essentially agree with Heilmann's assertion that the ancestors of birds must have been arboreal.
The origin of bird flight itself is still hotly debated. It is clear that feathers must have been a prerequisite for flight in birds (though flight may have not been a prerequisite for feathers). There has yet to be a consensus on whether flight involved from the ground up or the trees down, and Heilmann was largely responsible for popularizing the trees-down idea early on. The modern idea of the arboreal (or trees-down) hypothesis has changed little since Heilmann's time, and it states that bird flight would have originated by climbing birds gliding down from the tops of trees, making gliding a precursor to flapping or powered flight
Powered flight
Powered flight is flight achieved using onboard power to generate propulsive thrust and/or lift. Birds and insects use wings, in a variety of ways, to achieve powered flight. Man has developed several forms of powered aircraft. The term powered flight is also sometimes used excluding the natural...
. As these early birds became more efficient gliders, they would have begun to extend their range and capability by developing more powerful flight. In this proposed mode of bird evolution, Heilmann's "Proavis" is strongly implicated, which would have probably been a climbing, perching, early-stage gliding quadruped. This hypothesis for the origin of bird flight has had many adherents, including Walter J. Bock and Alan Feduccia.
Competing with the arboreal hypothesis is the idea that birds evolved from running bird ancestors, known as the cursorial (or ground up) hypothesis. This scenario may have involved ancient birds jumping or running along the ground and briefly becoming airborne, perhaps to avoid obstacles or catch insects. As these animals strove to overcome the force of gravity, powered flight may have appeared early on. They may also have used their ground speed to run up trees or other steep slopes, developing increasingly sophisticated flapping mechanism to assist with this. This model requires a highly cursorial and feathered ancestor. Proponents of the cursorial hypothesis cite the legs, feet and hands of Archaeopteryx as inheritance from a cursorial maniraptoran ancestor. This model has also had many adherents over the years, including John Ostrom and 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....
.
The arboreal hypothesis was popular in Heilmann's day, even prior to his research, as it had been advanced by Marsh. It fell out of favor following Ostrom's research in the 1960s and 1970s, which suggested that the ancestors of birds were fast–running bipedal animals, lending credence to the cursorial model. The focus shifted back to the arboreal model, when several Chinese non–avian theropods from the Early Cretaceous were found in the early 2000s. These new finds, represented chiefly by Epidendrosaurus and Microraptor
Microraptor
Microraptor is a genus of small, four-winged dromaeosaurid dinosaurs. Numerous well-preserved fossil specimens have been recovered from Liaoning, China...
, have been described as possessing features that indicate an arboreal lifestyle; Microraptor even has flight feathers on its legs, which suggest it was a glider. While the arboreal hypothesis is still popular in modern times, there are several proponents of the cursorial model and no consensus has been established. Adherence to the arboreal model is shared both by paleontologists who accept the dinosaurian ancestry of birds, and by the minority who still believe birds to have evolved from a non-dinosaurian group of reptiles. Although the arboreal model was somewhat popular before Heilmann's research, his writings helped to advance and popularize it and the idea continues to have a hold.
Beebe's leg-wings
In the fourth section of The Origin of Birds, Heilmann examines the Tetrapteryx hypothesis proposed by William BeebeWilliam Beebe
William Beebe, born Charles William Beebe was an American naturalist, ornithologist, marine biologist, entomologist, explorer, and author...
in 1915. This hypothesis was based on observations of bird embryos and hatchlings, which Beebe found to possess a presumably atavistic
Atavism
Atavism is the tendency to revert to ancestral type. In biology, an atavism is an evolutionary throwback, such as traits reappearing which had disappeared generations before. Atavisms can occur in several ways...
fringe of flight feathers on their hindlimbs. His main evidence came from examination of incipient quill feathers on the thigh of a four-day old white-winged dove
White-winged Dove
The White-winged Dove is a dove whose native range extends from the south-western USA through Mexico, Central America, and the Caribbean. In recent years with increasing urbanization and backyard feeding, it has expanded throughout Texas and into Louisiana...
. He theorized based on this embryological fringe and the recapitulation theory
Recapitulation theory
The theory of recapitulation, also called the biogenetic law or embryological parallelism—and often expressed as "ontogeny recapitulates phylogeny"—is a disproven hypothesis that in developing from embryo to adult, animals go through stages resembling or representing successive stages...
that birds had once passed through a "Tetrapteryx" stage in their distant evolution, which he represented as a hypothetical four-winged gliding animal.
Heilmann, though excited about Beebe's idea, found little evidence for these leg-wings when studying the nestlings in the Zoological Museum collection in Copenhagen
Copenhagen
Copenhagen is the capital and largest city of Denmark, with an urban population of 1,199,224 and a metropolitan population of 1,930,260 . With the completion of the transnational Øresund Bridge in 2000, Copenhagen has become the centre of the increasingly integrating Øresund Region...
. He also examined the nestlings of more basal bird species, such as the ostrich and the emu, searching for a trace of leg-wings there, again without success. Even after studying the nestlings of birds closely related to Beebe's doves, including pigeons, he still found no trace of leg-wings. Instead he found on the nestlings' thighs "a series of permanent feathers, and no atavism. If it were a genuine relic from such a very remote past, it would make its appearance, like a glimpse, in the embryo or squab, quickly to vanish again." Heilmann went on to outline the morphological difficulties involved in such a leg-wing, indicating that it could hinder survival. In effect, Beebe's Tetrapteryx theory was completely disregarded by Heilmann, and this remained the consensus in ornithological literature until much later. Despite this, Heilmann's Proavis illustrations sport a short border of feathers behind the thigh, which some authors have suggested was inspired by Beebe's Tetrapteryx idea.
Beebe was relatively undeterred by the scientific community's acceptance of Heilmann's rejection of his theory, as is evidenced by the fact that he was still writing about his Tetrapteryx hypothesis well into the 1940s. His adherence to his theory was well-placed, as in 2003 a revolutionary discovery was made in the early Cretaceous Jiufotang Formation
Jiufotang Formation
The Jiufotang Formation is an Early Cretaceous geological formation in Chaoyang, Liaoning which has yielded fossils of feathered dinosaurs, primitive birds, pterosaurs, and other organisms. . It is a member of the Jehol group. The exact age of the Jiufotang has been debated for years, with...
of 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"...
, China: Microraptor gui, the small, four-winged dromaeosaur that had led to renewed credence of the arboreal model for the origin of bird flight. This animal's most remarkable feature was the existence of long, pennaceous feathers on both its arms and legs, forming a set of four aerodynamic wings that its discoverers theorized were used for gliding. This discovery had the immediate effect of resurrecting the idea that leg feathers may have had some bearing on the origin of flight in birds, building on the idea originally proposed by Beebe and rejected by Heilmann.
Today, Beebe's 1915 description of his hypothetical four-winged bird ancestor is regarded as prescient, and there is no doubt that Microraptor looks startlingly similar to the almost-century old Tetrapteryx illustrations. After its discovery, Microraptor had the effect of both reinvigorating the arboreal hypothesis as well as finally putting an end to the widespread acceptance of Heilmann's disregard for the Tetrapteryx theory.