Herrerasaurus

Herrerasaurus

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Herrerasaurus was one of the earliest 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. All known specimens of this carnivore
Carnivore
A carnivore meaning 'meat eater' is an organism that derives its energy and nutrient requirements from a diet consisting mainly or exclusively of animal tissue, whether through predation or scavenging...

 have been discovered in rocks of late Ladinian
Ladinian
The Ladinian is a stage and age in the Middle Triassic series or epoch. It spans the time between 237 ± 2 Ma and 228 ± 2 Ma...

 age (middle 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...

 according to the ICS
International Commission on Stratigraphy
The International Commission on Stratigraphy , sometimes referred to by the unofficial "International Stratigraphic Commission" is a daughter or major subcommittee grade scientific daughter organization that concerns itself with stratigraphy, geological, and geochronological matters on a global...

, dated to 231.4 million years ago) in northwestern Argentina. The type species
Type species
In biological nomenclature, a type species is both a concept and a practical system which is used in the classification and nomenclature of animals and plants. The value of a "type species" lies in the fact that it makes clear what is meant by a particular genus name. A type species is the species...

, Herrerasaurus ischigualastensis, was described by Osvaldo Reig
Osvaldo Reig
Osvaldo Alfredo Reig, , was an Argentine biologist and paleontologist. He was born in Buenos Aires, Argentina....

 in 1963 and is the only species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 assigned to the genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

. The names Ischisaurus and Frenguellisaurus are synonymous
Synonym (taxonomy)
In scientific nomenclature, a synonym is a scientific name that is or was used for a taxon of organisms that also goes by a different scientific name. For example, Linnaeus was the first to give a scientific name to the Norway spruce, which he called Pinus abies...

 with Herrerasaurus.

For many years, the classification of Herrerasaurus was unclear, as the animal was initially known from very fragmentary remains; it has been hypothesized
Hypothesis
A hypothesis is a proposed explanation for a phenomenon. The term derives from the Greek, ὑποτιθέναι – hypotithenai meaning "to put under" or "to suppose". For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it...

 to be a 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:...

 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...

, a basal 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...

, a basal 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, or not a dinosaur at all. However, with the discovery of a mostly complete skeleton and skull in 1988, Herrerasaurus has been classified as either an early theropod or an early saurischian in at least five recent reviews of theropod evolution, with many researchers treating it at least tentatively as the most primitive member of Theropoda. It was a member of the Herrerasauridae
Herrerasauridae
Herrerasaurids are among the oldest known dinosaurs, appearing in the fossil record 231.4 million years ago . They became extinct by the end of the Triassic period. Herrerasaurids were small-sized carnivorous theropods or basal saurischians...

, a group of similar animals which were among the earliest of the dinosaurian
Evolution of dinosaurs
Dinosaurs evolved from the archosaurs 232-234 Ma in the Ladinian age, the latter part of the middle Triassic. Dinosauria is a well-supported clade, present in 98% of bootstraps...

 evolutionary radiation
Evolutionary radiation
An evolutionary radiation is an increase in taxonomic diversity or morphological disparity, due to adaptive change or the opening of ecospace. Radiations may affect one clade or many, and be rapid or gradual; where they are rapid, and driven by a single lineage's adaptation to their environment,...

.

Description




Herrerasaurus was a lightly built bipedal carnivore with a long tail and a relatively small head. Its length is estimated at 3 to 6 meters (10 to 20 ft), and its hip height at more than 1.1 meters (3.3 ft). It may have weighed around 210–350 kilograms (463–772 lb). In a large specimen at first thought to belong to a separate genus, Frenguellisaurus, the skull measured 56 centimeters (1.8 ft) in length. Smaller specimens had skulls which measured around 30 centimeters (1 ft) in length.

Skull


Herrerasaurus had a long, narrow skull
Skull
The skull is a bony structure in the head of many animals that supports the structures of the face and forms a cavity for the brain.The skull is composed of two parts: the cranium and the mandible. A skull without a mandible is only a cranium. Animals that have skulls are called craniates...

 that lacked nearly all the specializations that characterized later dinosaurs, and more closely resembled those of more primitive 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 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 :...

. It had five pairs of 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...

 (skull openings) in its skull, two pairs of which were for the eyes and nostrils. Between the eyes and the nostrils were two antorbital fenestra
Antorbital fenestra
An antorbital fenestra is an opening in the skull, in front of the eye sockets. This skull formation first appeared in archosaurs during the Triassic Period. Living birds today possess antorbital fenestrae, but the feature has been lost in modern crocodilians...

e and a pair of tiny, 1-centimeter-long (0.4 in) slit-like holes called promaxillary fenestrae. Behind the eyes were large infratemporal fenestrae. These holes helped to reduce the weight of the skull.

Herrerasaurus had a flexible joint in the lower jaw, allowing it to slide back and forth to deliver a grasping bite. This cranial specialization is unusual among the dinosaurs but has evolved independently in some lizard
Lizard
Lizards are a widespread group of squamate reptiles, with nearly 3800 species, ranging across all continents except Antarctica as well as most oceanic island chains...

s. The rear of the lower jaw also had fenestrae. The jaws were equipped with large serrated teeth for biting and eating flesh, and the neck was slender and flexible.

Limbs



The forelimbs of Herrerasaurus were less than half the length of its hind limbs. The upper arm
Humerus
The humerus is a long bone in the arm or forelimb that runs from the shoulder to the elbow....

 and forearm
Radius
In classical geometry, a radius of a circle or sphere is any line segment from its center to its perimeter. By extension, the radius of a circle or sphere is the length of any such segment, which is half the diameter. If the object does not have an obvious center, the term may refer to its...

 were rather short, while the manus
Manus (zoology)
The manus is the zoological term for the distal portion of the fore limb of an animal. In tetrapods, it is the part of the pentadactyl limb that includes the metacarpals and digits . During evolution, it has taken many forms and served a variety of functions...

 (hand) was elongated. The first two fingers and the thumb ended in curved, sharp claws for grasping prey. Its fourth and fifth digits were small stubs without claws.

Unlike most reptiles of its era, Herrerasaurus was fully bipedal. It had strong hind limbs with short thighs
Femur
The femur , or thigh bone, is the most proximal bone of the leg in tetrapod vertebrates capable of walking or jumping, such as most land mammals, birds, many reptiles such as lizards, and amphibians such as frogs. In vertebrates with four legs such as dogs and horses, the femur is found only in...

 and rather long feet, indicating that it was most likely a swift runner. The foot had five toes, but only the middle three (digits II, III, and IV) bore weight. The outer toes (I and V) were small; the first toe had a small claw. The tail, partially stiffened by overlapping vertebral projections, balanced the body and was also an adaptation for speed.

Derived and basal characteristics


Herrerasaurus is something of an enigma in that it displays traits that are found in different groups of dinosaurs, and several traits found in non-dinosaurian archosaurs. Although it shared most of the characteristics of dinosaurs, there were a few differences, particularly in regard to the shape of its hip and leg bones. Its pelvis was similar to that of saurischian dinosaurs, but it had a bony acetabulum
Acetabulum
The acetabulum is a concave surface of the pelvis. The head of the femur meets with the pelvis at the acetabulum, forming the hip joint.-Structure:...

 (where the femur
Femur
The femur , or thigh bone, is the most proximal bone of the leg in tetrapod vertebrates capable of walking or jumping, such as most land mammals, birds, many reptiles such as lizards, and amphibians such as frogs. In vertebrates with four legs such as dogs and horses, the femur is found only in...

 meets 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:...

) that was only partially open. The ilium
Ilium (bone)
The ilium is the uppermost and largest bone of the pelvis, and appears in most vertebrates including mammals and birds, but not bony fish. All reptiles have an ilium except snakes, although some snake species have a tiny bone which is considered to be an ilium.The name comes from the Latin ,...

, the main hip bone, was supported only by two sacrals
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...

, a 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:...

 trait, but the 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....

 pointed backwards, a derived
Derived
In phylogenetics, a derived trait is a trait that is present in an organism, but was absent in the last common ancestor of the group being considered. This may also refer to structures that are not present in an organism, but were present in its ancestors, i.e. traits that have undergone secondary...

 trait that parallels what is seen in dromaeosaurids 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. Additionally, the end of the pubis had a booted shape, similar to what is present in avetheropods, and the vertebral centra had an Allosaurus
Allosaurus
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...

-like hourglass
Hourglass
An hourglass measures the passage of a few minutes or an hour of time. It has two connected vertical glass bulbs allowing a regulated trickle of material from the top to the bottom. Once the top bulb is empty, it can be inverted to begin timing again. The name hourglass comes from historically...

 shape.

Classification


The upper cladogram
Cladogram
A cladogram is a diagram used in cladistics which shows ancestral relations between organisms, to represent the evolutionary tree of life. Although traditionally such cladograms were generated largely on the basis of morphological characters, DNA and RNA sequencing data and computational...

 presented here follows one proposed analysis by M.D. Ezcurra in 2010. In this review, Herrerasaurus is a primitive saurischian, but not a theropod. The lower cladogram is based on an analysis by S. Nesbitt, in 2011. This analysis indicated that Herrerasaurus was a basal theropod.
Herrerasaurus gives its name to its family, Herrerasauridae, a group of similar animals from the Late Triassic
Late Triassic
The Late Triassic is in the geologic timescale the third and final of three epochs of the Triassic period. The corresponding series is known as the Upper Triassic. In the past it was sometimes called the Keuper, after a German lithostratigraphic group that has a roughly corresponding age...

 which were among the earliest of the dinosaurian
Evolution of dinosaurs
Dinosaurs evolved from the archosaurs 232-234 Ma in the Ladinian age, the latter part of the middle Triassic. Dinosauria is a well-supported clade, present in 98% of bootstraps...

 evolutionary radiation
Evolutionary radiation
An evolutionary radiation is an increase in taxonomic diversity or morphological disparity, due to adaptive change or the opening of ecospace. Radiations may affect one clade or many, and be rapid or gradual; where they are rapid, and driven by a single lineage's adaptation to their environment,...

. Where it and its close relatives lie on the early dinosaur evolutionary tree is unclear. Most 21st century analyses have found them to be 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:...

 theropods, though they may in fact represent basal saurischians or even be non-dinosaurian, predating the saurischian-ornithischian split. Most analyses, such as those by Sterling Nesbitt and colleagues (2009, 2011), have found Herrerasaurus and its relatives to be very basal theropods. Others (such as Ezcurra 2010) have found them to be basal to the clade Eusaurischia, that is, closer to the base of the saurischian tree than either true theropods or sauropodomorphs, but not members of either group. The situation is further complicated by uncertainties in correlating the ages of late Triassic beds bearing land animals.

Other members of the 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...

 may include 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...

from the same Ischigualasto Formation
Ischigualasto
Ischigualasto is a geological formation and a natural park associated with it in the province of San Juan, north-western Argentina, near the border with Chile...

 of Argentina as Herrerasaurus, Staurikosaurus
Staurikosaurus
Staurikosaurus is a genus of herrerasaurid dinosaur from the Late Triassic of Brazil.-Discovery:The first known specimen of Staurikosaurus was recovered from the Paleontological Site Jazigo Cinco of the Santa Maria Formation in the geopark of paleorrota , Rio Grande do Sul, southern Brazil...

from the Santa Maria Formation
Santa Maria Formation
The Santa Maria Formation is a sedimentary rock formation found in Rio Grande do Sul, Brazil. It has a late Ladinian – early Carnian age , and is notable for its fossils of early dinosaurs, including the herrerasaur Staurikosaurus, the basal saurischian Teyuwasu, and the basal sauropodomorph...

 of southern Brazil, Chindesaurus
Chindesaurus
Chindesaurus is a genus of saurischian dinosaur named after Chinde Point , near where the genoholotype specimen was discovered in the Chinle Formation of Petrified Forest National Park, Arizona, by Bryan Small in 1985...

from the Upper Petrified Forest
Petrified Forest National Park
Petrified Forest National Park is a United States national park in Navajo and Apache counties in northeastern Arizona. The park's headquarters are about east of Holbrook along Interstate 40 , which parallels a railroad line, the Puerco River, and historic U.S. Route 66, all crossing the park...

 (Chinle Formation
Chinle Formation
The Chinle is a geologic formation that is spread across the U.S. states of northern Arizona, Nevada, Utah, western New Mexico, and western Colorado. The Chinle is controversially considered to be synonymous to the Dockum Group in eastern Colorado, eastern New Mexico, southwestern Kansas, the...

) of Arizona, and possibly Caseosaurus
Caseosaurus
Caseosaurus is a genus of dinosaur from the Late Triassic of North America. The type species, Caseosaurus crosbyensis, was formally described by Hunt, Lucas, Heckert, Sullivan and Lockley in 1998. It was found in the Dockum Formation of Texas, in strata dating to the Late Triassic...

from the Dockum Formation
Dockum Group
The Dockum is a Late Triassic geologic group found primarily on the Llano Estacado of western Texas and eastern New Mexico with minor exposures in southwestern Kansas, eastern Colorado, and Oklahoma panhandle. The Dockum reaches a maximum thickness of slightly over 650 m but is usually much...

 of Texas, although the relationships of these animals are not fully understood, and not all paleontologists agree. Other possible basal theropods, Alwalkeria
Alwalkeria
Alwalkeria is a genus of basal saurischian dinosaur from the Late Triassic of India. It was a small bipedal omnivore.This dinosaur was originally named Walkeria maleriensis by Sankar Chatterjee in 1987, in honor of famous British paleontologist Alick Walker and the Maleri Formation, in which its...

from the Late Triassic Maleri Formation of India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...

, and Teyuwasu
Teyuwasu
Teyuwasu is a dubious genus of dinosaur from the Late Triassic. Named by Edio-Ernst Kischlat in 1999, little is currently known about this genus; the type species, T. barbarenai, was named from leg bones discovered in geopark of paleorrota, Brazil. The specific name honors Dr. Mário Costa...

, known from very fragmentary remains from the Late Triassic of Brazil, might be related. Novas (1992) defined Herrerasauridae as Herrerasaurus, Staurikosaurus, and their most recent common ancestor. Sereno (1998) defined the group as the most inclusive clade including H. ischigualastensis but not Passer domesticus
House Sparrow
The House Sparrow is a bird of the sparrow family Passeridae, found in most parts of the world. One of about 25 species in the genus Passer, the House Sparrow occurs naturally in most of Europe, the Mediterranean region, and much of Asia...

. Langer (2004) provided first phylogenetic definition of a higher level taxon
Taxon
|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...

, infraorder Herrerasauria.

History


Herrerasaurus was named by paleontologist Osvaldo Reig after Victorino Herrera, an Andean goatherd
Goatherd
A goatherd or goatherder is a person who herds goats as a vocational activity. Similar to a fisherman who catches fish for a living, the drover here herds goats. Goatherds are popular in countries where goat populations are significant; for instance, in Africa and South Asia...

 who first noticed its fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

s in outcrops near the city of San Juan
San Juan, Argentina
San Juan is the capital city of the Argentine province of San Juan in the Cuyo region, located in the Tulúm Valley, west of the San Juan River, at above mean sea level, with a population of around 112,000 as per the ....

 in 1959. These rocks, which later yielded 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...

, are part of the Ischigualasto Formation
Ischigualasto
Ischigualasto is a geological formation and a natural park associated with it in the province of San Juan, north-western Argentina, near the border with Chile...

 and date from the late Ladinian
Ladinian
The Ladinian is a stage and age in the Middle Triassic series or epoch. It spans the time between 237 ± 2 Ma and 228 ± 2 Ma...

 to early Carnian
Carnian
The Carnian is the lowermost stage of the Upper Triassic series . It lasted from about 228.7 till 216.5 million years ago . The Carnian is preceded by the Ladinian and is followed by the Norian...

 stages of the Late 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...

 period. Reig named a second dinosaur from these rocks in the same publication as Herrerasaurus; this dinosaur, Ischisaurus cattoi, is now considered a junior synonym
Synonym (taxonomy)
In scientific nomenclature, a synonym is a scientific name that is or was used for a taxon of organisms that also goes by a different scientific name. For example, Linnaeus was the first to give a scientific name to the Norway spruce, which he called Pinus abies...

 and a juvenile
Juvenile (organism)
A juvenile is an individual organism that has not yet reached its adult form, sexual maturity or size. Juveniles sometimes look very different from the adult form, particularly in terms of their colour...

 of Herrerasaurus. Two other partial skeletons, with skull material, were named Frenguellisaurus ischigualastensis by Fernando Novas
Fernando Novas
Fernando Emilio Novas is an Argentine paleontologist working for the Comparative Anatomy Department of the Bernardino Rivadavia Natural History Argentine Museum in Buenos Aires...

 in 1986, but this species too is now thought to be a synonym.

Reig believed Herrerasaurus was an early example of a carnosaur
Carnosauria
Carnosauria is a group of large predatory dinosaurs that lived during the Jurassic and Cretaceous periods. While it originally contained a wide assortment of giant theropods that were not closely related, the group has since been defined to encompass only the allosaurs and their closest kin...

, but this was the subject of much debate over the next 30 years, and the genus was variously classified during that time. In 1970, Steel classified Herrerasaurus as a prosauropod. In 1972, Peter Galton
Peter Galton
Peter M. Galton is a British vertebrate paleontologist working in America, who has to date written or co-written about a hundred papers in scientific journals or chapters in paleontology textbooks, especially on ornithischian and prosauropod dinosaurs.With Robert Bakker in a joint article...

 classified the genus as not diagnosable beyond 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...

. Later, using 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...

 analysis, some researchers put Herrerasaurus and Staurikosaurus at the base of the dinosaur tree before the separation between ornithischians and saurischians. Several researchers classified the remains as non-dinosaurian.

A complete Herrerasaurus skull was not found until 1988, by a team of paleontologists led by Paul Sereno
Paul Sereno
Paul Callistus Sereno is an American paleontologist from the University of Chicago who discovered several new dinosaur species on several continents. He has conducted excavations at sites as varied as Inner Mongolia, Argentina, Morocco, and Niger...

. Based on the new fossils, authors such as Thomas Holtz and Jose Bonaparte
José Bonaparte
José Fernando Bonaparte, Ph.D. , is an Argentine paleontologist who discovered a plethora of South American dinosaurs and mentored a new generation of Argentine paleontologists like Rodolfo Coria...

 classified Herrerasaurus at the base of the saurischian tree before the divergence between prosauropods and theropods. However, Sereno favored classifying Herrerasaurus (and the Herrerasauridae) as primitive theropods. These two classifications have become the most persistent, with Rauhut (2003) and Bittencourt and Kellner (2004) favoring the early theropod hypothesis
Hypothesis
A hypothesis is a proposed explanation for a phenomenon. The term derives from the Greek, ὑποτιθέναι – hypotithenai meaning "to put under" or "to suppose". For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it...

, and Max Langer (2004), Langer and Benton
Michael J. Benton
Michael J. Benton is a British paleontologist, a Fellow of the Royal Society of Edinburgh, and professor of vertebrate palaeontology in the Department of Earth Sciences at the University of Bristol....

 (2006), and Randall Irmis and his coauthors (2007) favoring the basal saurischian hypothesis. If Herrerasaurus were indeed a theropod, it would indicate that theropods, sauropodomorphs, and ornithischians diverged even earlier than herrerasaurids, before the middle Carnian
Carnian
The Carnian is the lowermost stage of the Upper Triassic series . It lasted from about 228.7 till 216.5 million years ago . The Carnian is preceded by the Ladinian and is followed by the Norian...

, and that "all three lineages independently evolved several dinosaurian features, such as a more advanced ankle joint or an open acetabulum". This view is further supported by ichnological
Ichnology
Ichnology is the branch of geology that deals with traces of organismal behavior, such as burrows and footprints. It is generally considered as a branch of paleontology; however, only one division of ichnology, paleoichnology, deals with trace fossils, while neoichnology is the study of modern traces...

 records showing large tridactyl (three-toed) footprints that can be attributed only to a theropod dinosaur. These footprints date from the Ladinian
Ladinian
The Ladinian is a stage and age in the Middle Triassic series or epoch. It spans the time between 237 ± 2 Ma and 228 ± 2 Ma...

 (Middle Triassic) of the Los Rastros Formation in Argentina and predate Herrerasaurus by 3 to 5 million years.

The study of early dinosaurs such as Herrerasaurus and Eoraptor therefore has important implications for the concept of dinosaurs as a monophyletic
Monophyly
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...

 group (a group descended from a common ancestor). The monophyly of dinosaurs was explicitly proposed in the 1970s by Galton and Robert T. Bakker
Robert T. Bakker
Robert T. Bakker is an American paleontologist who helped reshape modern theories about dinosaurs, particularly by adding support to the theory that some dinosaurs were endothermic...

, who compiled a list of cranial and postcranial synapomorphies
Synapomorphy
In cladistics, a synapomorphy or synapomorphic character is a trait that is shared by two or more taxa and their most recent common ancestor, whose ancestor in turn does not possess the trait. A synapomorphy is thus an apomorphy visible in multiple taxa, where the trait in question originates in...

 (common anatomical traits derived from the common ancestor). Later authors proposed additional synapomorphies. An extensive study of Herrerasaurus by Sereno in 1992 suggested that of these proposed synapomorphies, only one cranial and seven postcranial features were actually derived from a common ancestor, and that the others were attributable 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...

. Sereno's analysis of Herrerasaurus also led him to propose several new dinosaurian synapomorphies.

Paleoecology



Although Herrerasaurus shared the body shape of the large carnivorous dinosaurs, it lived about 230 million years ago, a time when dinosaurs were small and insignificant. It was the time of non-dinosaurian reptiles, not dinosaurs, and a major turning point in the Earth's ecology. The vertebrate fauna of the Ischigualasto Formation and the slightly later Los Colorados Formation
Los Colorados Formation
The Los Colorados Formation is a sedimentary rock formation found in La Rioja, Argentina. It has a Norian age , and is notable for its fossils of early dinosaurs, including the coelophysoid Zupaysaurus and the "prosauropods" Coloradisaurus, Lessemsaurus, and Riojasaurus.-Dinosaurs:Indeterminate...

 consisted mainly of a variety of crurotarsal
Crurotarsi
The Crurotarsi are a group of archosauriformes, represented today by the crocodiles,...

 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 and synapsids. For instance, in the Ischigualasto Formation, dinosaurs constituted only about 6% of the total number of fossils. By the end of the 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...

 Period, dinosaurs were becoming the dominant large land animals, and the other archosaurs and synapsids declined in variety and number.

Studies suggest that the paleoenvironment
Natural environment
The natural environment encompasses all living and non-living things occurring naturally on Earth or some region thereof. It is an environment that encompasses the interaction of all living species....

 of the Ischigualasto Formation was a volcanically active floodplain covered by forests and subject to strong seasonal rainfalls. The climate was moist and warm, though subject to seasonal variations. Vegetation consisted of fern
Fern
A fern is any one of a group of about 12,000 species of plants belonging to the botanical group known as Pteridophyta. Unlike mosses, they have xylem and phloem . They have stems, leaves, and roots like other vascular plants...

s (Cladophlebis
Cladophlebis
Cladophlebis is an extinct genus of fern which grew during the late Paleozoic and Mesozoic eras . It was a common plant during that time in both the northern and southern hemispheres, and belonged to the order of plants called Filicales....

), horsetail
Horsetail
Equisetum is the only living genus in the Equisetaceae, a family of vascular plants that reproduce by spores rather than seeds.Equisetum is a "living fossil", as it is the only living genus of the entire class Equisetopsida, which for over one hundred million years was much more diverse and...

s, and giant conifers (Protojuniperoxylon). These plants formed highland
Highland (geography)
The term highland or upland is used to denote any mountainous region or elevated mountainous plateau. Generally speaking, the term upland tends to be used for ranges of hills, typically up to 500-600m, and highland for ranges of low mountains.The Scottish Highlands refers to the mountainous...

 forests along the banks of rivers. Herrerasaurus remains appear to have been the most common among the carnivores of the Ischigualasto Formation. It lived in the jungles of Late Triassic South America
South America
South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...

 alongside another early dinosaur, the one metre long 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...

, as well as Saurosuchus
Saurosuchus
Saurosuchus is an extinct genus of rauisuchian archosaur in the family Prestosuchidae. With a length of around 7 m , it was the largest rauisuchian, except perhaps for the less well known Fasolasuchus. Like other rauisuchians, Saurosuchus walked on four fully erect limbs...

, a giant land-living rauisuchia
Rauisuchia
Rauisuchia is a group of predatory and mostly large Triassic archosaurs. As a clade, Rauisuchia includes these Triassic forms and all crocodylomorphs, which are descendants of Triassic rauisuchians. The group in its traditional sense is paraphyletic, because it does not include crocodylomorph...

n (a quadrupedal meat eater with a 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...

-like skull); the broadly similar but smaller Venaticosuchus
Venaticosuchus
Venaticosuchus is a genus of Late Triassic quadrupedal crurotarsan archosaur. Originally it was thought to be the ancestor to the carnosaur dinosaurs ; however, now it is known to be more closely related to crocodilians than dinosaurs...

, an ornithosuchid
Ornithosuchidae
Ornithosuchidae is an extinct family of reptiles from the Triassic period that were distantly related to crocodilians. They are classified as crurotarsan archosaurs. Ornithosuchids were quadrupedal and facultatively bipedal, meaning they had the ability to walk on two legs for short periods of time...

; and the predatory chiniquodontids
Chiniquodontidae
Chiniquodontidae is a family of meat-eating advanced mammal-like reptiles that lived during the Upper Triassic of South America and perhaps Europe. A further possible representative, Aleodon, has been identified from the Middle Triassic of Africa...

. Herbivores were much more abundant than carnivores and were represented by rhynchosaur
Rhynchosaur
Rhynchosaurs were a group of Triassic diapsid reptiles related to the archosaurs.-Description:Rhynchosaurs were herbivores, and at times abundant , with stocky bodies and a powerful beak...

s such as Hyperodapedon
Hyperodapedon
Hyperodapedon is a genus of rhynchosaur from the late Triassic period . The type species of Scaphonyx , Scaphonyx fischeri that once thought to be a dinosaur, is now known to be based on dubious material and therefore should be a nomen dubium...

(a beaked reptile); aetosaur
Aetosaur
Aetosaurs are an extinct order of heavily armoured, medium- to large-sized Late Triassic herbivorous archosaurs. They have small heads, upturned snouts, erect limbs, and a body covered by plate-like scutes. All aetosaurs belong to the family Stagonolepididae...

s (spiny armored reptiles); kannemeyeriid
Kannemeyeriidae
Kannemeyeriidae is a family of large, stocky, beaked and sometimes tusked dicynodonts. They were the dominant large terrestrial herbivores through most of the Triassic period...

 dicynodonts (stocky, front-heavy beaked quadrupedal animals) such as Ischigualastia
Ischigualastia
Ischigualastia was a dicynodont that lived during the Carnian age of the Late Triassic Period. From the Ischigualasto Formation of Argentina, it was a member of the family Kannemeyeridae....

; and traversodontids
Traversodontidae
Traversodontidae is a family of herbivorous cynodonts. Traversodonts were primarily Gondwanan, with many species known from Africa and South America. Recently, traversodonts have also been found from Europe and eastern North America. Traversodonts first appeared in the Middle Triassic, diversified...

 (somewhat similar in overall form to dicynodonts, but lacking beaks) such as Exaeretodon
Exaeretodon
Exaeretodon is a genus of traversodontid cynodont; several species are known, from various formations. E. argentinus, E. frenguelli, and E. vincei are from the Carnian-age Ischigualasto Formation of Argentina. E. major and E. riograndensis are from the Ladinian-age portion of the Santa Maria...

. These non-dinosaurian herbivores were much more abundant than early ornithischian dinosaurs like Pisanosaurus
Pisanosaurus
Pisanosaurus is a genus of primitive ornithischian dinosaur from the Late Triassic of what is now South America. It was a bipedal herbivore described by Argentine paleontologist Rodolfo Casamiquela in 1967. Only one species, the type, Pisanosaurus mertii, is known, based on a single partial skeleton...

.

Paleobiology



The teeth of Herrerasaurus indicate that it was a carnivore
Carnivore
A carnivore meaning 'meat eater' is an organism that derives its energy and nutrient requirements from a diet consisting mainly or exclusively of animal tissue, whether through predation or scavenging...

; its size indicates it would have preyed upon small and medium-sized plant eaters. These might have included other dinosaurs, such as Pisanosaurus
Pisanosaurus
Pisanosaurus is a genus of primitive ornithischian dinosaur from the Late Triassic of what is now South America. It was a bipedal herbivore described by Argentine paleontologist Rodolfo Casamiquela in 1967. Only one species, the type, Pisanosaurus mertii, is known, based on a single partial skeleton...

, as well as the more plentiful rhynchosaur
Rhynchosaur
Rhynchosaurs were a group of Triassic diapsid reptiles related to the archosaurs.-Description:Rhynchosaurs were herbivores, and at times abundant , with stocky bodies and a powerful beak...

s and synapsid
Synapsid
Synapsids are a group of animals that includes mammals and everything more closely related to mammals than to other living amniotes. They are easily separated from other amniotes by having an opening low in the skull roof behind each eye, leaving a bony arch beneath each, accounting for their name...

s. Herrerasaurus itself may have been preyed upon by giant rauisuchid
Rauisuchia
Rauisuchia is a group of predatory and mostly large Triassic archosaurs. As a clade, Rauisuchia includes these Triassic forms and all crocodylomorphs, which are descendants of Triassic rauisuchians. The group in its traditional sense is paraphyletic, because it does not include crocodylomorph...

s like Saurosuchus
Saurosuchus
Saurosuchus is an extinct genus of rauisuchian archosaur in the family Prestosuchidae. With a length of around 7 m , it was the largest rauisuchian, except perhaps for the less well known Fasolasuchus. Like other rauisuchians, Saurosuchus walked on four fully erect limbs...

; puncture wounds were found in one skull.

Coprolite
Coprolite
A coprolite is fossilized animal dung. Coprolites are classified as trace fossils as opposed to body fossils, as they give evidence for the animal's behaviour rather than morphology. The name is derived from the Greek words κοπρος / kopros meaning 'dung' and λιθος / lithos meaning 'stone'. They...

s (fossilized dung) containing small bones but no trace of plant fragments, discovered in the Ischigualasto Formation, have been assigned to Herrerasaurus based on fossil abundance. Mineralogical and chemical analysis of these coprolites indicates that if the referral to Herrerasaurus was correct, this carnivore could digest bone.

Comparisons between the scleral ring
Sclerotic ring
Sclerotic rings are rings of bone found in the eyes of several groups of vertebrate animals, except for mammals and crocodilians. They can be made up of single bones or small bones together. They are believed to have a role in supporting the eye, especially in animals whose eyes are not spherical,...

s of Herrerasaurus and modern birds and reptiles suggest that it may have been cathemeral
Cathemeral
A cathemeral organism is one that has sporadic and random intervals of activity during the day or night in which food is acquired, socializing with other organisms occurs, and any other activities necessary for livelihood are performed...

, active throughout the day at short intervals.

Paleopathology


In a 2001 study conducted by Bruce Rothschild and other paleontologists, 12 hand bones and 20 foot bones referred to Herrerasaurus were examined for signs of stress fracture
Stress fracture
A stress fracture is one type of incomplete fracture in bones. It is caused by "unusual or repeated stress" and also heavy continuous weight on the ankle or leg...

, but none were found.

PVSJ 407, a Herrerasaurus ischigualastensis had a pit in a skull bone attributed to a bite by Paul Sereno
Paul Sereno
Paul Callistus Sereno is an American paleontologist from the University of Chicago who discovered several new dinosaur species on several continents. He has conducted excavations at sites as varied as Inner Mongolia, Argentina, Morocco, and Niger...

 and Novas. Two additional pits occurred on the splenial
Splenial
The splenial is a small bone in the lower jaw of reptiles, amphibians and birds, usually located on the lingual side between the angular and suprangular....

. The area around these pits are swollen and porous, suggesting the wounds were afflicted by a short-lived non-lethal infection. Because of the size and angles of the wound, its likely that they were obtained in a fight with another Herrerasaurus.

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