Herrerasaurus

''Herrerasaurus'' is a genus of saurischian dinosaur from the Late Triassic period. This genus was one of the earliest dinosaurs from the fossil record. Its name means "Herrera's lizard", after the rancher who discovered the first specimen in 1958 in South America. All known fossils of this carnivore have been discovered in the Ischigualasto Formation of Carnian age (late Triassic according to the ICS, dated to 231.4 million years ago) in northwestern Argentina. The type species, ''Herrerasaurus ischigualastensis'', was described by Osvaldo Reig in 1963 and is the only species assigned to the genus. ''Ischisaurus'' and ''Frenguellisaurus'' are synonyms.

For many years, the classification of ''Herrerasaurus'' was unclear because it was known from very fragmentary remains. It was hypothesized to be a basal theropod, a basal sauropodomorph, a basal saurischian, or not a dinosaur at all but another type of archosaur. However, with the discovery of an almost complete skeleton and skull in 1988, ''Herrerasaurus'' has been classified as an early saurischian in most of the phylogenies on the origin and early evolution of dinosaurs.

It is a member of the Herrerasauridae, a family of similar genera that were among the earliest of the dinosaurian evolutionary radiation..

Discovery

''Herrerasaurus'' was named by paleontologist Osvaldo Reig after Victorino Herrera, an Andean goatherd who first noticed its fossils in outcrops near the city of San Juan, Argentina in 1959. These rocks, which later yielded ''Eoraptor'', are part of the Ischigualasto Formation and date from the late Ladinian to early Carnian stages of the Late Triassic 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 and a juvenile of ''Herrerasaurus''.

Reig believed ''Herrerasaurus'' was an early example of a carnosaur, 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 classified the genus as not diagnosable beyond Saurischia. Later, using cladistic 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.

Two other partial skeletons, with skull material, were named ''Frenguellisaurus ischigualastensis'' by Fernando Novas in 1986, but this species too is now thought to be a synonym. ''Frenguellisaurus ischigualastensis'' was discovered in 1975, and was described by Novas (1986) who considered it a primitive saurischian, and possibly a theropod. Novas (1992) and Sereno and Novas (1992) examined the ''Frenguellisaurus'' remains and found them referable to ''Herrerasaurus''. ''Ischisaurus cattoi'' was discovered in 1960 and described by Reig in 1963. Novas (1992) and Sereno and Novas (1992) reviewed its remains and found them also to be referable to ''Herrerasaurus''.
A complete ''Herrerasaurus'' skull was found in 1988, by a team of paleontologists led by Paul Sereno. Based on the new fossils, authors such as Thomas Holtz and José Bonaparte 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, and Max Langer (2004), Langer and Benton (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, 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 records showing large tridactyl (three-toed) footprints that can be attributed only to a theropod dinosaur. These footprints date from the early Carnian Los Rastros Formation in Argentina, which predates ''Herrerasaurus'' by several million years.

The study of early dinosaurs such as ''Herrerasaurus'' and ''Eoraptor'' therefore has important implications for the concept of dinosaurs as a monophyletic group (a group descended from a common ancestor). The monophyly of dinosaurs was explicitly proposed in the 1970s by Galton and Robert T. Bakker, who compiled a list of cranial and postcranial synapomorphies (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. Sereno's analysis of ''Herrerasaurus'' also led him to propose several new dinosaurian synapomorphies.

Description

''Herrerasaurus'' was a lightly built bipedal carnivore with a long tail and a relatively small head. Adults had skulls up to long and were up to in total length and in weight. Smaller specimens were half the size, with skulls only about long.

''Herrerasaurus'' was fully bipedal. It had strong hind limbs with short thighs and rather long feet, indicating that it was 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. The forelimbs of ''Herrerasaurus'' were less than half the length of its hind limbs. The upper arm and forearm were rather short, while the manus (hand) was elongated. The first two fingers and the thumb ended in curved, sharp claws for grasping prey. The fourth and fifth digits were small stubs without claws.

''Herrerasaurus'' displays traits that are found in different groups of dinosaurs, and several traits found in non-dinosaurian archosaurs. Although it shares most of the characteristics of dinosaurs, there are a few differences, particularly in the shape of its hip and leg bones. Its pelvis is like that of saurischian dinosaurs, but it has a bony acetabulum (where the femur meets the pelvis) that was only partially open. The ilium, the main hip bone, is supported by only two sacrals, a basal trait. However, the pubis points backwards, a derived trait as seen in dromaeosaurids and birds. Additionally, the end of the pubis has a booted shape, like those in avetheropods; and the vertebral centra has an hourglass shape as found in ''Allosaurus''.

''Herrerasaurus'' had a long, narrow skull that lacked nearly all the specializations that characterized later dinosaurs, and more closely resembled those of more primitive archosaurs such as ''Euparkeria''. It had five pairs of fenestrae (skull openings) in its skull, two pairs of which were for the eyes and nostrils. Between the eyes and the nostrils were two antorbital fenestrae and a pair of tiny, slit-like holes called promaxillary fenestrae.

''Herrerasaurus'' had a flexible joint in the lower jaw that could slide back and forth to deliver a grasping bite. This cranial specialization is unusual among dinosaurs but has evolved independently in some lizards. 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.

According to Novas (1993), ''Herrerasaurus'' can be distinguished based on the following features: the presence of a premaxilla-maxilla fenestra, and the dorsal part of laterotemporal fenestra is less than a third as wide as the ventral part; the presence of a ridge on the lateral surface of the jugal bone, and a deeply incised supratemporal fossa that extends across the medial postorbital process; the subquadrate ventral squamosal process has a lateral depression, and the quadratojugal bone overlaps the posterodorsal quadrate face; the pterygoid process of the quadrate has an inturned, trough-shaped ventral margin, and the presence of a slender ribbed posterodorsal dentary process; the surangular bone has a forked anterior process for articulation with the posterodorsal dentary process; the humerus' internal tuberosity is proximally projected and separated from the humeral head by a deep groove (also present in coelophysoids); possesses enlarged hands, which are 60% of the size of the humerus+radius, and the humeral entepicondyle is ridge-like with anterior and posterior depressions; and the posterior border of the ilial peduncle forms a right angle with the dorsal border of the shaft on the ischium.

According to Sereno (1993), ''Herrerasaurus'' can be distinguished based on the following features, all of which are unknown in other herrerasaurids: a circular pit is present on the humeral ectepicondyle, a feature also present in '' Saturnalia''; a saddle-shaped ulnar condyle of the humerus, and the articular surface for the ulnare on the ulna is convex; the articular surface of the ulnare is smaller than that of the ulna, a feature unknown in ''Staurikosaurus'' and ''Sanjuansaurus''; the centrale is placed distal to the radiale; a broad subnarial process of the premaxilla, and a broad supratemporal depression (noted by Sereno and Novas, 1993); the basal tuber and the occipital condyle are subequal in width (noted by Sereno and Novas, 1993).

Classification

''Herrerasaurus'' was originally considered to be a genus within Carnosauria, which then included forms similar to ''Megalosaurus'' and ''Antrodemus'' (the latter is equivalent to ''Allosaurus''), even though ''Herrerasaurus'' lived many millions of years before them and therefore would have retained multiple primitive features. This carnosaurian classification was amended upon by Rozhdestvensky and Tatarinov in 1964, who classified ''Herrerasaurus'' within the family Gryponichidae inside Carnosauria. The same year, Walker published a differing opinion that ''Herrerasaurus'' instead was allied with Plateosauridae, although it differed in possessing a pubic boot. Walker also proposed that ''Herrerasaurus'' may instead be close to ''Poposaurus'' (now considered a pseudosuchian) and the unnamed theropod from the Dockum Group of Texas (now assigned to the rauisuchian ''Postosuchus''). In 1985, Charig noted that ''Herrerasaurus'' was of uncertain classification, showing similarities to both "prosauropods" and "carnosaurians". Romer (1966), simply noted that ''Herrerasaurus'' was a prosauropod possibly within Plateosauridae. In the description of ''Staurikosaurus'', Colbert noted that there were many similarities between his taxon and ''Herrerasaurus'', but classified them in separate families, with ''Herrerasaurus'' in Teratosauridae. In 1970, Bonaparte also proposed similarities between ''Herrerasaurus'' and ''Staurikosaurus'', and while classifying them both clearly as in Saurischia, he stated that they appeared as though they could not be placed in a current family. This was further supported by Benedetto in 1973, who named for the taxa the new family Herrerasauridae, which he classified as saurischians, possibly within Theropoda but not in Sauropodomorpha. However, in 1977 Galton proposed that Herrerasauridae only included ''Herrerasaurus'', and found it to be Saurischian ''incertae sedis''.

Proposed in 1987 by Brinkman and Sues, ''Herrerasaurus'' has at times been considered basal to Ornithischia and Saurischia, although Brinkmann and Sues still considered it to be inside Dinosauria. They supported this on the basis that ''Herrerasaurus'' has a large pedal digit V, and has a well developed medial wall on the acetabulum. Brinkmann and Sues considered ''Staurikosaurus'' and ''Herrerasaurus'' to not form a true group called Herrerasauridae, and that instead they were successively more primitive forms. Also, they considered the characters used by Benedetto to be invalid, instead representing only the plesiomorphies state that was found in both taxa. This was disagreed with in 1992 by Novas, who stated many derived synapomorphies of Herrerasauridae, such as a distinct pubic boot, but still classified them as basal to Ornithischia and Saurischia. Novas defined the family as the least common ancestor of ''Herrerasaurus'' and ''Staurikosaurus'' and all its descendants. A differing definition of Herrerasauridae as the most inclusive clade including ''Herrerasaurus'' but not ''Passer domesticus'' was first suggested by Sereno (1998), and more closely follows the original inclusion proposed by Benedetto. Another group, Herrerasauria was named by Galton in 1985, and defined as ''Herrerasaurus'' but not '' Liliensternus'' or ''Plateosaurus'' by Langer (2004), who used the node-based definition for Herrerasauridae.

In a revision of basal Dinosauria, Padian and May (1993) discussed the definition of the clade, and redefined it as the latest common ancestor of ''Triceratops'' and birds. They also discussed what this definition would do to the most basal taxa, such as Herrerasauridae, and ''Eoraptor''. Padian and May considered that since both Herrerasauridae and ''Eoraptor'' lack many diagnostic features of Saurischia or Ornithischia, that they could not be considered inside Dinosauria.

A later 1994 study by Novas instead classified ''Herrerasaurus'' within Dinosauria, and strongly supported its position within Saurischia, as well as provided synapomorphies that it shared with Theropoda. Novas found that the primitive features of lacking a brevis fossa and having only two sacral vertebrae were simply reversals found in the genus. In 1996, Novas went further by supporting a theropod position for ''Herrerasaurus'' with a phylogenetic analysis, which placed it closer to Neotheropoda than ''Eoraptor'' or Sauropodomorpha. Langer (2004) mentioned that this hypothesis was widely accepted, but that more later authors instead preferred to place ''Herrerasaurus'' as well as ''Eoraptor'' basal to Theropoda and Sauropodomorpha, a clade called Eusaurischia. Langer (2004) conducted a phylogenetic analysis, and found that it was much more likely that ''Herrerasaurus'' was a basal saurischian, than either a theropod or a non-dinosaurian. Langer's proposal was supported by multiple studies until the discovery of '' Tawa'', when Nesbitt ''et al.'' conducted a more inclusive analysis, and the resulting cladogram placed Herrerasauridae basal to ''Eoraptor'', but closer to ''Dilophosaurus'' than Sauropodomorpha. Unlike Nesbitt, Ezcurra (2010) conducted a phylogenetic analysis to place his new taxon '' Chromogisaurus'', and found that Herrerasauridae was basal to Eusaurischia.

In 2010, Alcocer and Martinez described a new taxon of herrerasaurid, '' Sanjuansaurus''. It could be distinguished from ''Herrerasaurus'' based on multiple features. In the phylogenetic analysis, ''Herrerasaurus'', ''Sanjuansaurus'' and ''Staurikosaurus'' all were in a polytomy, and Herrerasauridae was the most primitive group of saurischian, outside Eusaurischia, ''Eoraptor'' and '' Guaibasaurus''. In 2011, Martinez ''et al.'' described ''Eodromaeus'', a basal theropod from the same formation as ''Herrerasaurus''. In a phylogenetic analysis, ''Eoraptor'' was placed within Sauropodomorpha, Herrerasauridae was placed as the most basal theropods, and ''Eodromaeus'' was placed as the next most basal. A more recent analysis, by Bittencourt ''et al.'' (2014), placed Herrerasauridae in a polytomy with Theropoda and Sauropodomorpha, with ''Eoraptor'' also being in an unresolved position. This cladogram is shown below.

Other members of the clade may include ''Chindesaurus'' from the Upper Petrified Forest ( Chinle Formation) of Arizona, and possibly '' Caseosaurus'' from the Tecovas Formation of the Dockum Group in Texas, although the relationships of these animals are not fully understood, and not all paleontologists agree. Other possible basal theropods, ''Alwalkeria'' from the Late Triassic Lower Maleri Formation of India, and '' Teyuwasu'', known from very fragmentary remains from the Late Triassic of Brazil, might be related. Paul (1988) noted that it had been incorrectly suggested that ''Staurikosaurus pricei'' was a juvenile ''Herrerasaurus''. This claim was refuted when pelvic bones from a juvenile ''Herrerasaurus'' were discovered, which upon examination did not resemble the pelvic bones of ''Staurikosaurus''.

Paleobiology

The teeth of ''Herrerasaurus'' indicate that it was a carnivore; its size indicates it would have preyed upon small and medium-sized plant eaters. These might have included other dinosaurs, such as '' Pisanosaurus'', as well as the more plentiful rhynchosaurs and synapsids. ''Herrerasaurus'' itself may have been preyed upon by giant rauisuchids like ''Saurosuchus''; puncture wounds were found in one skull.

Coprolites (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 rings of ''Herrerasaurus'' and modern birds and reptiles suggest that it may have been cathemeral, active throughout the day at short intervals.

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, but none were found.Rothschild, B., Tanke, D. H., and Ford, T. L., 2001, Theropod stress fractures and tendon avulsions as a clue to activity: In: ''Mesozoic Vertebrate Life'', edited by Tanke, D. H., and Carpenter, K., Indiana University Press, pp. 331–336.

PVSJ 407, a ''Herrerasaurus ischigualastensis'', had a pit in a skull bone attributed by Paul Sereno and Novas to a bite. Two additional pits occurred on the splenial. The areas 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, it is likely that they were obtained in a fight with another ''Herrerasaurus''.Molnar, R. E., 2001, Theropod paleopathology: a literature survey: In: ''Mesozoic Vertebrate Life'', edited by Tanke, D. H., and Carpenter, K., Indiana University Press, pp. 337–363.

Paleoecology

The holotype of ''Herrerasaurus'' (PVL 2566) was discovered in the Cancha de Bochas Member of the Ischigualasto Formation in San Juan, Argentina. It was collected in 1961 by Victorino Herrera, in sediments that were deposited in the Carnian stage of the Triassic period, approximately 231 to 229 million years ago. Over the years, the Ischigualasto Formation produced other fossils ultimately referred to ''Herrerasaurus''. In 1958, A.S. Romer discovered specimen MCZ 7063, originally referred to '' Staurikosaurus'' in Carnian sediments. ''Herrerasaurus'' specimens PVL 2045 and MLP(4)61, were collected in 1959 and 1960, respectively, in sediments that were deposited in the Norian stage of the Triassic period, approximately 228 to 208 million years ago. However, these specimens are no longer regarded as pertaining to ''Herrerasaurus''. In 1960, Scaglia collected specimen MACN 18.060, originally the holotype of ''Ischisaurus cattoi'', in sediments deposited in the Carnian stage. In 1961, Scaglia collected ''Herrerasaurus'' specimen PVL 2558, in the Carnian beds of this formation. In 1990, the Cancha de Bochas Member produced more ''Herrerasaurus'' specimens, also from its Carnian beds. Specimen PVSJ 53, originally the holotype of ''Frenguellisaurus ischigualastensis'', was collected by Gargiulo & Oñate in 1975 in sediments that were deposited in the Carnian stage.

Although ''Herrerasaurus'' shared the body shape of the large carnivorous dinosaurs, it lived during a time when dinosaurs were small and few. 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 consisted mainly of a variety of crurotarsal archosaurs and synapsids. In the Ischigualasto Formation, dinosaurs constituted only about 10% of the total number of fossils, but by the end of the Triassic 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 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 ferns (''Cladophlebis''), horsetails, and giant conifers (''Protojuniperoxylon''). These plants formed lowland 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 alongside other early dinosaurs, such as '' Sanjuansaurus'', ''Eoraptor'', '' Panphagia'', and '' Chromogisaurus'', as well as rhynchosaurs (''Scaphonyx''), cynodonts (e.g., ''Exaeretodon'', '' Ecteninion'' and '' Chiniquodon''), dicynodonts ('' Ischigualastia''), pseudosuchians (e.g., ''Saurosuchus'', ''Sillosuchus'' and ''Aetosauroides''), proterochampsids (e.g., '' Proterochampsa'') and temnospondyls ('' Pelorocephalus'').

References

External links

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Introduction to ''Herrerasaurus''
from the University of California Museum of Paleontology

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Prehistoric saurischians
Dinosaur genera
Carnian genera
Late Triassic dinosaurs of South America
Triassic Argentina
Fossils of Argentina

Fossil taxa described in 1963