Herrerasaurus is likely a genus of saurischian dinosaur from the Late Triassic period. Measuring long and weighing around , 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 Carnian stage 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 about long and weighed about .
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 have 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 probably 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.