Herrerasaurus (meaning "Herrera's lizard," after the name of the rancher who discovered the first fossil of the animal) was one of the earliest dinosaurs. All known specimens of this carnivore have been discovered in northwest Patagonia, Argentina, in late Triassic Period rocks (early Carnian stage, around 228 million years ago). The type species, Herrerasaurus ischigualastensis, was described by Osvaldo Reig in 1963 and is the only species assigned to the genus.
For many years, the classification of Herrerasaurus was unclear, as the animal was initially known from very fragmentary remains; it has been hypothesized to be a basal theropod, a basal sauropodomorph, a basal saurischian, 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 surveys of theropod evolution. This medium-sized bipedal reptile is a member of the Herrerasauridae, a group of similar animals which were among the earliest of the dinosaurian radiation.
Herrerasaurus had a long, narrow skull that lacked nearly all the specializations that characterized later dinosaurs, being not that different from those of more primitive archosaurs such as Euparkeria. It had five pairs of fenestrae (skull openings) in its skull, two of which were for ocular and nasal openings. Between the eyes and the nostrils were two antorbital fenestrae 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 reduce the weight of the skull.
Herrerasaurus had a flexible joint in the lower jaw; this allowed the animal to slide its lower jaw back and forth and deliver a grasping bite. This cranial specialization is unusual among the 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.
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 M. 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. A complete Herrerasaurus skull was not found until 1988, by a team of paleontologists led by Paul Sereno. Based on the new fossils, authors such as Thomas Holtz and Jose 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 was indeed a theropod, it would indicate that theropods, sauropodomorphs, and ornithischians diverged even earlier than herrerasaurids, before the middle Carnian (age of the Ischigualasto Formation), 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 footprints that can be attributed only to a theropod dinosaur, dating from the Ladinian (Middle Triassic) of the Los Rastros Formation in Argentina and predating Herrerasaurus by 3 to 5 million years.
The importance of Herrerasaurus and Eoraptor lies in the fact that their remains allow for directly testing the idea of dinosaurs being a monophyletic group, i.e. all dinosaurs have a common ancestor. The monophyly of dinosaurs was explicitly proposed in the 1970s by Bakker, and nine cranial and about fifty postcranial synapomorphies (common anatomical traits derived from the common ancestor) have been listed. However, an extensive study of Herrerasaurus by Sereno indicates that only one cranial and seven postcranial synapomorphies in Bakker's original list are actually supported while additional synapomorphies were discovered.
Studies suggest that the paleoenvironment of the Ischigualasto Formation was a volcanically active floodplain covered by forests and subject to strong seasonal rainfalls. Vegetation consisted of ferns (Cladophlebis), sphenopsids (horsetails), and giant conifers (Protojuniperoxylon). The plants formed an upland riparian forest. 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 another early dinosaur, Eoraptor, as well as Saurosuchus, a giant land-living meat-eating rauisuchian; Venaticosuchus, an ornithosuchid; and the predatory chiniquodontid cynodonts. Herbivores were much more abundant than carnivores and were represented by rhynchosaurs such as Hyperodapedon (formerly Scaphonyx); aetosaurs; kannemeyeriid dicynodonts such as Ischigualastia, and traversodontids such as Exaeretodon. These non-dinosaurian herbivores were much more abundant than early ornithischian dinosaurs like Pisanosaurus and therefore more likely prey for Herrerasaurus than were the early dinosaurs.
The teeth of Herrerasaurus indicate it was a carnivore; its size indicates it would have preyed upon small and medium-sized animals. It may have fed on other dinosaurs, such as the herbivorous Pisanosaurus. However, since Herrerasaurus lived during an era when other dinosaurs were uncommon, more plentiful prey would have included rhynchosaurs and aetosaurs. Herrerasaurus itself may have been preyed upon by giant rauisuchids like Saurosuchus, as 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. The mineralogical and chemical analysis of these coprolites indicate that the carnivorous animal had the ability to digest bones.
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