Herrerasaurus

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 rocks of early Carnian age (late Triassic, around 228 million years ago) in northwestern Argentina. The type species, Herrerasaurus ischigualastensis, was described by Osvaldo Reig in 1963[1] and is the only species assigned to the genus. The names Ischisaurus and Frenguellisaurus are synonymous 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 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,[2][3] Herrerasaurus has been classified as either an early theropod or an early saurischian in at least five recent reviews of theropod evolution. Herrerasaurus is a medium-sized bipedal member of the Herrerasauridae, a group of similar animals which were among the earliest of the dinosaurian evolutionary radiation.

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),[3] and its hip height at more than 1.1 meters (3.3 ft).[4] It may have weighed around 210–350 kilograms (463–772 lb).[4] In a large specimen at first thought to belong to a separate genus, Frenguellisaurus, the skull measured 56 centimeters (1.8 ft) in length.[4] Smaller specimens had skulls which measured around 30 centimeters (1 ft) in length.

Skull
Herrerasaurus had a long, narrow skull that lacked nearly all the specializations that characterized later dinosaurs,[6] 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, 1-centimeter-long (0.4 in) slit-like holes called promaxillary fenestrae.[7] 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.[6] This cranial specialization is unusual among the dinosaurs but has evolved independently in some lizards.[8] 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
Herrerasaurus had relatively short forelimbs, which were less than half the length of its hind limbs. The upper arm and forearm were rather short, while the manus was elongated. The first two fingers and the thumb bore curved, sharp claws for grasping prey. Its fourth and fifth digits were small stubs without claws.[3][10]Herrerasaurus was bipedal. It had strong hind limbs with short thighs and rather long feet, indicating this animal was most likely a swift runner. The balancing tail, partially stiffened by overlapping vertebral processes, also indicates an adaptation for speed.[5]

Unlike most reptiles of its era, Herrerasaurus was fully bipedal. It had strong hind limbs with short thighs 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 (where the femur meets the pelvis) that was only partially open. The ilium, the main hip bone, was supported only by two sacrals, a basal trait,[5] but the pubis pointed backwards, a derived trait that parallels what is seen in dromaeosaurids and birds. Additionally, the end of the pubis had a booted shape, similar to what is present in avetheropods, and the vertebral centra had an Allosaurus-like hourglass shape.

Classification
Herrerasaurus gives its name to its family, Herrerasauridae, of the mid- to late Triassic, although where it and its close relatives lie on the early dinosaur evolutionary tree is unclear. They are possibly basal theropods or basal saurischians but may in fact predate the saurischian-ornithischian split.[12] The situation is further complicated by uncertainties in correlating the ages of late Triassic beds bearing land animals.[5] Other members of the clade may include Eoraptor from the same Ischigualasto Formation of Argentina as Herrerasaurus, Staurikosaurus from the Santa Maria Formation of southern Brazil,[13] Chindesaurus from the Upper Petrified Forest (Chinle Formation) of Arizona,[14] and possibly Caseosaurus from the Dockum Formation of Texas,[15] although the relationships of these animals are not fully understood, and not all paleontologists agree. Other possible basal theropods, Alwalkeria from the Late Triassic Maleri Formation of India,[16] and Teyuwasu, known from very fragmentary remains from the Late Triassic of Brazil, might be related.[17] Novas (1992) defined Herrerasauridae as Herrerasaurus, Staurikosaurus, and their most recent common ancestor.[18] Sereno (1998) defined the group as the most inclusive clade including H. ischigualastensis but not Passer domesticus.[19] Langer (2004) created a higher level taxon, infraorder Herrerasauria.