Temporal range: Late Triassic
Herrerasauria, meaning 'Herrera's lizard', is one of the oldest recorded dinosaur genus in the history of the species. Herrerasaurs appeared in the fossil record as early as 228 million years ago, in the mid to late Triassic Period. The species went extinct at the conclusion of the Triassic Period.
Herrerasaur physiology resembled the later-appearing theropods, who may in fact be the early dinosaurs' closest relatives. However, as opposed to the larger theropods, hererrasaurs were small to medium-sized carnivores. The best known representative of the group is the namesake Herrerasaurus ischigulastenis, which had a nearly complete skeleteon discovered in 1988 in San Juan, Argentina. The genus itself was first discovered in 1960s. Fossil remains have also been discovered in North America, indicating the creatures inhabited other continents as well prior to the continental shift.
Herrerasaurs have a unique physiology and anatomy compared to other genus of dinosaurs, and are not generally considered to be an ancestral species to any later dinosaur group. The herrerasaurid structure represents a mixture of primative and derived traits, indicative of its place in the early history of the dinosaurs. The acetabulum is only partly open, and the creature only has two sacral vertebrae, which is the lowest among all discovered dinosaurs. The hand structure of the creature is also of primitive design, in that herrerasaurs have five metacarpals and a third finger longer than the second. However, like the later theropods, the herrerasaurs have only hree long fingers, with curved claws. Herrerasaurs also have a hinged mandible like theropods, but this may have evolved independently.