Herrerasaurus - Classification

Classification

Herrerasaurus gives its name to its family, Herrerasauridae, a group of similar animals from the Late Triassic which were among the earliest of the dinosaurian evolutionary radiation. Where it and its close relatives lie on the early dinosaur evolutionary tree is unclear. Most 21st-century analyses have found them to be basal theropods, though they may in fact represent basal saurischians or even be non-dinosaurian, predating the saurischian-ornithischian split. Most analyses, such as those by Sterling Nesbitt and colleagues (2009, 2011), have found Herrerasaurus and its relatives to be very basal theropods. Others (such as Ezcurra 2010) have found them to be basal to the clade Eusaurischia, that is, closer to the base of the saurischian tree than either true theropods or sauropodomorphs, but not members of either group. The situation is further complicated by uncertainties in correlating the ages of late Triassic beds bearing land animals.

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, Chindesaurus from the Upper Petrified Forest (Chinle Formation) of Arizona, and possibly Caseosaurus from the Dockum Formation of 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 Maleri Formation of India, and Teyuwasu, known from very fragmentary remains from the Late Triassic of Brazil, might be related. Novas (1992) defined Herrerasauridae as Herrerasaurus, Staurikosaurus, and their most recent common ancestor. Sereno (1998) defined the group as the most inclusive clade including H. ischigualastensis but not Passer domesticus. Langer (2004) provided first phylogenetic definition of a higher level taxon, infraorder Herrerasauria.