New information on the systematics and postcranial skeleton of Herrerasaurus ischigualastensis(Theropoda: Herrerasauridae) from the Ischigualasto Formation (Upper Triassic) of Argentina

Abstract

Herrerasauridae from the Ischigualasto Formation (San Juan Province, Argentina) included Herrerasaurus ischigualastensis Reig, 1963, Ischisaurus cattoi Reig, 1963, and Frenguellisaurus ischigualastensis Novas, 1986. Review of type and referred specimens of those taxa suggests that Ischisaurus and Frenguellisaurus are junior synonyms of Herrerasaurus. At present, Herrerasaurus is the only Herrerasauridae documented in the Ischigualasto Formation, and the record of a cf. Staurikosaurus in those beds is here dismissed. Cladistic analysis counters previous hypotheses supporting the outgroup position of Herrerasauridae with respect to the remaining dinosaurs. Instead, new evidence supports this group as saurischian dinosaurs: the posterior process of the jugal is forked, a hyposphene-hypantrum articulation is present in dorsal vertebrae, the distal shaft of the ischium is rod-like, and six other saurischian synapomorphies are present. Moreover, herrerasaurids share with other theropods 11 synapomorphies including prong-shaped epipophyses on the cervical vertebrae, elongate prezygapophyses in distal caudals, humerus nearly 50% of femoral length, strongly reduced metacarpals IV and V, and distally enlarged pubis. In the context of the hypothesis accepted here, the presence of two sacral vertebrae and absence of a brevis fossa on the ilium in the Herrerasauridae are interpreted as apomorphic reversals within Dinosauria.