The skull and neck of the basal theropodHerrerasaurus ischigualastensis

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Abstract
We describe the skull and neck of Herrerasaurus ischigualastensis from specimens discovered recently in the Upper Triassic Ischigualasto Formation of northwestern Argentina. The skull has a rectangular profile and a transversely narrow snout. Marked supratemporal depressions for jaw adductor musculature on the skull roof and a well-developed, sliding intra-mandibular joint suggest that Herrerasaurus ischigualastensis was an active predator. The neck is relatively slender with prominent epipophyses on all of the cervical vertebrae. Diagnostic features of the species include the semicircular shape of the antorbital fossa and the long, slender posterodorsal process of the dentary. Other features of the skull, such as the loss of the postfrontal and small post-temporal opening, constitute dinosaurian synapomorphies. These synapomorphies, however, remain equivocal at the level of Dinosauria because they cannot be evaluated in immediate dinosaurian outgroups for which cranial information is lacking. Comparisons of the skull and neck of Herrerasaurus ischigualastensis to other basal dinosaurs reveals several derived similarities with saurischians and, in particular, theropods. These synapomorphies include a subnarial foramen (between premaxilla and maxilla) shared with saurischians and an intra-mandibular joint shared with theropods. The cervical and cranial anatomy of Herrerasaurus ischigualastensis, however, is remarkably primitive compared to that in other basal dinosaurs.