TAXONOMY AND ONTOGENY OF PROTEROSUCHIDAE (DIAPSIDA: ARCHOSAUROMORPHA): IMPLICATIONS FOR THE TEMPO AND MODE OF EARLY ARCHOSAURIFORM EVOLUTION

EZCURRA, M. D.; BUTLER, R. J.

Berlin

Congreso; 74th Annual Meeting of the Society of Vertebrate Paleontology; 2014

Archosauriforms include archosaurs and several Permo-Triassic diapsid groups, suchas proterosuchids, erythrosuchids, proterochampsids, euparkeriids and doswelliids. Thephylogenetic position of Proterosuchidae as the earliest-branching group ofarchosauriforms means that it is a critically important clade for understanding thesuccessful evolutionary radiation of this group during the Mesozoic. Proterosuchids arebest sampled from the earliest Triassic of South Africa, with more than 20 specimensknown. These specimens form the basis for four nominal species: Proterosuchus fergusi,Chasmatosaurus vanhoepeni, Chasmatosaurus alexanderi and Elaphrosuchus rubidgei.However, interpretations of the taxonomy of the South African proterosuchids have beencomplex and widely divergent, and P. fergusi is currently considered the only validspecies. Ongoing discovery of well-preserved specimens continues to improve samplingof South African proterosuchids and provides an impetus to revisit their taxonomy. Basedupon a comprehensive re-examination of proterosuchid specimens in collectionsworldwide we conclude that the holotype of 'P. fergusi' is undiagnostic. Qualitative andquantitative analyses indicate that C. vanhoepeni and C. alexanderi are valid species, andwe recognize a third new species of Chasmatosaurus based on a single, previouslydescribed specimen (NMQR 880). Using this new taxonomy we qualitatively andquantitatively analyzed the cranial ontogenetic development of C. vanhoepeni based on13 specimens. Through ontogeny, the skull of C. vanhoepeni became proportionallytaller, the infratemporal fenestra larger, and the teeth more numerous with apicobasallyshorter crowns. The elongated snout and enlarged premaxilla that are characteristic ofproterosuchids underwent isometric growth. Our results indicate that the species richnessof earliest Triassic archosauriforms is greater than previously appreciated, butarchosauriform morphological disparity remained low and did not expand until the lateOlenekian-early Anisian. The overall cranial morphology of juvenile proterosuchidsresembles that of the non-archosauriform archosauromorph Prolacerta, and adultspecimens acquire a more erythrosuchid-like morphology. Accordingly, peramorphicevents may have driven the early evolution of the archosauriform skull. This newinformation provides a basis for the future identification of more punctual heterochronicevents during the early evolution of Archosauriformes.