THE EARLY EVOLUTION OF BIRD-LINE ARCHOSAURS: A POSSIBLE NEW CLADE OF GLOBALLY DISTRIBUTED AVEMETATARSALIANS JUST OUTSIDE THE DINOSAUR-PTEROSAUR SPLIT

NESBITT, S. J.; BUTLER, R. J.; BARRETT, PAUL M.; STOCKER, MICHELLE R.; SIDOR, CHRISTIAN A.; ANGIELCZYK, KENNETH D.; EZCURRA, M. D.; SMITH, ROGER M. H.

Congreso; SVP anual meeting 2016; 2016

Bird-line archosaurs (=Avemetatarsalia, the clade containing dinosaurs, pterosaurs,and their kin) had their origin in the Triassic Period. However, that origin is poorlydocumented as fossils from their early evolutionary history are extremely rare and consistmostly of postcrania. Here, we report the discovery of a new reptile (femoral length = 17cm) from the lower portion of the Middle Triassic Lifua Member (Manda beds) of theRuhuhu Basin, southwestern Tanzania. Material referred to the new taxon includes apartial skeleton of a single individual including cervical, trunk, and caudal vertebrae,pectoral, pelvic, forelimb, and hind limb material (= ?Teleocrater? of A. Charig), andparts (skull elements, vertebrae, pectoral, pelvic, and limb elements) of a minimum ofthree individuals collected from a bonebed discovered in 2015 very close to Charig'soriginal partial skeleton. Character states of the limbs, vertebrae, and ilium indicate aclose relationship with early dinosauromorphs including: elongated cervical vertebrae, anilium with a slightly concave ischial peduncle and clear anterior crest, a weaklydeveloped anterior trochanter of the femur, an anteriorly compressed fibula with longstrap-like iliofibularis crest, and absence of osteoderms. Many character states suggestthat the new reptile taxon falls outside of the pterosaur-dinosaur clade (=Ornithodira).However, the distributions of some of these character states at the base of Archosauriaare unclear and some character states of the new taxon suggest a more basal relationshipoutside Archosauria (e.g., absence of two medial tubera of the proximal femur). Nomatter the position within or outside Archosauria, the new Lifua taxon shares seeminglyunique character states with the poorly known Dongusuchus from the Middle Triassic ofRussia (known from femora) and Yarasuchus from the Middle Triassic of India (knownfrom partial skeletons), rather than with other archosauriforms. As a result, these formsappear to represent a globally distributed clade of early diverging avemetatarsalians. Thelarger body size of the Manda form and its potential phylogenetic position outside ofpterosaurs and dinosauromorphs indicates that there was a size decrease at the origin ofOrnithodira. This new taxon, and other new discoveries from the Middle to Late Triassic,are elucidating the sequence of character acquisitions in Avemetatarsalia and fill a crucialgap in the evolutionary history that led to the flourishing of dinosaurs later in theMesozoic.