A MEGARAPTOR-LIKE THEROPOD (DINOSAURIA, TETANURAE) FROM AUSTRALIA: SUPPORT FOR FAUNAL EXCHANGE BETWEEN EASTERN AND WESTERN GONDWANA IN THE MID-CRETACEOUS

SMITH, N. D.; MAKOVICKY, P. J.; AGNOLIN, F. L.; EZCURRA, M. D.; PAIS, D. F.; SALISBURY, S. W.

Neuquen

Congreso; III Congreso Latinoamericano de Paleontologia de Vertebrados; 2008

Universidad Nacional del Comahue

The fossil record of Australian dinosaurs in general, and theropods in particular, isextremely sparse. We describe an isolated ulna from the Early Cretaceous (late Aptian?earlyAlbian) Eumeralla Formation of Australia that shares unique autapomorphies with the SouthAmerican theropod Megaraptor namunhuaiquii, including: a proximally expanded, blade-likeolecranon process that extends distally as a caudal olecranon crest, and a robust lateraltuberosity that extends distally along the ulnar shaft as a distinct crest.A phylogenetic analysis of basal Theropoda performed on a dataset consisting of 58taxa scored for 353 characters recovers Megaraptor and the Eumeralla material as derivedspinosauroids. Though statistical support for Spinosauroidea and less inclusive clades is low,several derived characters support this placement of Megaraptor, including: a hypertrophiedmanual ungual I-2; a hypertrophied ulnar lateral tuberosity; a manual phalanx I-1 with apronounced ventral groove; a compressed olecranon process; and an expanded, triangularcranial process on the proximal radius. Megaraptor thus represents the youngest knownmember of Spinosauroidea, and increases the overlap between typical ?middle? and ?Late?Cretaceous Gondwanan theropod faunas.The Eumeralla ulna represents the first occurrence of Spinosauroidea in Australia, andthe first Australian non-avian theropod with unquestionable affinities to taxa from otherGondwanan landmasses, suggesting faunal interchange between eastern and westernGondwana during the mid-Cretaceous. This counters claims of Laurasian affinities for EarlyCretaceous Australian dinosaur faunas, and of a geographic/climatic barrier isolating Australiafrom the other Gondwanan continents during this time. The temporal and geographicdistributions of Megaraptor and the Eumeralla ulna are less consistent with traditionalpaleogeographic models for Gondwanan fragmentation, but compatible with models positingconnections between South America and Antarctica in the mid-Cretaceous. The Eumerallamaterial adds a phylogenetically constrained data point from a poorly sampled geographicalarea, which may be valuable to future paleobiogeographic analyses. More completespecimens of Megaraptor and its close allies will be critical to testing the robustness of thesephylogenetic and biogeographic patterns.