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

Cleveland

Congreso; 68th Annual Meeting of the Society of Vertebrate Paleontology; 2008

The fossil record of Australian dinosaurs in general, and theropods in particular, is extremelysparse. We describe an isolated ulna from the Early Cretaceous (late Aptian?early Albian)Eumeralla Formation of Australia that shares unique autapomorphies with the SouthAmerican theropod Megaraptor, including: an expanded, blade-like olecranon process thatextends distally as a caudal olecranon crest, and a robust lateral tuberosity that extends asa distinct lateral crest. A phylogenetic analysis of basal Theropoda performed on a datasetconsisting of 58 taxa scored for 353 characters recovers Megaraptor and the Eumerallamaterial as derived spinosauroids. Though statistical support for Spinosauroidea and lessinclusive clades is low, several derived characters support this placement of Megaraptor,including: a hypertrophied manual ungual I-2; a hypertrophied ulnar lateral tuberosity; amanual phalanx I-1 with a pronounced ventral groove; a compressed olecranon process; andan expanded, triangular cranial process on the proximal radius. Megaraptor thus representsthe youngest known member of Spinosauroidea, and increases the overlap betweentypical ?middle? and ?Late? Cretaceous Gondwanan theropod faunas. The Eumeralla ulnarepresents the first occurrence of Spinosauroidea in Australia, and the first Australian nonaviantheropod with unquestionable affinities to taxa from other Gondwanan landmasses,suggesting faunal interchange between eastern and western Gondwana during the mid-Cretaceous. This evidence counters claims of Laurasian affinities for Early CretaceousAustralian dinosaur faunas, and of a geographic/climatic barrier isolating Australia from theother Gondwanan continents during this time. The temporal and geographic distributionsof Megaraptor and the Eumeralla ulna are less consistent with traditional paleogeographicmodels for Gondwanan fragmentation, but compatible with models positing connectionsbetween South America and Antarctica in the mid-Cretaceous. Accordingly, the Eumerallamaterial adds a phylogenetically constrained data point from a poorly sampled geographicalarea, which may be valuable to future paleobiogeographic analyses.