An abelisauroid dinosaur with a non-atrophied manus from the Late Cretaceous Pari Aike Formation of southern Patagonia

EZCURRA, MARTIN D.; AGNOLIN, FEDERICO; NOVAS, FERNANDO

ZOOTAXA

MAGNOLIA PRESS

Lugar: Auckland; Año: 2010 p. 1 - 25

1175-5326

We describe the new basal abelisauroid dinosaur Austrocheirus isasii gen. et sp. nov. from the Late Cretaceous Pari AikeFormation of southwestern Patagonia, Argentina. The preserved remains include manual bones, a distal tibia, and somepedal and axial elements. Austrocheirus is differentiated from other basal theropods by the presence of metacarpal IIIwith a dorsoventrally compressed shaft and posteriorly displaced collateral tendon fossae located at the same level of theproximal end of distal condyles, and pedal phalanges with a conspicuous longitudinal crest delimitating the dorsalmargin of the distal collateral tendon fossae. A cladistic analysis recovered the new species as more derived thanCeratosaurus and Berberosaurus, but within a polytomy at the base of Abelisauroidea, an assignment supported by twoabelisauroid synapomorphies: distal end of tibia with a planar vertical scar for the reception of the ascending process ofthe astragalus that occupies most of its anterior surface and is medially bounded by the longitudinally oriented facet; andscar for the reception of the ascending process with a median vertical ridge, which imbeds into a crescentic verticalgroove on the posterior surface of the ascending process of the astragalus forming an interlocking tibiotarsal articulation.Furthermore, Austrocheirus represents the first known medium-sized Late Cretaceous abelisauroid bearing nonatrophiedhands. The evidence provided here suggests that the strong reduction of the forelimb recorded in derivedabelisaurids is not directly correlated with their increased body-size, but it seems to be an evolutionary event exclusive tothis lineage within Ceratosauria.