Austrocheirus isasii - new Late Cretaceous abelisauroid

[Tim Williams <tijawi@yahoo.com>]

Ezcurra, MD, Agnolin, FL, and Novas, FE (2010) An abelisauroid dinosaur with a 
non-atrophied manus from the Late Cretaceous Pari Aike Formation of southern 
Patagonia.  Zootaxa 2450: 1â25

http://www.mapress.com/zootaxa/2010/2/zt02450p025.pdf

Abstract:

"We describe the new basal abelisauroid dinosaur _Austrocheirus isasii_ gen. et 
sp. nov. from the Late Cretaceous Pari Aike Formation of southwestern 
Patagonia, Argentina. The preserved remains include manual bones, a distal 
tibia, and some pedal and axial elements. _Austrocheirus_ is differentiated 
from other basal theropods by the presence of metacarpal III with a 
dorsoventrally compressed shaft and posteriorly displaced collateral tendon 
fossae located at the same level of the proximal end of distal condyles, and 
pedal phalanges with a conspicuous longitudinal crest delimitating the dorsal 
margin of the distal collateral tendon fossae. A cladistic analysis recovered 
the new species as more derived than _Ceratosaurus_ and _Berberosaurus_, but 
within a polytomy at the base of Abelisauroidea, an assignment supported by two 
abelisauroid synapomorphies: distal end of tibia with a planar vertical scar 
for the reception of the ascending process of the
 astragalus that occupies most of its anterior surface and is medially bounded 
by the longitudinally oriented facet; and scar for the reception of the 
ascending process with a median vertical ridge, which imbeds into a crescentic 
vertical groove 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 nonatrophied hands. The evidence provided here suggests 
that the strong reduction of the forelimb recorded in derived abelisaurids is 
not directly correlated with their increased body-size, but it seems to be an 
evolutionary event exclusive to this lineage within Ceratosauria."