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Austrocheirus isasii, a new abelisauroid from Argentina

Dear DMLers,
Ezcurra, Agnolin and Novas described a new abelisauroid from
Argentina, Austrocheirus isasii. The paper was published today
(accepted: 7 April 2010, by the way).

Ezcurra, M. D., Agnolin, F. L. & Novas, F. E. 2010. An abelisauroid
dinosaur with a non-atrophied manus from the Late Cretaceous Pari Aike
Formation of southern Patagonia. Zootaxa 2450: 1–25.

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.

Daniel Madzia
web: www.wildprehistory.org
mail: daniel.madzia@gmail.com
skype: danielmadzia