An abelisauroid dinosaur from the Middle Jurassic of Laurasia and its implications on theropod palaeobiogeography and evolution

EZCURRA, MARTIN D.; AGNOLIN, FEDERICO

PROCEEDINGS OF THE GEOLOGISTS ASSOCIATION

GEOLOGICAL SOC PUBL HOUSE

Lugar: Londres; Año: 2012 p. 1 - 8

0016-7878

We review here the phylogenetic relationships of a theropod distal end of tibia (MB. R.2351) from theMiddle Jurassic Stonesfield Slate (Taunton Formation, middle Bathonian) of Oxfordshire, England. Thisspecimen was previously described as a small basal tetanuran, but our reinterpretation suggests that itwas an early member of the Abelisauroidea. The new assignment is supported by the presence of anapomorphic vertical facet for the reception of the ascending process of the astragalus, sub-rectangularanterior scar of the astragalar ascending process, median vertical ridge in the scar for the reception of theascending process of the astragalus, and posterolateral process not distinctly offset from the lateralmargin of the shaft. In particular, the Stonesfield specimen shares an overall morphology and a uniquecombination of apomorphies with the Middle Jurassic Australian abelisauroid Ozraptor. Nevertheless,both specimens differ in some punctual features. MB. R.2351 constitutes the oldest evidence of anabelisauroid outside Gondwana and indicates that the group had achieved a Pangean distribution during,at least, the Middle Jurassic. Thus, the initial diversification of abelisauroids would have occurred earlierthan previously thought. Accordingly, the Middle and Late Jurassic Pangean distribution of abelisauroidsimplies that the absence of the group in Cretaceous Asiamerican assemblages would reflect a regionalextinction, in which a competitive replacement with coelurosaurs (e.g. tyrannosauroids) is surely one ofthe hypotheses that should be tested in future studies.