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

We review here the phylogenetic relationships of a theropod distal end of tibia (MB. R.2351) from the Middle Jurassic Stonesfield Slate (Taunton Formation, middle Bathonian) of Oxfordshire, England. This specimen was previously described as a small basal tetanuran, but our reinterpretation suggests that it was an early member of the Abelisauroidea. The new assignment is supported by the presence of an apomorphic vertical facet for the reception of the ascending process of the astragalus, sub-rectangular anterior scar of the astragalar ascending process, median vertical ridge in the scar for the reception of the ascending process of the astragalus, and posterolateral process not distinctly offset from the lateral margin of the shaft. In particular, the Stonesfield specimen shares an overall morphology and a unique combination 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 an abelisauroid 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 earlier than previously thought. Accordingly, the Middle and Late Jurassic Pangean distribution of abelisauroids implies that the absence of the group in Cretaceous Asiamerican assemblages would reflect a regional extinction, in which a competitive replacement with coelurosaurs (e.g. tyrannosauroids) is surely one of the hypotheses that should be tested in future studies.