On the cranial anatomy of the polycotylid plesiosaurs, including new material of Polycotylus latipinnis, Cope, from Alabama

Abstract

ABSTRACT The cranial anatomy of plesiosaurs in the family Polycotylidae (Reptilia: Sauropterygia) has received renewed attention recently because various skull characters are thought to indicate plesiosauroid, rather than pliosauroid, affinities for this family. New data on the cranial anatomy of polycotylid plesiosaurs is presented, and is shown to compare closely to the structure of cryptocleidoid plesiosaurs. The morphology of known polycotylid taxa is reported and discussed, and a preliminary phylogenetic analysis is used to establish ingroup relationships of the Cryptocleidoidea. This study also presents new material referable to Polycotylus latipinnis from the Mooreville Chalk Formation of Alabama. This skeleton is largely complete, and although the skull is fragmentary it does preserve several regions not previously represented. The preserved portions demonstrate that the palate of Polycotylus is similar to that of Trina-cromerum in the possession of robust epipterygoids, and similar to that of all other polycotylids in the possession of a large anterior interpterygoid vacuity. Polycotylus therefore displays the derived palate morphology common to all polycotylids, but also displays some plesiomorphic features. This impression is confirmed by the results of the cladistic analysis.