Braincase of a polycotylid plesiosaur (Reptilia: Sauropterygia) from the Upper Cretaceous of Manitoba, Canada

The braincase of a polycotylid plesiosaur referable to Dolichorhynchops from the Niobrara Formation (Santonian to the earliest Campanian) in Manitoba, Canada, is described. The well-preserved material without matrix permits observation of various anatomical features of the polycotylid braincase with unprecedented clarity. A virtual reconstruction of the braincase was created by using a three-dimensional scanning technique that produces accurate reconstruction of missing or damaged elements and demonstrates the three-dimensional relationships among the elements. Comparison with other plesiosaurian braincases revealed previously unrecognized characters and character states. The I-beam-shaped parasphenoid, the relatively short pituitary fossa, and the anterior process of the prootic are possible synapomorphies of the Polycotylidae or of its subclades. It could be a challenge to confirm the presence of the supraoccipital process and its median ridge, as well as the presence of the fenestra ovalis, in polycotylid skulls because of the location and orientation of these structures. Variable morphology of basioccipital-basisphenoid contact and the paroccipital process in Dolichorhynchops and related taxa require further investigation. The I-beam of the parasphenoid and the wide contact surfaces of the basioccipital and neighboring elements provide more resistance to flexion of the braincase. Distribution of the extensive pterygoid-basicranium contact, the shape of foramen magnum, anterior process of the prootic, and the short pituitary fossa do not confirm the current phylogenetic hypotheses. The location and orientation of the fenestra ovalis suggest the lack of a tympanic ear in this plesiosaur.