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

ABSTRACT

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.

ACKNOWLEDGMENTS

We thank CMN for technical assistance, especially M. Gilbert who prepared the scientific illustrations in Figures 3 and 4. K. Seymour of ROM facilitated our research and kindly granted the loan of the material to CMN. Canadian Fossil Discovery Centre in Mordern provided assistance with our fieldwork. Data on comparative specimens have been accumulated during past museum visits in North America and Europe by T.S., and she thanks the research and collection staff of these institutions, such as KUVP, MCZ, UCB, Royal Saskatchewan Museum in Regina, and United States National Museum, to mention a few. Thorough reviews and comments by F. R. O’Keefe, B. Schumacher, K. Carpenter, and R. Day improved the manuscript considerably. This work was supported by KAKENHI (Grant-in-Aid for Young Scientists “A,” 20684023) to T.S., and CMN research grants to X.C.W.

Handling editor: Robin O'Keefe.