Plioplatecarpus primaevus (Mosasauridae) from the Bearpaw Formation (Campanian, Upper Cretaceous) of the North American western interior seaway

One almost complete mosasaur skeleton and much additional material from the Upper Cretaceous Bear-paw Formation of South Central Saskatchewan permits the first adequate description and diagnosis of Plioplatecarpus primaevus Russell. It possesses the following unique (within mosasaurs) characters: 11 maxillary teeth; a large shield shaped septomaxilla forming a median internarial septum; a posterodorsally directed iliac process; and no obturator foramen. It can be distinguished from Plioplatecarpus marshi in the possession of a modest sized coracoid foramen, and both P. houzeaui and P. marshi in the possession of an unreduced presacral column of 30 vertebrae, all bearing functional zygapophyses, indicating that P. primaevus is the primitive sistergroup to these two taxa. Although extremely similar to UNO 8611-2, an unnamed specimen from Alabama, the lack of an eminence on the posterior surface of the quadrate shaft in the latter establishes UNO 8611-2 as the primitive sister group of P. primaevus + [P. houzeaui + P. marshi]. A quadrate eminence, long considered to be diagnostic of the genus, shows considerable variability in size and shape in P. primaevus, suggesting that it is not suitable as a specific diagnostic character. Other species of Plioplatecarpus (P. crassartus, P. depressus, ''Platecarpus'' somenensis, ''Mosasaurus'' scanicus, and undescribed species from Scabby Butte, Alberta and Anderson River, North West Territories), have not been sufficiently characterized to include in a phylogenetic analysis.