The Anatomy and Phylogeny of a New Large Plioplatecarpine Mosasaur From the Campanian Bearpaw Shale of Montana (USA)

In 2018, a large and associated plioplatecarpine mosasaur skull, pectoral girdle, and rib cage, whose total body length may have exceeded five meters, was uncovered in the Late Campanian Bearpaw Shale of Northeast Montana (USA). Phylogenetic analysis of this specimen, MOR 10855, recovers this individual as a basal member of the genus Plioplatecarpus. This specimen, is unique in that it is estimated to be nearly twice the size of any of the other species of Plioplatecarpus found in the Western Interior Seaway during this part of the Cretaceous. While the included phylogenetic study suggests MOR 10855 represents a new species within the genus Plioplatecarpus, as supported by the presence of diagnostic features in the frontal, quadrate, and scapula, a poor understanding of individual variation and ontogeny in the related species prevents the confident assignment of a new taxon at this time. In addition to the exceptionally well-preserved skull and anterior skeleton, this specimen also preserves two bite marks on the skull possibly inflicted by a similarly sized mosasaur. These bites exhibit evidence of healing, suggesting one of the first documented cases of non-lethal face biting in a plioplatecarpine mosasaur. The size disparity between this Plioplatecarpus specimen and other species of Plioplatecarpus known from the Bearpaw Shale (P. peckensis and P. primaevus) is reminiscent of size variance observed in some extant cetacean clades, including physeteroids and delphinids. The discovery of MOR 10855 suggests that this portion of the Western Interior Seaway was exceptionally productive during this part of the Cretaceous, and that ecological niche partitioning among the resident mosasaurs was probable