Description of plesiosaurs (reptillia: sauropterygia) from the Bearpaw Formation (campanian - maastrichtian) and a phylogenetic analysis of the elasmosauridae
Elasmosaurid and polycotylid plesiosaurs are known from the Upper Cretaceous (Campanian-Maastrichtian) Bearpaw Formation in Saskatchewan, Alberta, and Montana, including Terminonatator ponteixensis Sato and Trinacromerum herschelensis n. sp. The holotype of T ponteixensis is from Ponteix, Saskatchewan, and consists of a considerable portion of the skeleton, including the skull with braincase. The holotype of T herschelensis is a completely disarticulated skeleton from Herschel, Saskatchewan, but each element was preserved with minimal distortion. Because of the historical confusion in polycotylid taxonomy, assignment of T herschelensis as a species of Trinacromerum should be taken as provisional. Molluscan fossils and palynomorphs from the Ponteix site indicate that the holotype of T ponteixensis is from B. cuneatus to B. reesidei Zones, approximately 72 to 73 million years ago. Palynomorphs from the Herschel site indicate that the holotype of T herschelensis is from the Upper but not the uppermost Campanian. The Bearpaw plesiosaurs represent a plesiosaur fauna near the end of the extensive marine phase in the Western Interior. Plesiosaurs contemporaneous with, or later than, the Bearpaw age are known from Alberta, Arctic Canada, Texas, and Pacific and Atlantic Coasts. Phylogenetic relationships of elasmosaurid plesiosaurs are analyzed based on 215 osteological characters and 34 terminal units. Three kinds of character subsets were analyzed to evaluate effects of the deletion of quantitative characters and potentially non-phylogenetic features, and three kinds of taxon subsets were analyzed to see the effects of combination of specimens and missing data. In general, analyses of all of morphological characters and deletion of taxa with abundant missing data result in low number of most parsimonious trees (MPTs). Deletion of quantitative characters causes the loss of resolution particularly among elasmosaurids, and deletion of taxa with abundant missing data affects the number of MPTs and tree topology. The traditionally defined Elasmosauridae never formed a monophyletic group in any of the nine analyses, whereas the phylogenetically defined Elasmosauridae appeared as more inclusive than the original sense. Based on the regulations set by PhyloCode, six new clade names are defined, including a proposal for the revision of the definition of the Elasmosauridae.
Bibliography: p. 153-177
Sato, T. (2002). Description of plesiosaurs (reptillia: sauropterygia) from the Bearpaw Formation (campanian - maastrichtian) and a phylogenetic analysis of the elasmosauridae (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca. doi:10.11575/PRISM/12674