[N] 2006 An Elasmosaurid Plesiosaur from Japan
Sato, T., Bryant, H., Hasegawa, Y. & Manabe, M. (2006) An Elasmosaurid Plesiosaur from Japan and its phylogentic relationships. JVP 26(3) Abstracts pp.120
new genus and species of elasmosaurid plesiosaur is recently described based on a partial skeleton from the Lower Santonian (Upper Cretaceous) Tamayama Formation, Futaba Group, in Fukushima Prefecture, Japan. Indeterminate elasmosaurids are common in the Upper Cretaceous of the circum-Pacific region, but the holotype of the Japanese taxon is the first elasmosaurid diagnosable at the species level from the Western Pacific, and the oldest in the Northern Pacific.
Phylogenetic relationships of the new taxon and other elasmosaurids were analyzed using 176 characters coded for 33 operational taxonomic units (OTU). More than a half of the OTUs have more than 50% missing data, but there were only two MPTs. CI and RI were low in comparison with those in previous studies of plesiosaurian phylogeny, indicating a high level of homoplasy. In the strict consensus, all Cretaceous elasmosaurids except for Brancasaurus formed a clade, and the Japanese taxon appeared as a basal member of this clade, within which most species from the Upper Cretaceous of the North American Western Interior Seaway formed a subclade. Microcleidus and a clade of cryptocleidoids were successive outgroups of the large elasmosaurid clade.
Bootstrap support for these clades was very low (less than 50%), and the relationship within the elasmosaurid clade is very sensitive to the selection of OTUs and characters. For example, in a re-analysis in which a small number of OTUs represented only by juvenile specimens were removed, although most Cretaceous elasmosaurids still formed a clade, relationships within the clade changed drastically. Poorly known elasmosaurids from the Southern Hemisphere such as Tuarangisaurus and Aristonectes appeared outside of this clade. The subclade of Western Interior elasmosaurids was recovered but its internal relationships were different from the first analysis, and the Japanese taxon was more closely related to this subclade than in the first analysis.