Taxonomic utility of Early Cretaceous Australian plesiosaurian vertebrae

Abstract

Despite their broad occurrence in Australian basins, lower-level taxonomy of Australian Early Cretaceous plesiosaurs is hindered by the scarcity of skulls and other diagnostic elements. This paper presents a morphological and morphometric analysis of the vertebrae of Australian plesiosaurs and a comparison with selected non-Australian elasmosaurids (basal elasmosaurs, styxosaurines, aristonectines). Biometric analysis involved measurements that include centrum height (CH), centrum width (CW), centrum length (CL), width of zygapophyses, and others. Principal component analysis (PCA) and multivariate analysis of variance (MANOVA) were carried out on CH, CW, and CL. Morphometric ratios [Vertebral Length Index (VLI), Height Index (HI), and Breadth Index (BI)] were plotted along the length of the vertebral column and on bivariate graphs where the analysis is independent of vertebral position and correct sequencing. Direct comparison of homologous groups of vertebrae (especially anterior cervicals) easily distinguished an Australian polycotylid from all elasmosaurids in all plots. Most Australian elasmosaurid specimens plotted near basal 'Cimoliasaurus'grade elasmosaurs, where they clustered as a group relatively distinct from, but overlapping coeval non-Australian forms. However, one specimen plotted consistently with the non-Australian styxosaurines and Opallionectes andamookaensis plotted with the aristonectines. Although it is not possible to distinguish phylogenetic relationships from morphological convergence in the plotted positions, the analyses provide potential hypotheses to be tested with additional data. Hence, vertebrae have some taxonomic value for potential assignment of Australian plesiosaurs, even in the absence of skulls or other diagnostic material.