ANGIELCZYK, K.D.*; PARHAM, J.F.; California Academy of Sciences; Joint Genome Institute: Geometric Morphometric Analysis of Plastron Shape in the Western Pond Turtle (Emys marmorata): Implications for Conservation and Paleontology

We present results of a geometric morphometric study of plastron shape in Emys marmorata. We digitized 19 landmarks on the sulci between the plastral scutes using photos of over 700 specimens. After archiving the data, we examined three sources of shape variation, ontogeny, sexual dimorphism, and phylogeography. We also tested whether two fossil shells from the Miocene (~10 mya) and Pleistocene (0.5-1.0 mya) fall within the range of variation displayed by extant members of the species. Multivariate regression shows that size and plastron shape are correlated. Sexual dimorphism also is apparent; males and females display subtle but significant shape differences. To test for a phylogeographic signal, we binned specimens into geographically delimited groups based on four clades recognized in a recent molecular phylogeny of E. marmorata populations. Comparisons between the groups suggest that each differs significantly in plastron shape. Finally, the Pleistocene fossil reliably fell within the extant range of variation, but the Miocene fossil did not, perhaps reflecting postmortem deformation. Our preliminary results show that significant morphologic variation exists that corresponds to molecular variation in E. marmorata. In this sense, our data are an independent line of evidence corroborating the distinct histories of the four mitochondrial clades. If the apparent distinctiveness of the clades withstands further testing, conservation plans for E. marmorata will need to be altered to account for this diversity. Finally, our results suggest that geometric morphometrics may be useful for rigorously comparing fossil E. marmorata specimens to the range of extant morphologies, even in the absence of discrete characters.