PHYLOGENY CONSTRAINS THE HYDRODYNAMICS OF PLESIOSAUR PROPODIALS

Plesiosaurs are a diverse, globally distributed group of Mesozoic marine reptiles characterized by large, sub-equal fore- and hind flippers. Although previous studies have linked flipper planform shape and proportion to specific ecomorphotypes, it is unclear if the cross-sectional shape follows the same trend. Plesiosauromorphs, those with relatively long necks and small heads, tend to have drag-reducing features in their flippers such as higher aspect ratio while pliosauromorphs, those with relatively short necks and large heads, tend to have features associated with greater maneuverability such as lower aspect ratio. Since air- and hydrofoil cross-sectional shape is strongly related to performance, we hypothesized that drag-reducing features like camber are more pronounced in plesiosauromorphs than pliosauromorphs. We compared the distal cross-sectional shape of the propodial (humerus and femur) from diverse subclades including Elasmosauridae, Polycotylidae, Pliosauridae, and Rhomaleosauridae. Elasmosauridae is a plesiosauromorph while the other three are convergent pliosauromorphs including Polycotylidae, which is phylogenetically more closely related to Elasmosauridae. We used semi-landmarks to capture the shape and performed canonical variate analysis (CVA) on the data. Our preliminary results failed to show a statistically significant difference between the morphotypes or between limb types. However, there is a statistically significant difference across clades (p < 0.03), suggesting that the propodial cross-sectional shape is broadly more constrained by phylogeny or swimming efficiency in general and overshadows more subtle differences due to ecomorphology.