HEAD, J. J.; Smithsonian Institution: Ecological and phylogenetic variation in segmentation in snakes

Ecological influences, phylogenetic history, and the evolutionary mechanisms driving segment numbers in snakes are poorly understood, despite recent interest in the paleoecology of snake origins and their early evolution. I employ statistical and morphometric methods to examine the relationship between ecology, phylogeny, and segmentation in all major clades of extant snakes and the controversial pachyophiid snakes from the early Late Cretaceous. Linear regression and structural equation models compare precloacal vertebral and ventral scale counts to ecological variables including body size, locomotory habit, prey capture method, and prey composition. Results indicate that body size and specialized locomotory habits are the most significant predictors of segment numbers, exceptions being fully terrestrial locomotion and gigantism, where increasing body size is decoupled from increasing vertebral and scale counts in several clades. Linear regression of vertebral and scale counts onto patristic distances suggests that changes in segmentation are largely independent of phylogeny. Analysis of pachyophiid taxa indicates that numbers of precloacal vertebrae within this clade are consistent with adaptations to marine or estuarine habits, independent of their phylogenetic status. Finally, I use geometric morphometric analysis of vertebral shape to analyze regionalization within the precloacal column of extant alethinophidian snakes. Comparisons of regions between taxa with different numbers of vertebrae demonstrate a wide range of regional dominance in different taxa, but only weak evidence for terminal addition as a mechanism in the evolution of the snake axial skeleton.