Meeting Abstract
8.1 Monday, Jan. 4 The influence of mechanics on morphological disparity in the evolution of emydid turtle shell shape STAYTON, C. Tristan; Bucknell University tstayton@bucknell.edu
Turtle shells exhibit great morphological diversity in terms of both shape and size. However, previous studies have demonstrated that certain clades of turtles show greater per-taxon variation in shell shape than similar, related clades. One possible explanation for these differences in disparity involves the different functions that the turtle shell performs in different clades. Turtle shell shape undoubtedly influences how well shells perform certain functions (force resistance, moving through fluids, heat exchange, etc…), but few studies exist that quantify this relationship. Here I use geometric morphometrics to quantify shell shape and Finite Element (FE) methods to model the mechanical performance of emydid turtle shells in resisting physical loads and reducing drag. FE models of turtle shells were loaded at points on both the carapace and plastron, and stresses that developed as a result of that loading were quantified and compared between models. As previous studies have found, aquatic turtles generally possessed flatter shells that those found in terrestrial species. Flatter shells usually developed higher stresses for a given load, although in many aquatic species stresses were not exceptionally high. However, flatter shells also showed lower frontal and total wetted areas, implying less drag during swimming. Moreover, the Deirochelyinae, an entirely aquatic clade of emydid turtles, showed less per-taxon morphological disparity than its more ecologically diverse sister clade, the Emydinae. Aquatic emydids, needing to optimize both force resistance and drag reduction, are limited to a narrow range of shell morphologies, while terrestrial species, not requiring low-drag shells, show more disparity in shell shape. However, compromises in shell strength due to shape may be offset by evolutionary changes in shell size.
