Effects of Sutured Pelvic Elements on Turtle Shell Strength: A Comparison of Pleurodire and Cryptodire Shell Mechanics

The two extant clades of turtles, Pleurodira and Cryptodira, can be distinguished by the fact that the pelvic elements of pleurodire turtles are extensively sutured to both the carapace and plastron whereas the pelvic elements of cryptodire turtles are not. Pleurodire shells are also generally flatter than those of cryptodires. These distinctive features could potentially contribute to differences in the strength of the shell in these two clades, but quantitative assessments are lacking. We asked the following questions: How do the sutured pelvic elements of Pelomedusa subrufa (Pleurodira) affect the strength of the turtle's shell? Does the height of the turtle shell affect shell strength? Are there differences in the strength of the shells of pleurodires and cryptodires living in the same geographic regions? We developed finite element models of pleurodire and cryptodire shells and used them to assess the effects of various morphologies and loading regimes on shell stress. The pleurodire model without pelvic elements had higher maximum and average stresses for all load cases, and thus lower strength, than the model with pelvic elements, showing that sutured pelvic elements increase the strength of the shell. Increases in shell height increased shell strength. No differences in strength were found between pleurodire and cryptodire species living in the same geographic regions, indicating that the two clades might be evolving in response to the same selective pressures or requirements, but doing so in different ways. Overall, pleurodires and cryptodires are both able to increase the strength of their shells, but the morphological characteristics that they use to do so differ between the clades.