Side-Necked Versus Hidden-Necked: A Comparison of Shell Morphology Between Pleurodiran and Cryptodiran Turtles

Abstract: This study explores the potential morphological differences between pleurodiran and cryptodiran turtle shells. Pleurodires (Suborder Pleurodira), as suggested by their name, retract their heads into the shell by bending the neck to the side; this is different from cryptodires (Suborder Cryptodira), which withdraw their necks along a sagittal plane. Pleurodires also have evolved a unique pelvic morphology typified by two columnar pelvic elements sutured to both the carapace and plastron; in contrast, the pelvic girdle in cryptodires is loosely connected to the shell. These differences might put very different selective pressures on shell shape in these animals: The different head retraction strategies in the two groups could lead pleurodires to evolve wider and flatter anterior apertures while the differences in pelvic structure might permit the evolution of flatter carapaces (particularly in the posterior carapace) and narrower bridges as compared to cryptodires. We used 3D landmark data to characterize shell shape and used phylomorphospace plots to examine patterns of diversification within pleurodires. We also used analyses of variance to examine morphological differences between the shell shapes of cryptodire and pleurodire turtles. We found significant phylogenetic signal in pleurodire shell shape. Clustering of pleurodires in shape space primarily followed phylogeny, with all major clades occupying distinct regions of shell shape space. Pleurodires overlapped most strongly in shell shape with aquatic cryptodires, both swimmers (including sea turtles) and bottom-walkers (such as chelydrids). Future studies could seek additional support for the hypotheses, through mechanical tests of shell performance, that the mechanism of neck retraction or the presence of columnar pelvic girdles can influence the functional performance of turtle shells.