Fossorial Origin of the Turtle Shell

• Published in: Current biology 2016-07, Vol.26 (14), p.1887-1894
• Main Authors: Lyson, Tyler R., Rubidge, Bruce S., Scheyer, Torsten M., de Queiroz, Kevin, Schachner, Emma R., Smith, Roger M.H., Botha-Brink, Jennifer, Bever, G.S.
• Format: Article
• Language: English
• Subjects: Animal Shells - anatomy & histology; Animals; Biological Evolution; Fossils - anatomy & histology; Life History Traits; Locomotion; Phylogeny; South Africa; Turtles - anatomy & histology
• ISSN: 0960-9822; 1879-0445
• DOI: 10.1016/j.cub.2016.05.020

The turtle shell is a complex structure that currently serves a largely protective function in this iconically slow-moving group [1]. Developmental [2, 3] and fossil [4–7] data indicate that one of the first steps toward the shelled body plan was broadening of the ribs (approximately 50 my before the completed shell [5]). Broadened ribs alone provide little protection [8] and confer significant locomotory [9, 10] and respiratory [9, 11] costs. They increase thoracic rigidity [8], which decreases speed of locomotion due to shortened stride length [10], and they inhibit effective costal ventilation [9, 11]. New fossil material of the oldest hypothesized stem turtle, Eunotosaurus africanus [12] (260 mya) [13, 14] from the Karoo Basin of South Africa, indicates the initiation of rib broadening was an adaptive response to fossoriality. Similar to extant fossorial taxa [8], the broad ribs of Eunotosaurus provide an intrinsically stable base on which to operate a powerful forelimb digging mechanism. Numerous fossorial correlates [15–17] are expressed throughout Eunotosaurus’ skeleton. Most of these features are widely distributed along the turtle stem and into the crown clade, indicating the common ancestor of Eunotosaurus and modern turtles possessed a body plan significantly influenced by digging. The adaptations related to fossoriality likely facilitated movement of stem turtles into aquatic environments early in the groups’ evolutionary history, and this ecology may have played an important role in stem turtles surviving the Permian/Triassic extinction event. •Recently discovered stem turtles indicate the shell did not evolve for protection•Adaptation related to digging was the initial impetus in the origin of the shell•Digging adaptations facilitated the movement of turtles into aquatic environments•Fossoriality likely helped stem turtles survive the Permian/Triassic extinction The origin of the turtle shell is a major evolutionary transition whose initial function was unknown. Lyson et al. present a strongly supported idea that a burrowing ecology and adaptations related to digging favored the initial transformations on the road to the modern turtle shell. Only later was the shell coopted for protection.