Authors:Lyson et alAbstract: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.