Evolutionary Origin of the Turtle Shell

The origin of the turtle shell has perplexed biologists for more than two centuries [1]. It was not until Odontochelys semitestacea [2] was discovered, however, that the fossil and developmental data [3-8] could be synthesized into a model [9] of shell assembly that makes predictions for the as-yet unestablished history of the turtle stem group. We build on this model by integrating novel data for Eunotosaurus africanus-a Late Guadalupian (similar to 260 mya) [10] Permian reptile inferred to be an early stem turtle [11]. Eunotosaurus expresses a number of relevant characters, including a reduced number of elongate trunk vertebrae (nine), nine pairs of T-shaped ribs, inferred loss of intercostal muscles, reorganization of respiratory muscles to the ventral side of the ribs, (sub)dermal outgrowth of bone from the developing perichondral collar of the ribs, and paired gastralia that lack both lateral and median elements. These features conform to the predicted sequence of character acquisition and provide further support that E. africanus, O. semitestacea, and Proganochelys quenstedti represent successive divergences from the turtle stem lineage. The initial transformations of the model thus occurred by the Middle Permian, which is congruent with molecular-based divergence estimates [12-15] for the lineage, and remain viable whether turtles originated inside or outside crown Diapsida.