Tyler R. Lyson, Gabe S. Bever, Torsten M. Scheyer, Allison Y. Hsiang & Jacques
A. Gauthier (2013) Evolutionary Origin of the Turtle Shell.
Current Biology (advance online publication)
The origin of the turtle shell has perplexed biologists for more than two
centuries. It was not until Odontochelys semitestacea was discovered,
however, that the fossil and developmental data could be synthesized into
a model 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
(~260 mya) Permian reptile inferred to be an early stem turtle. 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 for the
lineage, and remain viable whether turtles originated inside or outside crown Diapsida.