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Evolutionary origin of turtle shell

From: Ben Creisler

A new online paper:

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)
doi: http://dx.doi.org/10.1016/j.cub.2013.05.003

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.