Paleontology • 2015
Evolutionary Origin of the Turtle Shell
Highlights
- Turtles and Eunotosaurus construct their shells in a uniquely similar fashion
- Phylogenetic and ontogenetic acquisitions of shell features are closely mirrored
- Initial transformations of the turtle shell occurred by the Middle Permian
Summary
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.
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Evolutionary Developmental Model for the Origin of the Turtle Shell
“Results of a phylogenetic analysis of shelled reptiles and characters
important in constructing a shell are plotted against the ontogeny of
pleurodire turtles. Thin sections through turtle embryos show the
initial outgrowth of (sub)dermal bone through the costals first
(carapace length [CL] = 13.0mm in the pleurodire Emydura subglobosa) and
then the neurals (CL = 18.0 mm in the pleurodire Pelomedusa subrufa).
The timing of ontogenetic transformations of those features (in red)
important in the construction of the shell (i.e., the number of dorsal
vertebrae or ribs does not change through ontogeny) is congruent with
the phylogenetic transformation of those same features based on our
recovered tree topology. Our model makes explicit morphological and
histological predictions for the lineage prior to the most recent common
ancestor of Eunotosaurus africanus and turtles that are met by the
morphology found in Milleretta rubidgei. Numbers above each node
represent bootstrap frequencies obtained in the phylogenetic analysis.”
Lyson, Tyler R.; Bever, Gabe S.; Scheyer, Torsten M.; Hsiang, Allison
Y.; Gauthier, Jacques A. 2013. Evolutionary Origin of the Turtle Shell. Current Biology. DOI: dx.doi.org/10.1016/j.cub.2013.05.003
How the turtle got its unique hard shell
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