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Re: Endoskeletal origin of turtle carapace (free pdf)
> We also examine the fossils of an outgroup of turtles to
> confirm that the structure equivalent to the turtle carapace developed
> independently of the true osteoderm. Our results highlight the
> hitherto unravelled evolutionary course of the turtle shell.
An interesting paper in that it confirms that not only the costal plates but
also the neural plates are outgrowths, not osteoderms.
Not only the authors but also the reviewers must all be embryologists. I can't
find another way to explain why *Eunotosaurus* is bypassed the way it is -- it
is mentioned once (for having ribs that are "expanded in shape"*), a paper from
1969 is cited, and that's it! The manuscript was received by Nature
Communications on "07 December 2012", more than two years after the recent
redescription of *Eunotosaurus* was published. I want a big stamp with "FAIL"
The authors make a lot of the fact that *Sinosaurosphargis*, a Triassic animal
on the euryapsid side of things, had ribs that must have been immobile or
nearly so and bear plate-like extensions (like turtles) at their distal ends.
*Sinosaurosphargis*, like *Henodus* (mentioned in the paper) and a few other
placodonts (not mentioned), had a complete shell composed of tiny osteoderms
_outside_ the ribs and even the gastralia. This would have the intriguing
implication that the turtles _lost_ this external shell (except, I guess, for
the peripherals), perhaps as it became unnecessary when the internal shell
became developed enough. Problem is, *Eunotosaurus* and *Odontochelys* have the
exact same number of trunk vertebrae (9; *Proganochelys* has 10), all of them
with ribs with plate-like extensions all over the lengths, and both have a
large number of small neck vertebrae, while *Sinosaurosphargis* -- as seen in
figs. 4 and S6 -- has a small number of neck vertebrae about the size of its
trunk vertebrae, of which it has upwards of 22.
* The paper would also have benefited a lot from being proofread by someone who
knows English better than the authors. They must have really struggled.
Japanese lacking a distinction between singular and plural, they didn't even
notice that "osteoderm" is a designation for one single bone in the dermis, not
a collective term for all dermal ossifications together.