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Re: Endoskeletal origin of turtle carapace (free pdf)

David Marjanovic <david.marjanovic@gmx.at> wrote:

> 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.

I could well be wrong, but my reading of the paper was slightly
different - though I certainly agree with your main points.  I thought
that the authors' hypothesis was that the construction of the trunk
skeleton in the saurosphargid-sauropterygian-testudinate clade allowed
the ribs to be readily converted into a carapace.  Thus, the
development of the carapace shared the same underlying "genetic
basis", but the carapace evolved independently in Saurosphargidae,
Placodontia, and Testudinata (turtles).

I'd really like to believe that turtles are extant survivors of a
"euryapsid" clade that also includes thalattosaurids, saurosphargids,
plesiosaurs and placodonts.  But the _Eunotosaurus_/parareptile link
and sauropterygian/lepidosauromorph link can't both be right.  And if
molecular phylogenetic analyses are correct (turtles as sister group
to archosaurs), neither scenario is correct (e.g.