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Re: questions about the Odontochelys study



--- On Sun, 11/30/08, Tim Williams <tijawi@yahoo.com> wrote:

 
> The phylogenetic link between turtles and sauropterygians
> is certainly weak (statistically speaking), though it is
> better than any alternate position - which is not saying
> much.  :-)

+++++++++++++++++++++++++++++++++++++++++

Admittedly, the discovery of _Odontochelys_ has me leaning a bit more towards 
Sauropterygia now, but I'd still argue that the procolophonid hypothesis is at 
least as strong. 

________________________________________
 

 On a separate issue, the presence of a plastron is said to
> be an indication of an aquatic origin of turtles, because it
> provides protection for the ventral body surface.  In
> terrestrial tetrapods, by contrast, the belly is not exposed
> to predators unless the animal is rolled over.  For that
> reason, the bellies of ankylosaurs are far less heavily
> armored than their backs.  However, one ankylosaur
> (_Liaoningosaurus_, based on a juvenile specimen) does have
> quite a large "shell-like" ventral abdominal plate
> to protect the underside close to the hips.  Rather than
> suggesting aquatic adaptations (!), it may indicate that
> juveniles of this species were more prone to be rolled over
> (such as by predators).  So would an adult _Liaoningosaurus_
> be expected to have such a large ventral plate as well?  If
> so, why?

+++++++++++++++++++++++++++++++++++++++++++++++++

Prior to the embryological discovery that turtles evolved the plastron first, a 
good alternative argument for plastral evolution was that it served as a brace 
for the carapace. By connecting both sides of the carapace together through the 
plastron, the chelonian shell was significantly strengthened. 

While this no longer seems feasible for turtles, it might have held for 
ankylosaurs; at least _Liaoningosaurus_. Perhaps the little guy got stepped on 
a lot. :)

I don't have the _Liaoningosaurus_ paper, so I'm not sure if this ventral 
plating ever connected to the dorsal armour, or if it was on its own.

Jason