Re: questions about the Odontochelys study

[Tim Williams <tijawi@yahoo.com>]

Jason <pristichampsus@yahoo.com> wrote:

> Placodonts if I remember correctly. Interestingly, if one
> removes sauropterygians from the mix, turtles fall back into
> the anapsid camp, thus calling into question whether or not
> turtle relations with sauropterygians are a result of
> homology, or just convergence.


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.  :-)

Also, although Rieppel & Riesz (1999) found that removing sauropterygians from 
the analysis caused turtles to fall back among anapsids (parareptiles), they 
also found that the derived characters that put turtles inside the Diapsida are 
found throughout the skeleton, and are not necessarily aquatic adaptations.

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?



Cheers

Tim