Re: questions about the Odontochelys study

[Tim Williams <tijawi@yahoo.com>]

Jason <pristichampsus@yahoo.com> wrote:


> 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. 


Also, I don't know if the inclusion of _Odontochelys_ into the matrix boosted 
the support for the Testudines+Sauropterygia clade above the measly 53% found 
previously.


> 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. 


Is this why terrestrial tortoises tend to retain the plastron?

 
> While this no longer seems feasible for turtles, it might
> have held for ankylosaurs; at least _Liaoningosaurus_.


By comparison, other juvenile nodosaur specimens (such as the "scuteling" from 
Fort Worth TX, and _Anoplosaurus_ from England) were found without any 
associated dermal armor, presumably because it had not yet ossified.  Or maybe 
the difference was taphonomic - i.e., something about the circumstances 
surrounding the preservation of _Liaoningosaurus_ allowed part of the dermal 
armor to be fossilized, despite it being not fully ossified?


> Perhaps the little guy got stepped on a lot. :)


My guess is that the armor made baby _Liaoningosaurus_ just a little bit 
crunchier when it was being chomped on by a predator.  Those little plates 
might have gotten stuck between the predator's teeth.  :-)

 
> 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.


There doesn't seem to be any connection; the only dorsal armor preserved 
appears to be a bunch of stegosaur-like plates close to the pectoral girdle.  
These subtriangular plates are interpreted as being vertical in life.  So 
overall, based solely on the preserved armor, the belly of _Liaoningosaurus_ 
seems to have been much better protected than the back and sides.  (Or again, 
this might be taphonomic.)


(BTW, in a previous message I mentioned that the left ventral plate was 
preserved.  My mistake - it was the right one.  The left one is only an 
impression - like the anterior ventral plates.)



Cheers

Tim