Re: New paper on fish fingers

[Tim Williams <tijawi@yahoo.com>]

David Marjanovic wrote:

> Not really. I see just three differences: *Odontochelys*
> lacks sutures between the costal plates, appears to have
> more fusion between the ribs and the costals (though
> it's squished flat, while *Chinlechelys* is not...), and
> *Chinlechelys* shares with more derived turtles the
> condition of having the dorsal ribs articulate at the
> boundaries between the vertebrae rather than on a single
> vertebra. In sum, I predict that my thesis will find:
> 
> --+--*Odontochelys*
>  `--+--*Chinlechelys*
>     `--+--*Proganochelys*
>        `--rest (Rhaptochelydia if you like).


I'm not arguing against your phylogeny.  But I think the original point was 
that there was "experimentation" in early turtle evolution.  I think this point 
still stands, given that_Odontochelys_ and _Chinlechelys_ would seem to 
exemplify very different shell morphologies, with one (at most) giving rise to 
the modern shell.

Here's the way I see it... One hypothesis for the evolution of the turtle shell 
posits that the carapace evolved via expansion of the ribs and vertebrae.  The 
other hypothesis proposes that the carapace evolved from fusion of osteoderms, 
which fused with the underlying skeleton. 

_Odontochelys_ has a plastron, but the dorsal armor is composed only of neural 
plates (no osteoderms), and so has been cited in support of the first 
hypothesis.  

_Chinlechelys_ (which also has a plastron) has a thin carapace and spike-like 
osteoderms adorning the neck and tail; it has been cited in support of the 
osteoderm origin of the carapace.  

Since both hypotheses cannot be right, at most only one of either 
_Odontochelys_ and _Chinlechelys_ can represent an ancestral state in the 
evolution of the modern turtle shell - making the other a separate 'dead-end' 
experiment in dorsal armor development.

As I said before, I don't know much about turtles (living or extinct), so I 
could be off kilter here.



Cheers

Tim