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Re: New paper on fish fingers
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:
> `--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
_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.