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Re: New paper on fish fingers
David Marjanovic wrote:
> But wait. It can easily be argued that the costal plates
> are there and fused to the ribs, rather than the ribs being
> broadened (as they are in *Eunotosaurus* and
> *Pumiliopareia*); that would mean it has the same carapace
> morphology as *Chinlechelys*, except that it lacks the
> peripherals, the nuchal and the pygal (which are undoubted
> osteoderms) as well as the neck & tail spikes.
Thanks for this scenario - and for the overall explanation. But at the risk of
betraying my ignorance of turtles, I'm still not clear on the matter of
osteoderms. Irrespective of the origin of the costal plates (whether
extensions of the ribs, or neomorphic ossifications), this is a separate issue
from the role of osteoderms in the origin of the turtle carapace... isn't it?
_Chinlechelys_ has a thin carapace, but some pretty hefty osteoderms along its
neck and tail. The authors (Joyce &c) suggest that the dorsal carapace evolved
via fusion of rows of osteoderms, which merged with the skeleton - resulting in
a "composite" structure made up of endoskeletal and dermal components.
But _Odontochelys_ has no osteoderms whatsoever. The turtles shell is
therefore hypothesized (by Li &c) to be wholly of skeletal/osseous origin (and
not a "composite").
These two scenarios regarding the origin of the modern turtle carapace can't be
reconciled - at least as far as the osteoderms are concerned. (Am I right
here?) So if the incipient (osteoderm-free) carapace of _Odontochelys_ gave
rise to the modern turtle carapace, how does the osteoderm-bearing turtle
_Chinlechelys_ fit in? Did turtles go through a phase where they evolved
dorsal osteoderms (like _Chinlechelys_), and then secondarily lost them
(perhaps as the osseous carapace expanded and thickened?). Or does
_Chinlechelys_'s shell morphology represent an evolutioonary 'dead-end' - in
other words, an early lineage of osteoderm-bearing turtles that evolved
independently of the line that gave rise to modern turtles?