Re: New paper on fish fingers

[Tim Williams <tijawi@yahoo.com>]

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?


Cheers

Tim