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Re: New paper on fish fingers



Jaime A. Headden <qilongia@yahoo.com> wrote:

>  On a more serious note, David (because this argument works BOTH ways) means 
> that your own persistent argument that the condition in one taxon is somehow 
> comparable to another when they show distinct morphologies, can only tell us 
> that these two taxa are different; and that, as Dr. Williams noted, the 
> conditions in either taxa at first glance indictate alternate, mutually 
> exclusive potential origins for the chelonian shell structure. At which point 
> is it as appropriate to argue that *Odontochelys* can be compared to 
> *Chinlechelys* if the ribs are UNKNOWN to be fused to osteoderms, or even if 
> the former has that many dorsal osteoderms to begin with. It's not that _I_ 
> care what came first, or which of these taxa is more indicative of the 
> ancestral testudinate condition, but that your own claims in argumentation to 
> Tim's are just as nondemonstrative.

I do not know if there are examples in turtles (fossil or Recent) in
which the costal plates are formed form an ossification center
different from the rib. Does that happen in Chinlechelys? In fossils
there should be seen sutures to counter this. If in other parts the
shell is formed by osteoderms, I don't see why the alternatives need
to be exclusive, and the osteoderm hypothesis should be true. In the
rib area the osteoderms may not independently form because the nearby
ossifying center of the rib reached the area before.

If we are going to be guided by parsimony, I do not see the difference
in the ribs of both turtles, and should rather suppose they are just
the same, and homologous, irrespective of the presence of osteoderms
elsewhere on the shell. I think that an arrangement in which the
differences seen are minimized is always preferrable as they imply
less changes, and so less steps.

By the way, if there results that the costal plate is formed by the
ossification center of the rib in all turtles, can we tell any
difference with the ribs of Eunotosaurus? Don't know about the many
differences between both that justify seeing them as not-related, but
then it would be less applicable saying they "just have broadened
ribs" as different from the chelonian condition. Why cannot the ribs
of Eunotosaurus also reach the dermis and ossify from it?. At least,
"broadened ribs" should be a character to suggest affinities between
turtles and Eunotosaurus (don't know if it is already used).