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Re: questions about the Odontochelys study
There seems thus to be examples of river, terrestrial and pelagic
(Archelon) turtles with shell reduction, so it does not seem to be
correlated with a given habitat. Don't know if we can correlate it
with faster locomotion, but seeing an aquatic pleurodire run much
faster than a tortoise, I do not know if some turtle with such as the
snapping can outrun some turtles with well-developed carapace.
There are pachyostotic amniotes, as many said above, that were
correlated with aquatic habitats. However, I have seen cetacean and
pinniped skulls and they are relatively trabecular and appear to be
less ossified, instead of more, than those of terrestrial mammals. As
far as I know such a pattern is also found in many fishes (granted,
many of them do not have to deal with a bag filled of air). One could
say that pachyostosis is better for the bottom (as for many turtles
and likely placodonts), and light skeleton for the pelagic realm, but
deep sea fishes have a relatively very light skeleton (and
musculature) (again, perhaps does not apply because of the lack of
air-filled organs in deep sea fishes, but in any case, many diving
amniotes reach dephts with a very reduced air volume in the lungs,
almacenating oxygen in the blood -although, again, perhaps
pachyostotic amniotes are precisely those without such elaborations.).
As far as I know, many salamanders are bottom-dwellers in rivers and
are not pachyostotic, and I do not think all of them are lungless...