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Re: questions about the Odontochelys study



Augusto Haro wrote:

No, I do not have reasons, but neither do I have for the other way.
The pleurodire strucked me as too fast.

You'll have to explain what you mean by "too fast". Incidentally, do you know what species of pleurodire it was?


Of course, velocities should be measured to give a more serious statement.

Good point. Some velocities are available, but I don't believe any have been measured for species that run along the substrate subaqueously. For terrestrial locomotion, escape sprints to the water by trionychids are among the highest measured velocities for turtles (R. Blob, pers com; also see work by Zani and Pram).


I did not know of something similar of a "high march" for the matamata, but in any case,
an upright stance does not necessarily have to do with velocity.

True, but in the case of punting locomotion, it will increase velocity, all else being equal (because the effective stride length increases). Of course, that "all else being equal" bit is tricky - so whether or not velocity is actually increased is hard to say for certain. The expectation, though, is that punting will be faster with a more erect stance.


With respect to permitting a greater amount of limb movility, it is
right when shell coverage is reduced around the limbs, but I think it
has nothing to do when related to the space between costal
plates/ribs, as for example in Archelon. In any case, there are also
pelagic sea turtles, which also performs subaqueous flight, with a
complete shell coverage without fenestras.

True, I was referring to margin reduction, not perforation of the carapace by fenestra as in Archelon. That's a different situation.


I would also say, perhaps the plastron reduction, even when permitting
enhanced limb motility (which I do not view as a great advantage,
given that these turtles do not need to be upright to feed with their
long necks and jaws, and to walk the common way of other aquatic
turtles with a more developed shell coverage would suffice), was not
an adaptation acquired through natural selection.

Perhaps not, but the rarity in reduction, and the correlation with bottom-dwelling, predaceous forms suggests that the plastron reduction might be selection-based.


The snapping turtle is too agressive, the matamata well camouflaged,
so perhaps the shell is less necessary for those turtles in defense.
Then, perhaps if the carapace reduced a little, there was not such a
great problem.

Neither of those has a reduced carapace; only the plastron is reduced. In any case, you're probably correct about the lower cost.


I think carapace reduction perhaps is just result of an
heterochronic process, of lack of development, and that can (or not,
being relatively neutral) fix into populations if the pressure of
predation is not so big. These pressures can be great for
non-aggresive, non camouflaged, and small turtles, but not so much for
others.

True, but lacking a known heterochronic correlation, and given that the effect appears to be non-random (it correlates with other traits across turtle phylogeny), a locomotion benefit hypothesis seems just as reasonable (coupled, again, with the low-cost angle). In other words, if the cost is low, then weak selection might reduce the plastron, or stochastic effects might do the same. Hard to determine one way or the other, but given that there is an apparent functional advantage for matamatas, at least, I hypothesize that the plastron reduction in Chelus is functional and selective. I propose the same, albeit with less confidence, for Chelydrids.


On a sidenote, the shape (and presence) of the carapace of many aquatic turtles might be hydrodynamically important, and this may impact how often the carapace is reduced, and its manner of reduction.

Cheers,

--Mike


Michael Habib, M.S. PhD. Candidate Center for Functional Anatomy and Evolution Johns Hopkins School of Medicine 1830 E. Monument Street Baltimore, MD 21205 (443) 280 0181 habib@jhmi.edu