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Re: questions about the Odontochelys study



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

It was the one with two "barbels" in the chin, I think the name is
Hydromedusa tectifera, .

> 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).

Are there many velocities measured for clades of aquatic turtles?

>> 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.

I do not understand why punting will increase the effective stride
lenght. I mean, I can see the correlation with limb lenght (of which I
cannot say anything with respest to the turtles we are comparing), but
at equal proportional limb lenght, and expansion of the arc of
movement in the antero-posterior sense, I can not see differences. For
example, I am not sure that if we put a mouse and lizard of relatively
the same size, limb lenght, and excursion range, we should see the
mouse winning in velocity by being more erect. Granted, in this
example the anaerobic, more powerful fiber type of the lizard may
introduce a further difference that we should like to discount.

>> 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.

Or not, perhaps the more erect posture is a result of the prior
reduction of the plastron. After all, most river turtles are
predaceous and bottom dwelling (I mean, they stay most of their time
in the bottom while on water than swimming, they do not pursue prey),
as far as I know.

>> 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.

True, you are refering as carapace only to the carapax, didn't know
that strict meaning was also present colloquially in English.

>> 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.

You mean an heterochronic correlation with other heterochronies along
the rest of the anatomy? In such a case, I do not know of other
heterochronies in turtles with reduced carapace, but do not think they
are needed. I suppose the reduction is achieved in turtles by a delay
in ossification instead of resorption (somebody who knows may clarify
this), and this may simply imply developmental arrest for these
skeletal parts. Now on selection vs. drift, I think selection must be
strong to overcome drift, if not, it is easy for stochastic processes
such as drift to dismantle the slowly-to-achieve change in allele
frequencies produced by selection. I have some difficulty in seeing
the usefulness of parasagital walking on the bottom, so as to give it
much advantage over other turtles that also dwell into the bottom with
a less reduced plastron.

> 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.

Yes, that may be. It is said that it is hydrodynamically important for
avoiding turbulence in water to have a skin deformable by water (in
cetaceans) or scales in fishes. I do not know if this has to be with
velocity (I should think yes) or if turbulence can be reduced
regardless of velocity, you may know better. In the first case, are
sea turtles fast? For reduction in Dermochelys perhaps the heat
exchange through skin can be a reason, although I think blood vessels
can transverse the bone, and bones were associated with heat exchange
in stegosaur plates (do not know if that hypothesis still holds)

But as the reduction occurs in turtles from very different habitats
and locomotion styles, including terrestrial ones, I do not think a
selective/adaptationist explanation will be useful if applied to all
the cases. Now, supporting a little these non-adaptationist
possibilities, let's talk about the thyroid fenestra. This fenestra
enlarges in some common ancestor of all Recent turtles, yet seeing the
carapace as heavy may lead us to think that this reduction in
ossification is not reduction in weigth of watever to make the animal
nimbler. No muscle passes through the fenestra, so perhaps attaching
to its borders is most resistant than attaching to a bone surface, but
I do not know.