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Re: questions about the Odontochelys study
Augusto Haro wrote:
It was the one with two "barbels" in the chin, I think the name is
Ah, gotcha. Very interesting species. Don't know of any velocity
measurements for them, but they are pursuit predators and pretty
strong. Interestingly enough, it may be the best living outgroup to
the matamata - with makes ancestral state reconstruction for the
punting stuff I was talking about before a real pain. There are a few
fossil Chelidae that might be able to help (can't recall how much
material is there).
Are there many velocities measured for clades of aquatic turtles?
There are a handful of decent measurements, mostly for terrestrial
locomotion. Swimming velocities are known with some precision for a
few trionychids, some sliders, and possible pig-nosed turtles.
Cruising velocities are also known for sea turtles, at least roughly.
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.
It's the other way around - greater limb length will tend to improve
punting, to an extent. Punting is an odd form of locomotion, in that
it incorporates aspects of both walking and running, from a
biomechanical standpoint. To the extent that it acts like walking,
there is gravitational potential utilization, and this improves with
limb length. It's the same reason that maximum walking speed
increases in humans with longer lower limb lengths - individuals with
shorter limbs transition to jogging gaits as lower velocities than
those individuals with longer limbs. Of course, part of what makes
the whole thing odd is that there is also buoyancy involved, which is
also a form of gravitational potential, so the calculations get a bit
On a side note: if there is anyone out there interested in
collaborating on some work on punting turtle kinematics, I've got a
hand-raised matamata that should be willing to run in a flow tank.
For example, I am not sure that if we put a mouse and lizard of
the same size, limb lenght, and excursion range, we should see the
mouse winning in velocity by being more erect.
Different sort of problem, in part because you seem to be referencing
top speed rather than walking efficiency. It's also a tough analogy
because the lizard introduces the effects of anguilliform motion. In
the case of punting turtles, I am only suggesting that the erect
stance helps because it increases the effective limb length for
gravitational potential transfer, not that erect gaits are uniformly
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.
Quite a few river turtles are pursuit predators or swim in the water
column while foraging. My hypothesis only refers to those few taxa
that are adapted specifically to bottom walking and "running". Still,
you could be correct - we don't know with certainty that the plastron
was not reduced first.
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
Seems reasonable enough, but it doesn't actually speak to an adaptive
versus drift-based paradigm - the aforementioned mechanism works
equally well in either.
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.
Whereas I see the possibility for a distinct advantage in mobility.
Just as you are correct in asserting that even modest drift can
prevail when selection is weak, even a small selective advantage can
fix a trait - the performance benefit doesn't have to be huge,
especially if costs are low.
In any case, both hypotheses are reasonable. Testing for drift is
fiendishly hard in this sort of circumstance, but a functional
advantage might be tested for with some simple comparative studies of
locomotor efficiency and/or velocity. The tricky bit would be finding
bottom-running/punting species with full plastron development - I'm
not sure there are any.
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.
The flow regime will depend on velocity, but most turtles probably
swim in a roughly similar Reynolds Number range, so the actual impact
of the shell on flow will probably depend more on specific gait, limb
kinematics, and agility of the animal involved (as well as substrate
effects for bottom-runners). For example, trionychids are relatively
fast swimmers, especially in a sprint, but they are still drag-based
swimmers with substantial surge losses during swimming. A sea turtle,
by contrast (as well as pig-nosed turtles, which happen to be the
sister group of trionychids, interestingly enough) is an aquaflyer,
and experiences substantially less surge-associated acceleration as a
In the first case, are
sea turtles fast?
They are relatively quick, yes. I have some papers with specific
speeds listed, but not handy at the moment. I do know at least one
paper indicates that trionychids, while among the fastest freshwater
turtles, are apparently slower than aquaflying species (sea turtles,
in particular), but this seems to be mostly a reference to cruising
speed. I'm not sure if there are any good records of sea turtle
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
I agree; but there are a few cases where plausible selective
hypotheses are present and worth testing. Pure drift, by contrast, is
difficult to get a good handle on (but that doesn't make it untrue).
Michael Habib, M.S.
Center for Functional Anatomy and Evolution
Johns Hopkins School of Medicine
1830 E. Monument Street
Baltimore, MD 21205