[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
Re: climbing dromaeosaurs and friends
Sorry about the delay in response, but I've had some trouble deciding
what tact to take. Some aspects of your email seemed almost personal, and I
didn't always understand what point you where trying to make.
For example, why did you think that your argument from authority
("...but this should not be a surprise to anyone given an understanding of
the morphology of its hip joint.") was persuasive, but when I did the same
("The utter lack of acceptance...") it was worthy of your belittlement
("Although I do not accept Larry Martin's reconstruction of Archaeopteryx,
let's not dismiss it just because it is unpopular--let's rely on evidence.")
I was aware when I made the remark that it was an argument from
authority. Surely it did not escape your attention that the remark in
question came in the middle of an email that was already quite long. A full
explanation of the functional morphology would have added considerably to
its length. The simple fact of the matter is that the dinolist does not and
cannot possibly maintain true scientific rigorousness in every posting, lest
each posting be 3 to 20 pages apiece and be peer reviewed. The list is a
device where scientists and lay people alike can share ideas about
paleontology, but there are practical constraints on length. My comments on
parasagitally constrained limbs were brief because there is little dissent
over this point, not because of a lack of understanding
of the functional morphology behind it.
If you were unhappy over the level of detail I'd provided on this
aspect, you are always welcome to request it, but unless I'm over reacting,
your remarks seemed like an attack on my standards of evidence.
Your discussion on pterosaur limbs (and the paper you mentioned) is
interesting, and frankly I think you are right. They are not, however,
terribly cogent to my hypothesis, which had to do with the mode of
preadaptation to avian flight of terrestrial theropods.
There are certain unambiguous characters that arboreal scansors have the
theropods lack. One is the distance of the center of mass from the
substrate during normal locomotion. The center of mass generally lays not
on the ventral portion of the thoraco-abdominal cavity, but rather somewhere
in the center of it. This is fairly easy to estimate, by using cylinder
models and solving a series of force equations that balance out at some
centerpoint, "x". What you find is that in _all_ arboreal scansors (except
scansorial birds which can make soft alula-enhanced landings, and therefore
aren't subject to selection
related to falling off branches to their death) the distance of the center
of mass (CM) is never more than 2x from the substrate during normal
locomotion. In theropods the value ranged from 2.8x to almost 4x. In
Archaeopteryx this value is atrocious, exceeding 4x.
Of course it's possible that Archaeopteryx may have been an adept
lander, in which case it would have been under avian-style selective forces,
but this seems unlikely, do to its primitive flight morphology and lack of
an alula. Even advocates of an arboreal habitat generally don't think it
was an adept lander. As such, the differences in hindlimb morphology
between it and scansors would have made arboreal landings and scansorial
leaps much more difficult.
There are many other features of theropods that are inconsistent with
scansoriality. Maniraptorans especially have a particularly long manus,
while good climbers generally have much shorter distal limb segments, to
increase leverage while climbing. This is probably why parrots and even the
hoatzin climb using their beaks and feet (with the
hoatzin using is clawed forelimbs to grasp and hold itself in place).
Although I suppose I can't rule this type of climbing out a priori for
theropods incl. Archaeopteryx, it seems unlikely in these smaller headed and
longer necked animals.
The exception to this rule is brachiators. Brachiators take advantage
of pendular motion to increase locomotory efficiency, so lengthen the their
distal limb elements. Applying brachiation as a hypothesis to laterally
folding maniraptoran forelimbs is a task I
leave to others.
You also wrote: "I think it also tells us that the wing in
Archaeopteryx was not attached to the hindlimb in Archaeopteryx, which also
should not be news to anyone." It certainly shouldn't be news to anybody.
It also shouldn't be news that the wing may not
have even been contiguous with the body, despite essentially all artistic
reconstruction?s showing it doing so. Despite the remarkably well preserved
primaries and tail feathers, there is not a shred of evidence for tertials
continuing the flight surface to the body.
Perhaps they simply weren't preserved (in _every_ specimen), but this
popularly unscrutinized aspect of Archaeopteryx anatomy deserves some more
You also wrote: "However, it does not tell us that the hindlimb of
Archaeopteryx was necessarily limited to move only in a parasagital plane. I
think that the hindlimb had to be able to move out of the plane (in all
theropods) just as it does in all sorts of extant animals
who limbs normally move in a parasagital plane."
First off, I assume that by "all sorts of extant animals" you mean
"lots," as opposed to "all." There are many animals that cannot move their
limbs out of the parasagital plane. Ungulates spring immediately to mind,
although large sub-ungulates like probscideans also
have a greatly reduced degree of lateral limb extension.
But the real crux of the debate is the morphology of the dinosaurian
femoral head and acetabulum. To put any pressure on a splayed hind limb
requires putting pressure on the medial wall of the acetabulum, which is a
problem for dinosaurs, since they don't have a
medial wall in their perforated acetabuli. The large femoral head which is
offset at 90 degrees to the long axis of the femur (unlike the vast majority
of animals, extinct or extant) also make it nearly impossible to impart
lateral rotation, since the head would run
into the acetablular ceiling, while the lower part of an abducting femoral
head would run afoul of the puboischial symphasis. There is simply no way
to do it.
There is one last thing. While adduction and abduction must have been
severely constrained, the plain of theropod limb motion was not truly
parasagital. If you can get a hold of foam casts of a femur and illium,
you'll see that the femurs "anterior" side is actually oriented 10-15
degrees from the midline, and so is the plane of motion. So at the
beginning of limb excursion, the foot further from the midline than at the
end. Presumably this is why some authors refer to theropods "waddling."
Get more from the Web. FREE MSN Explorer download : http://explorer.msn.com