Re: climbing dromaeosaurs and friends

["Scott Hartman" <scott_hartman@hotmail.com>]

Chris,
    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 
wonderful communicative
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 
attention.
     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."

Scott
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