re: bipedal crocodylomorphs

["Jaime A. Headden" <qilongia@yahoo.com>]

David Peters (davidrpeters@earthlink.net) wrote:

<The notion of bipedalism among various diapsids is over 100 years old and I'm
not sure who first coined the idea ? but if you're looking for analogs and
evidence, IMHO, the worst way to answer this question is to go searching among
synapsids (sorry Jaime). They're different in every way.>

  Eh, I have no problem with people disagreeing with me, but "different in
everyway" is not precisely true. One quadrupedal tetrapod and another show
similar constraints towards bringing the limbs beneath the body, as in
archosauromorphs and many different groups of "basal" sauropsids and synapsids.
The hypothesis is that these animals should show similar features if they
developed their vertical limbs in a similar manner and thus should also show
similar limbs if they developed bipedalism in the same manner. 

  So far, the constraints on long ilia with bipeds do not all correllate one
one one with one another, since there are many quadrupeds among tetrapods (all
of them, bar a few descendant groups), and not all of them have long ilia. So
one then looks at those species with long ilia and what else they have in
common. Excluding synapsids is yet another unparsimonious a priori descision
that affects the outcome. Despite hominoid spinal orientation, the relationship
of the femur and the ilia/subiliac pelvic bones have retained their
plesiomorphic relationship: ischia ventral and posterior to femur, pubes
ventral and anterior to femur, and ilia anterior and dorsal (cranial) to femur.
Thus, I disgree that excluding synapsids helps answer the hypothetical raised
above. Given the limb organization of dicynodonts and, say, pareiasaurs, and
their similarities from "rootward" parareptiles versus "crownward" ones
relative to them (i.e., procolophonids are to pareiasaurs as cistecephalids are
to kannemeyeriids, sprawling versus semi-upright), these distinctions in
synapsids are paralleled (and thus informative) in sauropsids.

<Rather, seek those answers among the diapsid taxa that you know are bipedal
and then work backward phylogenetically  to the quads. Between them try you'll
be able to determine what marks them as different morphologically.>

  I gave several examples of diapsids with the requisite condition (I noted
alligators, chameleons, drepanosaurs, etc.).

<Do the same with Eoraptor, Sharovipteryx and Scleromochlus, three (how could
they be anything other than) bipedal diapsids. And maybe Pseudhesperosuchus
just for grins.>

  Well, in comparison with other long legged, long armed, and upright limbed
animals with broadened scapulae showing apparent cursorial features of the
forelimb (as in hadrosaurs) it is very possible that *Silesaurus* was a
quadruped that could faculatively assume a bipedal posture. I do not think it
was an exclusive biped, and lizards should be the very model of this, even
"short-trunked" lizards. Also noted elsewhere, *Sharovipteryx* doesn't
preserved a complete forelimb nor does it allow even a semi-accurate
reconstruction of what's there. Finally, forelimb anatomy of *Eoraptor* implies
a bipedal posture with possible quadrupedal locomotion (mesaxonal manus, for
example). Well, then there are pterosaurs, which show a quadrupedal terrestrial
locomotion in every trace known and unstable bipedal motion in every model
described. This all implies an elongated preacetabular ala means "jack squat"
about bipedalism, especially since it's present in so many more quadrupeds.

  Cheers,

Jaime A. Headden

  Little steps are often the hardest to take.  We are too used to making leaps 
in the face of adversity, that a simple skip is so hard to do.  We should all 
learn to walk soft, walk small, see the world around us rather than zoom by it.

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)

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