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Re: bipedal crocodylomorphs

"Jaime A. Headden" wrote:

> 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.

DP: When you say "bringing the limbs beneath the body" that can mean two 
different things. If you bring the knees beneath the hips and the feet follow, 
that's one style. If you keep the knees sprawling and also bring the feet 
beneath the
hips, that's another style.

> 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.).

DP: alligators: bipedal ancestors. chamaleons and drepanosaurids: bipedal at 
times on branches, at least hypothetically in the latter case.

> <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.

DP: Silesaurus: bipedal ancestor.  Paper coming out soon.

> 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

DP: Forelimb has been found. Paper (not mine) out soon.

> 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).

DP: That's okay.

> Well, then there are pterosaurs, which show a quadrupedal terrestrial
> locomotion in every trace known

DP: Except for the early one. The "sauria aberrante".

> and unstable bipedal motion in every model
> described.

DP: Except mine, which falsifies the others. Unstable? What does that mean. In 
humans, walking has been described as a controlled fall. Is that what you mean? 
Aircraft and birds with a negative dihedral are described as unstable, but that
works to their advantage in making fast turns. Is that what you mean?

> This all implies an elongated preacetabular ala means "jack squat"
> about bipedalism, especially since it's present in so many more quadrupeds.

"The cladogram will guide you Luke. Use the cladogram." - Obi Wan.


>   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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