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RE: bipedal crocodylomorphs



John Hutchinson (jrhutch@socrates.berkeley.edu) wrote:

<I haven't seen Huene's original _Saltoposuchus_ (iirc) paper and am wondering
if it [elongation of the preacetabular ala, JAH] stems from there, or something
else I've missed in the disparate crocodylomorph literature.>

to which Tim Williams (twilliams_alpha@hotmail.com) replied:

<I think the idea of bipedal sphenosuchians was more typological than anything
else.>

  Surveying crocs and "thecodonts," I find that elongated preacetabular alae
are quite prevalent in Archosaurs; it is present in some specimens of
*Alligator* for instance, and to such a degree as to be only about 20% shorter
than we see in *Macelognathus*. In fact, in Archosauriforms, since even
*Euparkeria* has a distinct "tuber" on the craniodorsal margin of the ilium
laying dorsal and somewhat forward of the pubic peduncle. John was particularly
curious to how this got started into inferences of bipedality, and frankly I
don't know, but it will make a good work in background. I am currently looking
at limb mechanics among various archosauromorphs in comparitive research
regarding *Megalancosaurus,* and so my comments are relatively brief. I will
say that aside from what Darren Naish wrote back in 2003 (link below) that some
exceptions make me doubt the association of these iliac specializations to
strict bipedal motion.

  http://dml.cmnh.org/2003Oct/msg00345.html

  there are a LARGE host of quadrupeds that develop a distinct or massive
preacetabular ala, including none other than most of Mammalia. Of these,
relatively few (Anthropoidea, Macropodidae, etc.) can develop fully bipedal
motion, and only one subgroup (Hominoidea) can maintain locomotory equilibrium
bipedally using more than one gait (walking, pacing, running -- well, humans
running are outside of equilibrium, but close enough). Otherwise, such ilia are
known in drepanosaurs, chameleons, dicynodonts, pareiasaurs, and so forth,
including nearly all of Dinosauria. The dinosaurs with the largest expansions
of their ilia include the thyreophoran and ceratopsian ornithischians,
brachiosaurid sauropods, and birds among theropods; of these, only birds are
bipedal. Bipedal ornithsichians usually have a shallow, strap-like
preacetabular ala rather than a broad blade-like one as in saurischians, and
most theropods have a dolichoiliac pelvis (longer postacetabular ala than pre-)
which argues for a greater dependancy on the lateral and rear thigh muscles
than the fore (more on this below).

  So what DOES the preacetabular ala DO for the hindleg? I do not think that
proponents of bipedalism have neccessarily answered this, focusing on shared
bipeds' traits than surveying the traits they ascribed to bipeds among all
other animals. The first thing the ala does is provide the anchor for the M.
iliotibialis, which inserts on the cnemial crest; you can bet the tibia of the
animal with an ala will likely have more than a ridge for the insertion of this
muscle. The forward position of this muscle means there is more room for the
occupation of the ilium for the M. iliofemoralis, which inserts at the
trochanteric crest lateral to the femoral head and provides the motion
neccessary for everting the femur laterally and in part protracting it
(rotating it forward) at the acetabulum (which is where the origin is above).
If these muscles are farther apart and have greater bases for the origins,
relative function increases and angles reveal new, potentially stabilizing
abilites. None of these translates directly to femoral retraction (rotating it
rearward), thus forward motion is not seriously implied in these muscles'
effect on the thigh. Now, this is neccessarily brief and I am MASSIVELY
simplifying the muscle systems for the sake of research time and project
"goodies". Plus, Hutchinson in his _ZJLS_ papers and Hutchinson and Garcia to
many degrees covered many aspects over the last few years (so none of this is
new to John). So then, if the elongated preacetabular ala means larger muscles
for protracting the femur and stabilizing it, what does this do for bipedalism?
I have a theory, and it can be summed up in two words made infamous by the
late, great Chris Farley:

  JACK SQUAT

  Okay, that's not true. There IS a positive effect, as in stability and
control in a turn, and in quadrupeds this motion and stabilizing effect serves
in good tandem with the forelimb, so there tends to be a good reason why of all
the tetrapods to develop this feature, they are quadrupeds or evolved from
quadrupeds. Thus there is likely to be no special selective advantage to
develop the process AS a biped, not when it is greater than the size of the
postacetabular ala, site of the major femoral retractors (Mm. iliofibularis and
flexor tibialis externus, and to some degree the M. iliofemoralis). Bipeds have
already had the process, and thus adapt it but at the same time more greatly
develop the rear femoral muscles, far outstripping the fore.

<Unlike its Triassic brothers, _Hallopus_ was originally regarded as a 
coelurosau.  But is any forelimb material known for _Hallopus_?>

  A humerus as an impression and one as a poorly exposed shaft, both radii and
ulnae, both sets of proximal carpals (radiale and ulnare) and both sets of
metacarpals with associated phalanges. The scapula is present but lacks the
margin for articulation with the coracoid and the distal (dorsal) margin. None
of the sternal/clavicular elements are present.

<"The relatively slender pectoral girdle and forelimb suggest that 
_Erpetosuchus_ may have been a facultative biped.>

  My only counter argument for this is that cursorial ungulates show extensive
slenderization of the limb elements and may not precisely shorten the proximal
segments for the distal segments of the limb. Elongation of the forelimb,
increased mobility of the scapula, and "cannonization" of the limb elements are
MY personal criteria for cursorial features in forelimbs, as seen in
artiodactyls and some archosaurs (for this, wait for more work from Hartmann
and myself -- independantly).

  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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