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Re: Bounding *Thecodontosaurus* (was Re: Eustreptospondylus Q's)



Justin S Tweet wrote:

<It's possible that juveniles of *Thecodontosaurus* had very different
limb proportions from adults, i.e. arm length increasing relative to
leg length. Either way, a 2.5 meter long adult of *Thecodontosaurus*
probably was more than a little larger (heavier) than a big hare.>

  I hear ya'. Scaling up from a juvenile *Thecodontosaurus*, the leg
limb proportions are no longer within the capacity to "bound"; the
morphology suggests that the closests relatives to Theco are
*Massospondylus*, *Yunnanosaurus*, or *Anchisaurus*, all of whom have
ver short adult forearms. The skull resembles the juvenile head of
Masso and the adult head of Anchi quite well. Anyway, I know of no
case where an animal that bounds in an early part of life loses that
ability when older. Rabbits don't begin hopping for a while, but
certainly antelopids can "bound" almost from the hour they're born.

<If we use slender forearms and very long hind limbs as a starting
point for looking for bounding possibilities, I agree that juvenile
*Thecodontosaurus* fits well.>

  The pelvis, actually, and the metatarsals, are the strongest
criteria, I think, while the other components are not nearly so.
Rabbits and pronghorns both have very long metatarsals in relations to
their femora, and very short (front to back) ilia; the muscles that
pull the femur up and tibia forward (m. iliofemoralis, and m.
femorotibialis) are closely appressed to the femur, and quite narrow,
and short. They jerk, the bones pull _fast_, and all lower bones are
moved in a greater arc than if the respective muscles and bones were
longer. This is why animals with longer femora than tibiae can't run
as well or fast as those with the opposite condition.

<Also, certain members of Protoceratopidae (or basal members of
Neoceratopia, depending on one's opinion on the monophyletic status of
Protoceratopidae) appear to fit fairly well.>

[snip]

<For that matter, the limbs of *Protoceratops* have that look to them,
but because of the size of *Protoceratops* I find the idea of
rabbit-hopping *Protoceratops* more funny than serious.>

  In the case of Lepto and Proto, I believe it was quantified at one
point whether Proto could hop or not, perhaps in the same paper that
disproved its ability to gallop. For one, the metatarsus and
metacarpus are quite broad and the ends of the bones splayed. If the
animal put the amount of weight it would use when "bounding" it's mass
would rip the tendons binding the hand and/or feet together, and would
virtually cripple it. A pronghorn has a considerable less amount of
mass than a proto. Especially with that head---it might just fall
over. The bones for bounders must be flexible, and with a slight
curve, not strait, so both rabbits and pronghorns show.

<I wish pachycephalosaurians were better known.>

  "Sandy" the *Pachycephalosaurus* (studied by Triebold), is in
possession of a nearly complete skeleton. *Stegoceras* and
*Homalocephale* as well.

<Let's not forget heterodontosaurids, either. They seem to be better
suited for bounding than *Thecodontosaurus*, aside for quadrupedality.
I like the idea of them walking on all fours and running on hind legs.>

  Heteros had shorter hindlegs, comparatively, than did prosauropods.
I doubt they could bound, and I had considered them. More slender and
curvaceous hindlimbs, though, but you expect that from these fellows.
Everything's gotta be cool! So pop your shades on out there, all you
Hetero skeletons, this one's for you!

==
Jaime A. Headden

Qilong, the website, at:
http://members.tripod.com/~Qilong/qilong.html
---
All comments and criticisms are welcome!

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