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Re: Cretaceous taeniodont

John Bois (jbois@umd5.umd.edu) wrote:

<Body plan.  Dinosaurs were primitively bipedal.  Presumably, this was
part of the package that contributed to their early success.  I was just
re-reading Orwell's brilliant essay on mining.  Two-legged transport in
the confines of tunnels is a nightmare.  This could be part of the reason,
at least.  Many large dinsoaurs were secondarily on four. The fact that no
small dinosaurs did this (so far) _may_ indicate that others had the edge
here.>

  Whoa, major Sereno-esque conclusioning here. Let me qualify this
statement with a few facts that counter it.

  1.  the primitive dinosaur or near dinosaur *Silesaurus* has long
forelimbs that resemble cursorial quadrupeds in their columnarity and
slenderness. This implies faculative quadrupedality.

  2.  basal ornithischians like *Lesothosaurus* have long trunks, short
sacra, and relatively short legs, with long arms; the manus in basal
ornithischians are typically robust with large epiphyses. This implies
faculative quadrupedality.

  3. basal sauropodomorphans have short hindleds relative to long trunks
and robust forelimbs and manus. This implies faculative to obligate
quadrupedality.

  4. *Marasuchus* has very long forelimbs, as does *Lewisuchus,* which is
itselve similer to point 1 (see my post on the phylogeny of *Silesaurus*
for a more thourough discussion of biomechanical uses of the limb
structure) in that the long slender hindlegs resemble those of quadrupedal
lacertilians with longer outer digits.

  5. Not so well known but in keeping with "prosauropods" and basal
ornithischians, basal theropods/saurischians like *Herrerasaurus* and
*Eoraptor* have an mesaxonal manus digit arrangement, meaning the
outer-most digits are longer (3 and/or 4 are longer than more medial
digits), unlike the conventional manipulatory entaxonal manus, in which
digit 2 is longer than 3 or 4 (in dinosaurs, external reduction of fingers
obscures the conventional meanings of the terms mesaxonal, entaxonal, and
ectaxonal, but these are used in reference to the original five digits,
prior to any reductionism or frame-shift that may or may not have
occured); the arm in these basal theropods/saurischians are also very long
and slender, though they show an increase in grasping ability (Sereno,
1993) while the outer digits become reduced. It is not unlikely that
*Herrerasaurus,* less so than *Eoraptor,* was capable of _some_
quadrupedal motion. This is especially, as noted, true of *Eoraptor.*

  While it is true that mesotarsy and the medially and horizontally
directed femoral head of true dinosaurian bipeds implies basal bipedality
as this occurs multiple times in dinosaurs, basal dinosaurs had more
elevated, cranially directed femoral heads, sigmoid femoral shafts, a
medially directed fourth trochanter, simple and almost certainly more
permanently flex knee joint, and were less-conventionally mesotarsal ankle
with splayed metatarsals, large block-like tarsals, and unfixed and
peg-in-socket astragalar/calcaneal joint with rounded distal fibulae.
These features, found in nearly every truly basal dinosaur (or more basal
form) listed above, imply less than cursorial or bipedally advantageous
locomotory mechanics during the hind limb stride, much as that is seen in
quadrupedal reptiles. The conclusion is that it is unlikely that dinosaurs
in general were ancestrally bipedal, but were in fact experimenting in
bipedality, which took off early only in theropods, and may have led to
the increase in sauropod size and the evolution of small bipedal
ornithischian cursors, armor, and large size in ornithischians.

=====
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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