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Re: climbing dromaeosaurs and friends



You wrote:

<It is far more likely that wings evolved in plesiomorphic, small,
long-forelimbed archosaurs than in derived, large, short-forelimbed,
cursorial bipedal theropods.>

  Just archosaurs? Relative forelimb elongation (where the forelimb
comes close to 1:1 relative to the hindlimb) in Archosauromorpha occurs
in a grand total of four groups: Drepanosauridae, Pterosauria,
Neosauropoda, & Maniraptora; there are some archosaurs which approach
this, and I think (?) that some Crocodylomorpha might also compare.
*Coelophysis* has a fairly short forearm, as do the herrerasaurids,
though admittedly, the herrerasaurids come closer to that, but still
the manus resembled the ectaxonal of a terrestrial animal, rather than
a mesaxonal manus of a grasper. It is more efficient in grasping to
concentrate grip and twisting at the point of resistance, and with a
manus that has an inverted pollex, or like the ornithomimids where it
is everted, this will occur nearer the pollex than the last manal
digit. It is the opposite in lizards and such becuase torsion in the
wrist is used to propel the body, and the pivot is external, thus the
outer digits are longer and this is where the manus concentrates stress
and torsion (ectaxonal). Thus, when one has a longer outer digit (e.g.,
in herrerasaurs and *Eoraptor*) one is not likely to be grasping unless
force is lateral to the manus, and this is not reflected in the
morphology of the manus, which flexes medially [Sereno, 1993. The
pectoral girdle and forelimb of the basal theropod *Herrerasaurus
ischigualastensis*. _JVP_ 13(3):425-450]. This principle is true of
*Syntarsus* (QG1 -- Raath, 1969. A new coelurosaurian dinosaur from the
Forest Sandstone of Rhodesia. _Arnoldia_ 28 (4):25pp) by extrapolation
of the same dynamics, though the second digit is longest, not the
third, and as George has suggested, the pollex may have turned slightly
inward. This is a more advanced grasping manus, but it fails in
reflection of the length of the forelimb, which could not be used in
any scansorial method due to the size disparity between limbs. That is,
a scansor requires stability fore and aft of the center of gravity to
prevent toppling, and this pertains to vertical climbers as well;
stability is reduced when primary locomotion is concentrated on the
hindlimbs, and the stride length of one being greater than the other
presupposes a bipedality. It would be a relatively long period of
history between the limbs of a moderately equal-to-subequal ratio in
the limbs [of *Herrerasaurus*] to a strongly hindlimb-biased form as
the coelophysids.

  Now, I'm not contradicting the scenario, as I feel the concept of an
ancestral arboreality in dinosaurs is a well-supported one when one
considers the common features of the most basal saurischians and
ornithischians. My feeling is that ancestral flight may not be the key,
just certain characteristics following arboreality or perhaps scansoriality....

=====
Jaime A. Headden

  Aaaaaaaaaaaaaaaaaaaahhhhhhhhhhhhhhhhhhr-gen-ti-na
  Where the Wind Comes Sweeping Down the Pampas!!!!

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