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Theropod Wrist Evolution



  In regards to the little discussion on near-avian theropod wrist development:

  My previous participation on this subject is known, I certainly am not shy 
about discussing it.
But to re-iterate, there is a distinctive serious of theropods that, ignoring 
biomechanical
specialization and abberant transformation in a few theropod clades, 
development of the theropod
wrist follows a distinctive pattern from the basal-most theropods and 
non-theropodan dinosaurs up
to the near-avian and avian theropods.

  The best first example is *Coelophysis*, a form with large, block-ish and 
oblate carpals
arranged in two rows without appression of either row to manus or the 
ulna/radius. There is a
small intra-carpal transverse joint, something that allows for both axial 
twisting of the wrist
(pronation/supination) and volar-dorsal flexion, but not ulno-radial flexion. 
This is in keeping
with other basal dinosaurs, including *Heterodontosaurus* and 
*Thecodontosaurus*, where the wrist
is decidedly primitive and similar to the basic grasping manus, as seen in 
mammals. Other forms
with volar-dorsal flexion and axial torsion include lizards and various small 
quadrupedal
tetrapods in general. This is the prevalent condition among tetrapods, and that 
it exists in a
basal bipedal animal is suggestive that the pre-dinosaurian manus may have been 
quadrupedal in
design and function.

  As a trend through theropods, one can find a singular series of which the 
aberrant forms are few
and restricted in variety compared to the "main" trend in evolution. These 
sideways branches also
bear numerous other features that indicate divergent arm architecure is not 
pertinent to the
evolution of the modern avian wing, and therefore does not allow them to be 
used in the series.

  Coelophysidae [volar-dorsal flexion, axial torsion, supination/pronation]
    |
    | - - -> Carnotaurinae [axial torsion?, supination/pronation?] 
(block-shaped carpals?)
    V
  "Megalosaurs" + Spinosauroidea [reduced proximal carpals, no axial torsion, no
    |        supination/pronation, enlarged medial distal carpals]
    V
  Allosauroidea [enlarged medial distal carpals, trochleated distal carpals, 
reduced or absent
    |        volar-dorsal flexion, incipient ulnar-radial flexion]
    V
  Tyrannosauridae [flatter, oblate distal carpals, no axial torsion, carpals 
appressed to
    |        proximal metacarpals]
    |
    | - - -> Ornithomimosauria [flat, discoidal carpals, proximal carpals 
enlarged, carpals
    |        appressed to proximal metacarpals, each distal carpal covers 
proximal surface of
    |        respective metacarpal entirely]
    V
  Troodontidae, Oviraptorosauria, Dromaeosauridae [hyper-enlarged and distinct 
semilunate medial
    |        distal carpals, deep trochleated distal carpal, increased 
ulnar-radial flexion,
    |        folding mechanism with manus lateral to ulnar shaft, distal 
carpals cover
    |        metacarpals I+II]
    |
    | - - -> Therizinosauroidea [reduced semilunate distal carpal, shallow 
trochleated distal
    |        carpals, reduced ulnar-radial flexion, distal carpals reduced in 
expression across
    |        proximal metacarpal surface]
    V
  Pygostylia and above [incipient and complete distal carpal fusion to 
metacarpals, only ulnar-
             radial flexion, wing-folding mechanism allows manus to cross-over 
ulnar lateral
             surface, etc.]

  In the two major variants to the trend, the segnosaurs and the 
ornithomimosaurs, there are
predatory qualities that develop into non-predatory qualities of the limbs: in 
the
ornithomimosaurs, the digits flex in the same plane without a good deal of 
third digit mobility,
and a first digit opposability, but loss of any grasping function; the claws 
are decurved, and
generally as wide or wider than deep distal to the flexor tubercle. In 
segnosaurs (or at least
therizinosauroids) the arm is robust, but the claws decurved, and build for 
forcing the arm
against a great deal of resistance as in pulling the claws _through_ or _along_ 
something
(anything from tree trunks, earth mounds, and sauropod hides).

  From the trend, it seems that the arm of the basal dinosaurs do not in anyway 
appear to allow
for a "wing-like" form in it's ancestry. The reverse is apparent. (I say 
nothing of "truth;" such
is relative to experience, and as such is nothing but a generality, and any one 
person's "truth"
is probably most likely another's Kipple.) Whereas I can see the primitive 
dinosaurian limb as a
novelty from a quadrupedal ancestor -- also suggested in relative manal digit 
proportions as in
*Eoraptor,* *Herrerasaurus,* ornithischians, and sauropodomorphs -- and as an 
indicator of a
climbing ability, nothing suggests a basal "flying" or "wing-like" condition of 
the arms to prompt
a basal pre-dinosaurian bird in dinosaurian ancestry. Hence, the main premise 
of BCF does not
appear to meet my criticsm, though most lines of evidence do appear to be 
relevant and "factual"
to our knowledge of the fossil record.

  As for Secondary Flightlessness, in the diagram above, one places 
*Archaeopteryx* at the same
level as troodontids, oviraptorosaurs, and dromaeosaurids, and the finer 
details of evolution are
not in the scope of this post [email]. The data above neither supports or 
contradicts it, thus it
_is_ possible. I have no formal opinion of the theory.

  Critics, open fire; I welcome any comments, fairwheather or stormy, on this 
mein to my comments,

=====
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.

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