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Resolving the Digit Loss Debate?



Participants are so vast, and the suggested daily quota
insufficient for me to reply singly, that I'm compounding _my_
reply. So don't feel left out if you're name isn't in the cc:
column where I usually put it.

  Resolution of digit loss, functional approach:

  I can't see where a digit would be retained unless the digit
formed a function essential to various functional activities,
including prey manipulation, locomotion, etc. In these
considerations, that sauropods have a robust fifth metatarsal
and metacarpal is irrelevant to the issue of loss in other
groups; the basal condition is possession of all digits, on both
fore- and hind-feet. In bipeds, the trend is to loose manal
digits due to the lack of use; this is true even in us, whereas
we developed not 3 mya from arboreal forms who retained and
required all five digits on all four limbs to locomote
quadrupedally through the brush and bracken and canopy of their
habitats. Theoretically, we retain function by selecting for use
of the digits to manipulate prey; natural digit loss is so rare
that it is mathematically unlikely to affect the population
trend, or even of a select group.

  Sauropods essentially retain the plesiomorphic trend in having
a functional fifth pedal digit, and a long fifth metacarpal with
digit; based on any hypothetical ancestor interpreted in the
manner of the fossil record, this is a reversal of the
dinosauromorph trend to reduce the outermost digits. This is
present in the supposedly basal theropods *Eoraptor* and
*Herrerasaurus*, and it is present in basal ornithischians. So,
it seems to me, that any considerations of where loss becomes
phylogenetically informative for the mutual arrangement of
Theropoda, Sauropodomorpha, or Ornithischia is improper. Each
group has descendants that lost these digits. This is true even
of the first, but not essential to the problem at hand [no pun
intended]: As said before, in any form were efficiency is
required, elements will be lost that do not contribute to the
function overall. In this way, the legs of ornithomimids can be
essentially considered supremely adapted for running. In a way,
this can be said for dryomorphs [dryosaurids, camptosaurids,
iguanodontids, and hadrosauroids].

  Early wing development in dinosaurs?:

George proposes that the wing was selected for early in dinosaur
evolution, and that this is what drove manal digit reduction.
This can be tested in examining the limbs of basal dinosaurs and
their closest outgroups.

A sprawling, essentially capable "flapping" arm requires a
lateral glenoid fossa, a distally inflected humeral/elbow joint,
and proportions of the limbs mechanically suited to providing
greater energy to the arm distally than proximally, as well as
in the way to mechanically direct energy sagittally instead of
parallel to the body axis. In Dinosauromorpha and rauisuchians,
and much of the basal Archosauria, the glenoid faces ventrally
and caudally, with marginal lateral exposure, resulting in a
semi-erect humeral position, but not capable of excursion above
-50[degrees] to the sagittal plane. The elbow is not inflected,
but directed more or less distally. And direction of limb motion
in the shoulder and elbow is uniplanar, with the humerus moving
back and out, and the forearm strictly in line with the humerus.
The same for the manus; the carpus is complex with various loose
elements that allow twisting of the manus. There are external
digits.

Based on *Protoavis* as a basal bird with all five manal digits,
I will not first argue that it is not a bird, that's not my
project; however, I can say that the metacarpals are
ridiculously similar to twisted (possesses a contorted shaft)
and diseased bone and I'd take a really long look at them before
suggesting they bore quill knobs. Only the ulna looks like it
has real quill knobs. Second to the points above, it lacks the
manus/metacarpus to propodium/epipodium ratio to interpret the
forelimb having any mechanical ability in a downstroke. A
relatively unspecialized critter in the arm.

  However, basal dinosaurs and dinosauromorphs generally have
even worse limb ratios and the mechanical ability appears to be
in holding the manus rigid to the forearm while the humerus
provides most of the essential muscular effort. There is no
capability of dorsal excursion in the dinosauromorph shoulder
[as known :) ] until maniraptorans appear in the fossil record
at the end of the Jurassic. Previously, the scapula is
proximally broadened with a supraglenoid shelf that prevents a
good deal of humeral expression, and distally expanded and
providing a large-angle for the scapula to the trunk, suggesting
that some intra-shoulder mobility was present, and each arm was
independent of the other. These seem to be a holdover of the
development of dinosauromorphs from animals that walked
quadrupedally: having short arms means the shoulder has to swing
that much more to match the pace of the hindlimbs.

  Effect of digit loss on wing development:

  For an arm to have a mechanically capable wing, the leading
edge is more important to the trailing edge, even in the
skeletal frame. It can be theorized [in fact I think it may have
been] that the alular digit in birds developed from a functional
specialization of the first theropodan digit, even though it
lacked feathers, as in *Archaeopteryx* (Did our own Jim
Cunningham support the use of the first manal claw as a sort of
alula? Speak up, Jim, if that was you, or not :) ). The trailing
edge will loose extraneous elements that are not essential to
the use of the wing: extra phalanges, extra digits, etc.. George
supports use of the fourth and fifth digits as supporting frames
for the wing, but this is not necessary: an aerofoil designed to
withstand aerodynamic pressures exerted in flight does not need
further skeletal support. Hence the reduction of the third manal
digit even in very early birds, while the first and second
retain functional claws, etc.

  So to suggest that there was a functional wing in basal
Dinosauria does seem to counter the mechanical ability of the
arm, more so that than any fossil data at present. There is a
trend in arm lengthening, independent (to me) of the reduction
of digits, throughout Theropoda. This trend involves lengthening
of the manus and forearm generally, and the ability of the
shoulder orient more laterally, and the elbow into be inflected
cranial to the humeral axis, which increases the leverage of the
forearm compared to the shoulder, and that of the manus, over
the rest of the dinosaurs. It seems improbable that any basal
dinosaur was flying or capable of even a minor flap, or
developed from a sprawling posture, unless birds do not descend
from dinosaurs but from some [as yet undesignated]
archosauromorph (sensu Martin, Feduccia, etc.).

=====
Jaime A. Headden

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

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