Re: more stuff

["Jaime A. Headden" <qilongia@yahoo.com>]

I wrote:

<<you can easily arrive at a retroverted hallux by twisting the shaft
of the first metatarsal, which retains a long contact with the second
metatarsal, and the contact would in more advanced birds shift to the
sole of the foot and further orient in line with the third toe.>>

Nick Longrich wrote:

<There's another way to get a reversed digit 1 and a good example is
within easy arm's reach. Manual digit I in humans may or may not have
some long-axis torsion (don't have any refs or cleaned specimens to
examine handy) but at least part of its opposability made possible by
rotating the bone on its long axis relative to the other metapodials.>

  I would argue that the oposability of the first manal digit in
homonoids is entirely different in morphology, as there is a joint
between the first metacarpal and the first distal carpal and a series
of tendons that do not exist in the avian foot (relativity, not
homology, of elements)

<A limited degree of facultative reversability may therefore have been
present in the first digit and may have been how the digit began to
develop opposability. Based on what I've seen of them, the articulated
dromaeosaur feet the AMNH has are not good arguments for an unreversed
hallux in the Dromaeosauridae.>

  Here my point is that the long ascending taper of the first
metatarsal in these forms (as well as those you list below) provides a
contact to the second metatarsal that may in fact defer any ability for
the digit to rotate. In birds this occurs in the joint, as you imply,
however the utility of a reversible digit is for the purpose [as far as
evolutionary scenarios can affirm] for perching, or grasping with the
foot in simple mechanics, and a terrestrial form like dromaeosaurs,
even assuming they are secondarily flightless, could not perceive then
to retain a uselessly reversed or resersible toe without reasonable
exaptation.

  The toe in many terrestrial birds is lost or reduced. Galliforms have
a long but highly-positioned hallux, which no longer appears to serve a
primary perching ability.

<Anyways, some birds have unreversed the first digit, e.g. swifts, and
it is held more or less medially (as opposed to reversed) in some other
birds, so this stuff about first digits not being able to unreverse
from the opposed position is inaccurate.>

  Swifts have a reversible first pedal digit, which rotates along with
the fourth, but as you note can only orient toward the medial --
ectropodactylous. Not so much a grasping foot as a balancing foot. It
can assume a sort of psuedo-anisodactyl or a zygodactyl arrangement.

<Also, the first toes of dromaeosaurs, caenagnathids, therizinosaurs
and troodontids are all different, so it's dangerous to make sweeping
generalizations and assumptions about bird evolution without sampling
the whole range of variation, just as for example dromaeosaurs,
troodontids, oviraptorids, Caudipteryx, and alvarezsaurids all had
their own unique tails, or pelves, or whatever so you can't just look
at specialized dromaeosaurs and draw all these conclusions about bird
origins like we've tended to in the past.> 

  My point about the therizinosaur pes was not a generality but simply
to point out that the first metatarsal may have contacted the second
metatarsal, in response to Henri's query. I read my post and had
thought this was suggested, but forgot to actually say it, and realize
that whatever I had implied, was probably not taken, so this is my
error.

  Troodontids, on the other hand, have very small halluces, they are
positively puny relative to the other toes, and would have possibily
have been quite useless. The metatarsus is long and could be termed a
pseudo-cannon as in camels, and it is possible these are
highly-cursorial theropods.

=====
Jaime "James" A. Headden

  Dinosaurs are horrible, terrible creatures! Even the
  fluffy ones, the snuggle-up-at-night-with ones. You think
  they're fun and sweet, but watch out for that stray tail
  spike! Down, gaston, down, boy! No, not on top of Momma!

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