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Re: Reversed Hallux



Matthew Bonnan wrote:

<<From what I've seen in photographs of Archaeopteryx,
although it does appear to have a reversed hallux, the
location of this toe is located closer to the ankle
joint than that of modern perchers where the hallux is
close to or in line with the rest of foot.

  I suspect that is why Kevin Padian and others have
been reluctant to grant Archaeopteryx the powers of
perching, because you'd really have to rock the ankle
quite a bit back to get the hallux close to a
branch.>>

and Larry Febo wrote:

<Pardon me "interrupting" this thread. And I wasn`t
intending to start another arguement so soon, but as
long as this subject is being brought up,....Dosen`t
it seem unlikely that Theropod dinosaurs, with their
halluxes (hallucies??) located high up towards the
ankle had evolved "toward" the state of a lowered
hallux suitable for perching??
 
  I mean reptiles started with all the toes in pretty
much the same plane. I can see them lose digit five
and begin to reverse digit one for the purpose of
better grip and perching on tree branches, and later
loosing this function on the ground, the hallux rising
(through natural selection) perhaps to be "out of the
way" for a cursorial mode. I can`t see ecological
reasons for a hallux to descend, especially to reverse
and descend while the beasts were already on the
ground, to evolve into the ideal perching state. I
just can`t see the natural forces involved in this
scenario.>

  The particular toe we're talking about, in
dinosauriforms and dinosauromorphs and even
ornithodirans does indeed nearly contact the ground,
and for all functional reasons, could be considered
(or nearly so) a walking digit; however, by the
Herrerasauridae point in dino evolution, we see a
foreshortening of the first metatarsal, as is slowly
shrinks from the ground up to the tarsal side.

  (And correct me if I'm wrong) but by just after this
stage, the metatarsal stopped ossifying in the middle,
leaving only the hallugeal (sp?) and tarsal ends, the
latter which is often lost, as Currie, I believe, has
reported on _finding_ these little bony bits. The loss
of nearly the entire bone makes it much more plastic,
and it can shift in any direction. Thus, there is no
reversal, because the whole bone ain't there anymore.
We're two whole steps down the road.

  Orientation of the halluz, on the other hand, does
not neccesarily refer to perching, and though
certainly birds are the only animals _I_ can think of
who perch, not all birds with reversed halluces perch,
just that level in the Passeriform direction. Balance,
for instance, after loss of tail, coupled with the
forward knee, may be a prime reason for re-orienting
the hallux.

  Perching is required, by the way, by tendon /
ligamental placement relative to the tarsus; this was
mentioned on the list, and in a paper, can't get the
refs, sorry.

  As for Archie, I believe, wasn't the hallux found to
be 'reversible' and not 'reversed', suggesting it
could orient it forward, sideways, and maybe a little
backward? Sorry, I need to scrounge the reference up,
but I think it was during the Ostrom-fest this year.

  My Y3000.

===
- Greek proverb: "Knowledge is Inherent;
  Stupidity is Learned." -

Jaime A. Headden

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