Re: The retroverted hallux of birds

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

First post in a while ... I've been busy and rather well preoccupied.

Henri Rönkkö wrote:

<How is it attached to the rest of the limb? Halluxes in usual animals
sprout from more proximal limb, it seems to me. But the avian
retroverted hallux sprouts from the same level as other toes.

I once read, from Sereno's paper on Sinornis, I think, that metatarsal
I is attached to metatarsal II, near the distal end of the latter. This
sounds quite strange to me!>

  The first metatarsal in birds is a large J-shaped bone in unfused
metatarsi, that fuses until only the articular hinge is identifiable.
The condition is, I believe, variable in some taxa. Others lack it,
like ratites. Sereno and Rao argue that *Sinornis* is the first percher
because of the fully retroverted hallux preserved on both pes. As
demonstrated at SVP in Denver oh so many years ago! ('99) 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. The
metatarsal (i) retains a ball-and-socket joint, which adds tremendous
flexibility. My only difficulty is in finding lit. on the tendons of
the toes. Metatarsal I remains a distinct element even in some advanced
fossil birds, but ornithurines all have fully fused metatarsals, if I'm
not mistaken. Iberomesornithids and enantiornithines don't (?). I
haven't read all the fossil bird lit, so forgive any mistakes I may
have made.

-------

  Henri, the prevalent hypothesis on fusion of dorsal and sacral
vertebrae (no bird has lumbar vertebrae -- this is a mammalian thing of
dorsals that lack any rib contacts) in birds is to resist stress of
muscle tension during the wingbeat cycle, which would theoretically
pull the back in four separate directions (down, up, left, and right)
and would give the animal quite a workout in trying to keep breathing.
I don't feel competing hypotheses (which don't get much press, anyway)
are substantial enough to be likely, as far as my own research has suggested.

=====
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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