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Re: Reversed Hallux in Theropods: a query
In a message dated 95-11-24 16:55:05 EST, DSmith0531@aol.com writes:
>It seems that the reversed hallux of theropods figures prominantly in
>discussions about arboreal theropods and the evolution of birds. However,
>from every illustration and skeletal reconstruction I have seen, this
>"reversal" is not so reversed as to create a grasping toe! More often than
>not, it seems, it points backward (like a much reduced normal toe would),
>forward (like a digit specialized for grasping would). In some cases, it
>appears that it might be directed to the side a bit, but still faces more
>backward than forward.
The trouble is, you've seen it reconstructed only in cursorial theropods, in
which it was invariably reduced and repositioned. But if you want to see what
the retroverted hallux really looked like in the arboreal forms, take a look
at Luis Chiappe's paper on his Patagonian enantiornithan bird specimen
(sorry--exact reference is not handy, but check out any of his most recent
bird-paper reference lists and you'll find it). The hallux ungual is
_markedly_ larger than the unguals of the other toes, and it points forward,
in clear opposition to the other toes. This was a grasping foot, no question
about it. It is almost difficult to imagine, in fact, how this bird could
have walked with a claw like that stuck on its "heel."
>Now the questions:
>1) Is the "reversed" hallux really reversed after all? If so, why isn't it
>reconstructed that way? If not, why is it such a big deal?
>2) If the hallux were to be used as a grasping tool (e.g., for perching),
>wouldn't it have to be at least as large as the main toes (digits II-IV)?
> The purpose of a backwards pointing toe would be to keep the animal from
>falling forward, but the dismally small digit I's of theropods seem
>ill-suited for such a task - even for the small critters!
Heh, heh! See above.
>I must say that (gulp) on this point (and this point only), I agree with
>Larry Martin. I just don't see how the this little toe could be of much
>whether the animal was arboreal or not.
And indeed it wasn't--to a cursorial theropod.
But in all theropods, cursorial or not, that retained metatarsal I, it
articulated with the _back_ of metatarsal II, which even had a shallow groove
as the joint. There is a fair amount of rotation that the tip of metatarsal I
could undergo within the groove, so some liberty is used when making drawings
of that region of the skeleton, and digit I can be turned out sideways and
even pointed forward.
>Any clarifications would be most appreciated!