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Re: Reversed Hallux in Theropods: a query



In a message dated 95-11-25 08:42:48 EST, ornstn@inforamp.net (Ronald
Orenstein) writes:

>>The alternative is the BADD theory, in which the hallux loses its proximal
>>end, shrinks, retroverts for no evident reason to a quite specific site on
>>the back of metatarsal II, and then sits there with a dangling digit for
>>about 35 million years on almost every known theropod from ceratosaurians
to
>>tyrannosaurians awaiting future use as a perching organ in the one theropod
>>lineage that evolved into birds.
>>
>
>Except for the last 16 words this appears to me no more unlikely than the
>retention of cannon bones in horses, internal hind limb elements in
>cetaceans or a number of other vestigial structures that have apparently
>hung around for a long time in certain forms.  And of course this is not the
>only alternative - the other alternative is that in these animals the hallux
>retained some minor function, whatever that might have been, consistent with
>a terrestrial existence.

Well, of course, the last 16 words say it all. There are vestigial structures
and there are vestigial structures, of course. A vestigial organ like an
appendix, tucked away internally and not particularly interfering with the
animal's normal way of life (except when it gets appendicitis) can of course
sit there for megayears. But a vestigial pedal digit, which adds unnecessary
weight to the foot and which interferes with the animal's day-to-day walking
and running, is a positive liability. I'm pretty sure (correct me on this,
bird experts) that all extant ratite birds (ostriches, emus, cassowaries,
rheas, etc.) have actually _lost_ the hallux entirely as an external digit
(some might still have it as an internal splint metatarsal). Speaking of
splints, burined in their feet most theropods carried around a splint
metatarsal V, whose vestigialization predates by megayears the retroverted
hallux.

There are theropod footprints in which the hallux still touches the ground
toward the rear of the foot (none in which it is sideways, as far as I know),
but they're scarce, and I could argue that in such forms the hallux was "on
the way out" but not yet there. Usually all you see are the three main digits
II-IV.

So in most theropods, the hallux was carried off the ground and out of the
way. My guess is that it found a use in that group as a dew claw--otherwise
it would have been lost entirely, as in ratites and ornithomimids--and that
it may have been firmly bound to the metatarsus, so that in life only the
ungual and perhaps the preungual phalanx actually showed externally. So it
didn't actually "dangle uselessly" (a little bit of hyperbole here), but it
surely had very little potential to re-enlarge and re-acquire a set of
adductor and abductor muscles and their associated tendons that would work
the digit for perching.