Re: Dinosaur hallx(es)

[Dinogeorge@aol.com]

In a message dated 97-08-24 03:04:00 EDT, m_troutman@hotmail.com (Matthew
Troutman) writes:

<< When looking at the evidence alot of it does not add up. Perhaps in 
 some theropods (from Compsognathus down) the toe is like a hominids 
 thumb. It is mobile backwards for holding onto things and mobile 
 medially for times of running. Just my thoughts on the matter I could be 
 wrong Icould be right.  >>

The hallux in cursorial, terrestrial theropods is to a great degree
vestigial, so you cannot tell from a quick inspection just how effective or
functional it was in ancestral theropods (whatever they were). In theropods
with which we're all familiar, it was probably just imbedded in the flesh of
the foot and lacked any grasping ability at all; it was just a dew claw,
perhaps having uses in intraspecific combat, that stuck out sideways from the
back of metatarsal II.

If theropods had always been cursorial, terrestrial animals, in which
reduction of the medial and lateral digits of the foot is very common
(presumably to reduce the foot's moment of inertia by getting rid of unneeded
bone, muscle, and connective tissue), reduction of the first digit would
certainly have paralleled that of the fifth. The fifth digit in all theropods
(and convergently in most other dinosaurs) is reduced to a splint metatarsal
that articulates proximally with the ankle. But the first digit follows a
very different pattern. In all theropods it articulates close to the >distal<
end of the metatarsal bundle, at the >back< of metatarsal II. If the first
digit were reduced simply to save weight and lower the moment of inertia of
the foot in a cursorial animal, then it would have remained as far >up< the
foot as possible. Why would evolution have gone to all the trouble of moving
the digit >down< the foot (where it increases the moment of inertia) and to
the back of the foot, if it's only necessary to reduce the digit? Why would
evolution >retain< a functional claw on the digit--which is long gone from
digit V? Surely evolution could not know in advance that these would be
favorable features for the digit when it came time to evolve a perching foot!

No, the retroverted hallux >preceded< the reduced hallux of cursorial
theropods. It moved down and to the back of the foot because the foot evolved
a perching function >before< theropods became cursorial, when theropods were
small, arboreal animals on their way to becoming flying birds. In each
separate lineage of cursorial theropods--which quit the arboreal, flying
lifestyle to become terrestrial cursors--the retroverted hallux reduced
convergently, because that is the prevalent fate of that digit in a cursorial
animal. But it was stuck down at the back of the foot, and that's where it
stayed.