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No comment (most of this is WAY over my head!), just an answer to a

GSP1954@aol.com wrote:

> Why the avetheropod-bird clade would initially emphasize the development of
> III rather than IV is obvious. Loss of digit one in even the embryos would
> leave a big gap between the pisiform in the side of the wrist and metacarpal
> IV, unless the other digits shifted laterally. So IV would be were V was, and
> III would be were IV was. If the shift is not made from the get go, it is
> going only going to have to occur at some point later. Also, IV will be
> entirely lost. To follow the usual tetrapod finger growth pattern would
> require IV to grow large in embryos, then completely lose it later on (Burke
> & Feduccia say that some lizards sharply reduce the size of IV, but it is not
> completely lost. Does anyone know what happens to digit IV in horse embryos?)
>  This would be a waste of growth energy, and natural selection does not work
> to make finger burds convenient for embyrologists to count, but to maximize
> efficiency of growth. The severe asymmetry of finger growth in
> theropods-birds should have forced them to reconfigure the growth pattern, so
> that III is initially emphasized rather than IV, and the latter is never more
> than a stub before it is eliminated.

According to MacFadden' FOSSIL HORSES, horses never had digit I, except
in _Hyracotherium_'s forefoot where it was a simple splint. 
_Hyracotherium's_ hindfoot doesn't seem to have had either I or V. 
Digit V was also lost from the forefoot over time.  Even in
_Hyracotherium_, digit III is the main one.  It became more so over
time, and II and IV deteriorated.  In modern _Equus_, II and IV are both
represented by simple splints of bone, except in atavistic individuals
where one or both grow into reduced hooves.  What happens in horse
embryos I'm not sure, but I would guess that II and IV simply stop
growing at some early stage of development.

-- JSW