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Re: dinosaur and perissodactyl digits

John R. Hutchinson wrote:
> I'm sure it's out there in the literature, but does anyone know how
> horse/other perissodactyl limbs develop? Modern horses just have digit III,
> which means they must have switched something around.

According to HORSES (Simpson 1951) and FOSSIL HORSES (MacFadden 1992),
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 present on the forefoot, but it was by
far the smallest of the four toes.  It was lost fairly rapidly, and was
pretty much gone by the early Oligocene.  Even in _Hyracotherium_, digit
III is the main one.  It became more so over time, and II and IV
deteriorated accordingly.  By the Pliocene, there were several genera in
which the side toes no longer touched the ground.  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. 

> <sarcasm>Gee whiz, could biology possibly be complex? Could simple
> explanations and reliance on strict dichotomies mask underlying complexity?
> Wow, that would be such a revelation.</sarcasm> Sigh.


-- JSW