Re: vertebrate numbers of digits

[Dennis C Hwang <dchwang@itsa.ucsf.edu>]

On Tue, 28 Oct 1997 wa105@mead.anglia.ac.uk wrote:

> Jeffrey martz wrote:
> 
> >     All vetetebrates had five digits ancestrally...
> 
> This is the second time this has been said in this thread.  It is not true, 
> as 
> you'll realise if you think for a moment.  Ancestral vertebrates had no 
> digits 
> at all, and half of the world's verts still use fins.  Early tetrapods had 
> varying numbers of digits.  Ancestral amniotes probably had five.  The 
> earliest 
> lissamphibians probably had five on the hind limb, but I have no idea whether 
> they had four or five on the forelimb. 

Actually, IIRC, hasn't it been noted that the early tetrapods
(_Ichthyostega_ and _Acanthostega_) actually had up to eight digits per
foot?
 
> It is a puzzle to me why tetrapod lineages have often reduced their numbers 
> of 
> digits, but very rarely increased them.  (Icthyosaurs are the only group I 
> know 
> of.)  Polydactyly (additional fingers) occurs in humans and other tetrapods 
> and 
> often results in few if any ill effects.  So it is surprising that this 
> evolutionary avenue has so rarely been explored.

It's an interesting question.  In my experience, the incidents of
polydactyly (in humans) tend to relate either to prenatal teratogen
exposure (such as retinoic acid or thalidomide) or are linked to a number
of other birth defects, such as hypoplasia of the ribs and limbs
(Ellis-van Creveld syndrome).

Of course, there's a high incidence of polydactyly in (calico?) cats, too,
and I don't know if that's associated with any other genetic changes
(other than the fur pigmentation mosaicism).  The only other polydactylous
cat I've seen was a small black kitten with severe neurologic deficits as
well.

--Dennis
dchwang@itsa.ucsf.edu
xenopathologist at large!
Deathwalker for President:  for some *real* health care reform.