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vertebrate numbers of digits

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. 

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.

                                                  Just my thoughts,
                                                                      Bill Adlam