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RE: Ichthyostega digits



From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of philidor11 
 > Figure 4 on the following page shows that ichthyostega apparently had 7 digits, 3 small and close together, 4 larger behind: 
  > http://phylogeny.arizona.edu/eukaryotes/animals/chordata/ichthyostega/ichthyostega.html
Can someone tell me, or point the direction to find out, whether 7 digits is a widespread condition, and whether this has  
> significance for the separate 5th digit which in one group is a thumb? 
 Thanks.  
 
Other primitive stegocephalians had many digits: Acanthostega had eight fingers, Tulerpeton had six fingers.  No post-Devonian stegocephalian (i.e., tetrapods and their closest allies) had more than five digits per chiridium (hand or foot), except in pathological conditions.  See the related pages for the various Devonian critters linked on:
 
As for Ichthyostega: it is a foot, not a hand, so the "clumping" of digits is in the region of the big toe, not the thumb (and there actually are probably four digits there: the small medialmost one is somewhat obscured underneath the next one in).  The precise homologies of the individual digits is problematic at present, but is of interest to developmental biologists. 

                Thomas R. Holtz, Jr.
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