DIGIT DEVELOPMENT ARTICLE

[darren.naish@port.ac.uk]

Steve Brusatte wrote...

> Awhile back (oh, probably June) we had a nice discussion onlist about
> the loss of digits in theropods, and whether or not there was an
> evolutionary "pressure" keeping these digits from re-evolving.  

If you are interested in digit development there was a fascinating 
review article entitled 'Why only 5 digits?' (I think) in a recent issue of 
_Trends in Evolution & Ecology_ (incidentally, there's also a recent 
issue with an overview of cetacean origins - it's a good job I never did 
submit that article on archaeocetes to _Geology Today_:))

It is more concerned with polydactyly than the loss of digits but is 
nonetheless a good read. The main thrust of the article is discussion of 
pleiotropism as a constraint on digit reduction/addition [pleiotropism 
= effects of a gene on more than one structure]. In nearly all tetrapods, 
digit development in embryos occurs at the same time as the rest of the 
skeleton forms. Losses or additions of digits in the embryo therefore 
apparently have (often detrimental) effects elsewhere. Polydactyly in 
domestic dogs, for example, results in gastrointestinal deformities and 
other problems and lead to short lifespans compared to non-polydactyl 
dogs, even those of much smaller body size. There is also a famous 
case-study of a bunch of polydactyl guinea-pigs that had severe 
deformities in virtually every organ system, apparently because of 
pleiotropism.

In lissamphibians however, digit formation is separate from that of the 
rest of the skeleton and thus frogs and salamanders can do pretty much 
anything in terms of digit development without adverse pleiotropic 
effects. Thus the prepollex and prehallux might be true digits (thus 
some frogs are truly six-toed). There is obviously more to it than this 
and I'm providing a simplified summary based on memory.

Needless to say this is all somehow relevant to digit loss in theropods 
and birds. This may be discussed in the article - I'll have to reread 
(time, not embryological development, is my constraint).

DARREN NAISH 
PALAEOBIOLOGY RESEARCH GROUP
School of Earth & Environmental Sciences
UNIVERSITY OF PORTSMOUTH
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