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Re: mammal radiation ONLY after dinosaur extinction



John Bois wrote:

>       How do you know which adaptations lead to contemporary mammalian 
>diversity?  This is merely _your_ opinion.  One more to my liking is 
>Blair Hedges' writing in _Nature_ a couple of weeks ago and commenting 
>on his own molecular evidence and Archibald's fossils: "The existence of 
>the early ungulates really demonstrates that the evolutionary radiation 
>of the modern orders of placental mammals was well under way [100 
>million years ago]." (Also on pp. 1102 of the 24 May 
>_Science_ issue).

I think there is some confusion here between genetic diversity and
ecological or even phenotypic diversity.  Most of these mammals are
known as teeth only so our information is limited - but I think all
that Hedges' quote means is that the placental mammals of the Mesozoic
had already begun to diversify into separate lines equivalent to
modern groups.  It does NOT mean that anything like the diversity of
niche types, body sizes etc that existed even in the Paleocene existed
during the Cretaceous.  Obviously there must have been niche
diversification in Cretaceous placentals but this would have been in a
much narrower compass than that available to them after the K/T (even
if the only major new increase was in body size, this opens up a huge
range of diversity in phenotype and ecology).  I suspect that most
Mesozoic mammals would have fit nicely in the average zoo's small
mammal house with most visitors being unable to tell them apart -
however different their teeth may have been.  For a modern equivalent
consider the extreme physical similarity between tupaiids and some
Asian squirrels that live in the same area - mammals belonging to
widely divergent orders within the placentals.

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Ronald I. Orenstein                           Phone: (905) 820-7886 (home)
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