Re: _Eomaia_ and dung-eating vultures...

[Jeff Hecht <jeff@jeffhecht.com>]

At 12:49 AM +0200 4/28/02, David Marjanovic wrote:
>  > > Just how can one tell that a mammal fossil is placental or not?
>
> One thing is certain -- that *Eomaia* did not reproduce like living
> placentals: It retains epipubes ("marsupial bones" -- actually a
> synapomorphy of something like mammals in general), and its pelvic outlet is
> very narrow. Therefore it is assumed that it behaved like a pouchless
> marsupial in this respect (there are some alive), and that this is the
> plesiomorphy for live-bearing mammals.

While doing some housecleaning, I came across a paper by Michael 
Novacek et al in the 2 October 1997 Nature (V. 389, p. 483) titled 
"Epipubic bones in eutherian mammals from the Late Cretaceous of 
Mongolia" which reports "the first record of epipubic bones in two 
distinct eutherian lineages, dating from 75 million years ago.

The Eomaia paper cites Novacek, but does not address the issues about 
the evolution of placental reproduction raised by the presence of the 
epipubic bone. Looking at the phylogeny given for Eomaia, it seems 
likely that modern "placental" reproduction did not evolve 
immediately after the split between placentals and marsupials. That 
raises some interesting evolutionary questions, and implies a 
potentially significant chunk of the phylogentic tree of eutherians 
between Eomaia and true placentals has gone extinct. -- Jeff Hecht