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RE: No Cretaceous placental mammals?

> From: owner-DINOSAUR@usc.edu [mailto:owner-DINOSAUR@usc.edu]On Behalf Of
> Guy Leahy
>
>
> If this interpretation is correct, perhaps the
> end-Cretaceous impact event had a more profound
> influence on mammal evolution than previously thought?
>
> http://www.msnbc.msn.com/id/19335501/
>
The paper in question is:
Wible, J.R., G.W. Rougier, M.J. Novacek & R.J. Asher. 2007. Cretaceous 
eutherians and Laurasian origin for placental mammals near
the K/T boundary. Nature 447:1003-1006.

They find that no crown-group eutherian is known from the Cretaceous, and thus 
the minimum divergence date is in the earliest
Cenozoic. (However, the authors admit that it might well be earlier).

The new eutherian phylogeny, produced with the inclusion of _Maelestes 
gobiensis_ of the Djadokhta, and using a considerable spread
of Cretaceous eutherians and Paleogene mammals finds:
* monophyletic Euarchontaglires, Laurasiatheria, Xenarthra, and Afrotheria (the 
latter two as sister taxa, and together the sister
taxon to laurasiatheres)
* the sister group to Placentalia is a clade comprised of _Purgatorius_, 
_Protungulatum_, and _Oxyprimus_
* more distal outgroups are lepticids, zalambdalestids, _Deccanolestes_, a 
Asioryctotheria + Cimolestida clade (the latter includes
_Maelestes_), an _Eozhelestes_ + _Paranyctoides_ clade, zhelestids, 
_Montanaletes_, _Bobolestes_, and an (_Eomaia_ +
_Prokennalestes_)+ _Murtoilestes_ clade
* Eutherians that survived the K/Pg extinction include: _Cimolestes_, 
lepticids, the _Purgatorius_ clade, and the placental
ancestors.

This parallels previous work on metatherian phylogeny, where crown group 
metatheres (aka Marsupalia proper) are also restricted to
post-Cretaceous fossils, with a minimum divergence date in the earliest 
Cenozoic.

                Thomas R. Holtz, Jr.
        Senior Lecturer, Vertebrate Paleontology
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
        Mailing Address:
                Building 237, Room 1117
                College Park, MD  20742

http://www.geol.umd.edu/~tholtz/
http://www.geol.umd.edu/~jmerck/eltsite
Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-405-0796