RE: No Cretaceous placental mammals?

["Thomas R. Holtz, Jr." <tholtz@geol.umd.edu>]

Responding to a couple of different posts:

> From: owner-DINOSAUR@usc.edu [mailto:owner-DINOSAUR@usc.edu]On Behalf Of
> Richard Hing
>
> >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.
>
> Hi there, do you have the references to hand on this?
>
Sure, it's the paper in which _Sinodelphys_ is described:

Luo, Z.-X., Q. Ji, J.R. Wible, C.-X. Yuan. 2003. An Early Cretaceous 
Tribosphenic Mammal and Metatherian Evolution. Science
302:1934-1940.

Marsupalia = crowngroup Metatheria = Didelphimorphia + Paucituberculata has a 
time of divergence 65-62 Ma in that study.

> From: owner-DINOSAUR@usc.edu [mailto:owner-DINOSAUR@usc.edu]On Behalf Of
> Rob Janes
>
> if i understand this correctly, they have not found direct ancestors of
> modern placental mammals, only relatives on dead ends.  and yet some genetic
> mapping profiles indicate that placental mammals diverged from the rest well
> back in the cretaceous.

Not exactly. No one doubts that Eutheria (= placentals and everything more 
closely related to  placentals than to marsupials)
diverged from Metatheria (used definition above, but switch the words 
"marsupials" and "placentals"...) in the mid-Mesozoic. There
are fossils of basal eutherians and basal metatherians throughout the 
Cretaceous.

The real debate here is whether the divergences between the major groups of 
placentals happened well before the K/Pg boundary, or
near it, or after it. That is, was the last common ancestor of elephants, 
tigers, sloths, moles, and humans in danger of being eaten
by _Deinonychus_, or a baby _T. rex_, or some early Paleocene monitor lizard?

> it's possible for both statements to be true, but only if we haven't found
> the fossils yet.

It is certainly possible, as the authors admit.