Quoting "Jerry D. Harris" <jharris@dixie.edu>:
Hedman, M.M. 2010. Constraints on clade ages from fossil outgroups. Paleobiology 36(1):16-31. doi: 10.1666/0094-8373-36.1.16. ABSTRACT: ...this method indicates that if all Mesozoic eutherians are regarded as outgroups of Placentalia, then the last common ancestor of modern placental mammals and their Cenozoic allies lived between 65 and 88?98 million years ago, depending on the assumed cladogram and the number of outgroups included in the analysis.
Somewhere between 65 million and 8.9 billion years ago? I'd say that's a fair bet...
-- **************************************************************** Nicholas J. Pharris