when do most of the existing groups of mammals appear in the fossil record
Evolution of mammals Recent molecular phylogenetic studies suggest that most placental orders diverged late in the Cretaceous period, about 100 to 85 million years ago, but that modern families first appeared later, in the late Eocene and early Miocene epochs of the Cenozoic period.[98][99] Fossil-based analyses, on the contrary, limit the placentals to the Cenozoic.[100] Many Cretaceous fossil sites contain well-preserved lizards, salamanders, birds, and mammals, but not the modern forms of mammals. It is likely that they simply did not exist, and that the molecular clock runs fast during major evolutionary radiations.[101] On the other hand, there is fossil evidence from 85Â million years ago of hoofed mammals that may be ancestors of modern ungulates.[102]