<Li> Mitochondrial DNA's mutation rate in mammals varies from region to region--some parts hardly ever change and some change extremely quickly and even show large variations between individuals within the same species . </Li> <Li> Mammalian mitochondrial DNA mutates so fast that it causes a problem called "saturation", where random noise drowns out any information that may be present . If a particular piece of mitochondrial DNA mutates randomly every few million years, it will have changed several times in the 60 to 75M years since the major groups of placental mammals diverged . </Li> <P> 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 . Fossil - based analyses, on the contrary, limit the placentals to the Cenozoic . 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 . On the other hand, there is fossil evidence from 85 million years ago of hoofed mammals that may be ancestors of modern ungulates . </P> <P> Fossils of the earliest members of most modern groups date from the Paleocene, a few date from later and very few from the Cretaceous, before the extinction of the dinosaurs . But some paleontologists, influenced by molecular phylogenetic studies, have used statistical methods to extrapolate backwards from fossils of members of modern groups and concluded that primates arose in the late Cretaceous . However, statistical studies of the fossil record confirm that mammals were restricted in size and diversity right to the end of the Cretaceous, and rapidly grew in size and diversity during the Early Paleocene . </P>

When do most of the existing groups of mammals appear in the fossil record
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