Placentalia contains the vast majority of extant mammals, which are partly distinguished from monotremes and marsupials in that the fetus is carried in the uterus of its mother to a relatively late stage of development.

2022 studies of Bertrand, O. C. and Sarah L. Shelley have identified palaeoryctids and taeniodonts as basal placental mammal clades.

[13] Estimates for the divergence times among these three placental groups mostly range from 105 to 120 million years ago (MYA), depending on the type of DNA, whether it is translated, and the phylogenetic method (e.g. nuclear or mitochondrial),[14][15] and varying interpretations of paleogeographic data.

[13] In addition, a strict molecular clock does not hold, so it is necessary to assume models of how evolutionary rates change along lineages.

The evolution of crown orders such modern primates, rodents, and carnivores appears to be part of an adaptive radiation[25] that took place as mammals quickly evolved to take advantage of ecological niches that were left open when most dinosaurs and other animals disappeared following the Chicxulub asteroid impact.

Mammals also exploited niches that the non-avian dinosaurs had never touched: for example, bats evolved flight and echolocation, allowing them to be highly effective nocturnal, aerial insectivores; and whales first occupied freshwater lakes and rivers and then moved into the oceans.

An exception is smaller placentals such as rodents and primates, who left Laurasia and colonized Africa and then South America via rafting.