Stagodont marsupials, placentals and multituberculate mammals scurried underfoot. Modern mammals are subdivided into therians (modern members are placentals and marsupials) and monotremes. There are three extant kinds of mammals&colon; monotremes, placentals and marsupials, all with internal fertilization. The fossil has skeletal features that are closer to modern placentals than to marsupials. The first placentals and marsupials evolved in the Paleocene. Genetic analysis suggests a divergence date between the marsupials and the placentals at . Fossil-based analyses, on the contrary, limit the placentals to the Cenozoic. This would make pantodonts crown-group ungulate placentals and not related to cimolestids at all. This stiffening would be harmful in pregnant placentals, whose abdomens need to expand. The placentals give birth to relatively complete and developed young, usually after long gestation periods. These debates extend to the definition of and relationships between the major groups of placentals. Only placentals, and perhaps "Megazostrodon" and "Erythrotherium", would lose these. This uncertainty blurs the early evolution of placentals. In male placentals, the penis is used both for urination and copulation. Modern mammals (class "Mammalia") are divided into Monotremes, Marsupials, and Placentals.