The infraclass Placentalia /plæsənˈteɪliə/ is one of the three extant subdivisions of the class of animals Mammalia; the other two are Monotremata and Marsupialia. Placentalia contains the vast majority of extant mammals. Placentals are partly distinguished from other mammals in that the fetus is carried in the uterus of its mother to a relatively late stage of development. The name is something of a misnomer considering that marsupials also nourish their fetuses via a placenta,[1] though for a relatively briefer period, giving birth to less developed young who are then kept for a period in the mother's pouch.
Analysis of retroposon presence/absence patterns has provided a rapid, unequivocal means for revealing the evolutionary history of organisms: this has resulted in a revision in the classification of placentals.[6] There are now thought to be three major subdivisions or lineages of placental mammals: Boreoeutheria, Xenarthra, and Afrotheria, all of which diverged from common ancestors.
The exact relationships among these three lineages is currently a subject of debate, and four different hypotheses have been proposed with respect to which group is basal or diverged first from other placentals. These hypotheses are Atlantogenata (basal Boreoeutheria), Epitheria (basal Xenarthra), Exafroplacentalia (basal Afrotheria) and a hypothesis supporting a near simultaneous divergence.[8] Estimates for the divergence times among these three placental groups range from 105 to 120 million years ago (MYA), depending on the type of DNA (e.g. nuclear or mitochondrial)[9] and varying interpretations of paleogeographic data.[8]
As of 2020[update], the genome has been sequenced for at least one species in each extant placental order and in 83% of families (105 of 127 extant placental families).[12]
True placental mammals (the crown group including all modern placentals) arose from stem-group members of the clade Eutheria, which had existed since at least the Middle Jurassic period, about 170 MYA. These early eutherians were small, nocturnal insect eaters, with adaptations for life in trees.[5]
True placentals may have originated in the Late Cretaceous around 90 MYA, but the earliest undisputed fossils are from the early Paleocene, 66 MYA, following the Cretaceous–Paleogene extinction event. The species Protungulatum donnae is sometimes placed as a stem-ungulate [13] known 1 meter above the Cretaceous-Paleogene boundary in the geological stratum that marks the Cretaceous–Paleogene extinction event [14] and Purgatorius, sometimes considered a stem-primate, appears no more than 300,000 years after the K-Pg boundary;[15] both species, however, are sometimes placed outside the crown placental group, but many newer studies place them back in. eutherians[further explanation needed].[16] The rapid appearance of placentals after the mass extinction at the end of the Cretaceous suggests that the group had already originated and undergone an initial diversification in the Late Cretaceous, as suggested by molecular clocks.[17] The lineages leading to Xenarthra and Afrotheria probably originated around 90 MYA, and Boreoeutheria underwent an initial diversification around 70-80 MYA,[17] producing the lineages that eventually would lead to modern primates, rodents, insectivores, artiodactyls, and carnivorans.