Placentalia is one of the three extant subdivisions of the class of animals Mammalia; the other two are Monotremata and Marsupialia. The 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, though for a relatively briefer period, giving birth to less developed young who are then kept for a period in the mother's pouch.
Placental mammals are anatomically distinguished from other mammals by:
- a sufficiently wide opening at the bottom of the pelvis to allow the birth of a large baby relative to the size of the mother.
- the absence of epipubic bones extending forward from the pelvis, which are found in all other mammals. (Their function in non-placental mammals is to stiffen the body during locomotion, but in placentals they would inhibit the expansion of the abdomen during pregnancy.)
- the rearmost bones of the foot fit into a socket formed by the ends of the tibia and fibula, forming a complete mortise and tenon upper ankle joint.
- the presence of a malleolus at the bottom of the fibula.
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. 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 living orders of placental mammals in the three groups are:
- Magnorder Afrotheria (elephant shrews, tenrecs, golden moles, hyraxes, elephants, and manatees)
- Superorder Afroinsectiphilia
- Superorder Paenungulata
- Magnorder Boreoeutheria
- Superorder Euarchontoglires (treeshrews, colugos, primates, rabbits, hares, and rodents)
- Grandorder Gliriformes
- Grandorder Euarchonta
- Superorder Laurasiatheria (hedgehogs, shrews, moles, whales, bats, dogs, cats, seals, and hoofed mammals)
- Order Eulipotyphla (hedgehogs, gymnures, shrews, moles, and solenodons)
- Order Chiroptera (bats)
- Order Artiodactyla (even-toed ungulates: cattle, antelope, deer, camels, pigs, whales, etc.)
- Order Perissodactyla (odd-toed ungulates: horses, donkeys, zebras, rhinoceroses, and tapirs)
- Mirorder Ferae (pangolins, dogs, cats, bears, seals, mongooses, etc.)
- Superorder Euarchontoglires (treeshrews, colugos, primates, rabbits, hares, and rodents)
- Magnorder Xenarthra (armadillos, anteaters, and sloths)
The exact relationships among these three lineages is currently a subject of debate, and three 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), and Exafroplacentalia (basal Afrotheria). 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) and varying interpretations of paleogeographic data.
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.
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 was thought to be a stem-ungulate  known 1 meter above the Cretaceous-Paleogene boundary in the geological stratum that marks the Cretaceous–Paleogene extinction event  and Purgatorius, previously considered a stem-primate, appears no more than 300,000 years after the K-Pg boundary; both species, however, are now considered non-placental eutherians. 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. The lineages leading to Xenarthra and Afrotheria probably originated around 90 MYA, and Boreoeutheria underwent an initial diversification around 70-80 MYA, producing the lineages that eventually would lead to modern primates, rodents, insectivores, artiodactyls, and carnivorans.
However, modern members of the placental orders originated in the Paleogene around 66 to 23 MYA, following the Cretaceous–Paleogene extinction event. The evolution of crown orders such modern primates, rodents, and carnivores appears to be part of an adaptive radiation 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. As they occupied new niches, mammals rapidly increased in body size, and began to take over the large herbivore and large carnivore niches that had been left open by the decimation of the dinosaurs. Mammals also exploited niches that the 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. Primates, meanwhile, acquired specialized grasping hands and feet which allowed them to grasp branches, and large eyes with keener vision which allowed them to forage in the dark.
The evolution of land placentals followed different pathways on different continents since they cannot easily cross large bodies of water. An exception is smaller placentals such as rodents and primates, who left Laurasia and colonized Africa and then South America via rafting.
In Africa, the Afrotheria underwent a major adaptive radiation, which led to elephants, elephant shrews, tenrecs, golden moles, aardvarks, and manatees. In South America a similar event occurred, with radiation of the Xenarthra, which led to modern sloths, anteaters, and armadillos, as well as the extinct ground sloths and glyptodonts. Expansion in Laurasia was dominated by Boreoeutheria, which includes primates and rodents, insectivores, carnivores, perissodactyls and artiodactyls. These groups expanded beyond a single continent when land bridges formed linking Africa to Eurasia and South America to North America.
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