Temporal range: Paleocene-Holocene, 65–0Ma
|House Mouse, Mus musculus|
Placentalia is a group of mammals. The majority of living mammals are placental: the other two living mammal groups are the Monotremata and the Marsupialia. The placentals are distinguished from other mammals in that the fetus is nourished during gestation via a placenta while, in general, this is not the case with other mammals (Bandicoots are an exception). They reproduce sexually, and the young is carried in the mother until fully developed. Placental mammals are viviparous.
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 mid-Jurassic period. These early eutherians are small, nocturnal insect eaters, with adaptations for life in trees.
True placentals probably originated in the Late Cretaceous around 90 million years ago, but the earliest undisputed fossils are from the early Paleocene, 66 million years ago, following the disappearance of the dinosaurs. The stem ungulate Protungulatum donnae  is known 1 meter above the Cretaceous-Paleogene boundary that marks the disappearance of the dinosaurs  and the stem primate Purgatorius appears no more than 300,000 years after the K-Pg boundary  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 million years ago, and Boreoeutheria underwent an initial diversification around 70-80 million years ago, producing the lineages that eventually would lead to modern primates, rodents, insectivores, artiodactyls, carnivorans, and so forth. Consistent with this, a single tooth of Protungulatum has recently been discovered below the K-Pg boundary.
However, modern members of the placental orders originated in the Paleogene, following the extinction of the dinosaurs. The evolution of crown orders- modern primates, modern rodents, modern carnivores and so forth- 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 the 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 roles that had been left open by the disappearance of the dinosaurs. Mammals also exploited niches that the dinosaurs had never touched. Bats evolved flight and echolocation, allowing them to be highly effective nocturnal, aerial insectivores. Whales occupied freshwater lakes and rivers, and then moved into the oceans. Primates, meanwhile, evolved specialized grasping hands and feet to allow them to grasp branches, and large eyes to allow them to forage visually in the dark.
Because mammals (except for marine mammals) cannot easily cross large bodies of water, the evolution of placentals happened differently on different continents. In Africa, then an island continent, the Afrotheria launched a major adaptive radiation, which would lead to elephants, elephant shrews, tenrecs, golden moles, aardvarks, and manatees. South America, likewise isolated by water, saw a major radiation of Xenarthra, which includes modern sloths, anteaters, and armadillos, as well as the extinct ground sloths and glyptodonts. Laurasia was dominated by Boreoeutheria, which includes primates and rodents, insectivores, carnivores, perissodactyls and artiodactyls. Later, as land bridges formed to link Africa to Eurasia and South America to North America, these groups became more widespread. Rafting appears to play a role in letting smaller placentals disperse, allowing rodents and primates to colonize Africa and then South America.
Placental mammals are distinguished from other eutherians by:
- the presence of a malleolus at the bottom of the fibula, the smaller of the two shin bones.
- a complete mortise and tenon upper ankle joint, where the rearmost bones of the foot fit into a socket formed by the ends of the tibia and fibula.
- a wide opening at the bottom of the pelvis, which allows the birth of large, well-developed offspring. Marsupials and nonplacental eutherians have a narrower opening that allows only small, immature offspring to pass through.
- the absence of epipubic bones extending forward from the pelvis, which are not found in any placental, but are found in all other mammals – nonplacental eutherians, marsupials, monotremes, and earlier mammaliaforms - as well as in other cynodonts that are closest to mammals. Their function is to stiffen the body during locomotion. This stiffening would be harmful in pregnant placentals, whose abdomens need to expand.
Placentals are divided into three major groups:
- Boreoeutheria, e.g. badgers, rabbits, chimpanzees, guinea pigs, dogs
- Xenarthra, e.g. armadillos, anteaters
- Afrotheria, e.g. elephants, hyraxes, manatees
Molecular studies based on DNA analysis have revised the understanding of relationships among placental groups during the 21st century. Classification systems based on molecular studies reveal three major groups or lineages of placental mammals: Afrotheria, Xenarthra, and Boreoeutheria, all of which diverged from common ancestors in the Cretaceous. The exact relationships between 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). Boreoeutheria in turn contains two major lineages- Euarchontoglires and Laurasiatheria.
Estimates for the divergence times among these three placental groups range from 105 to 120 million years ago, depending on the type of DNA (e.g. nuclear or mitochondrial) and varying interpretations of paleogeographic data.
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