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Placental Mammals

Placentalia

Placental mammals are warm-blooded vertebrates belonging to the infraclass Placentalia. They are one of the three extant subdivisions of the class Mammalia, the other two being Monotremata and Marsupialia. Placentals are so named because they undergo complete embryonic development inside their mother’s uterus, nourished by a temporary vascular organ called the placenta. They are also the only living group under Eutheria or Pan-Placentalia, the clade containing all mammals more closely related to placentals than the other group, the marsupials.

Although marsupials, too, are initially nourished by a mother’s placenta, the organ is less developed in them, limiting the gestation period to a few weeks. In contrast, placentals are nourished till much later stages of development and are born as relatively independent neonates.

Unique Anatomical Features

Placental mammals have certain anatomical features that set them apart from other mammals.

Taxonomy

Based on molecular studies and sequencing data, about 4000 described species of placentals are classified under three broad subdivisions: Boreoeutheria, Xenarthra, and Afrotheria, all of which diverged from common ancestors. 

Although the relationship between the three lineages is uncertain, four hypotheses are proposed regarding which group diverged first from other placentals (basal). While the Atlantogenata hypothesis considers Boreoeutheria as the basal group, the Epitheria and Exafroplacentalia hypotheses place Xenarthra and Afrotheria as the same. Based on the type of DNA and interpretations of paleogeographic data, the three placental groups diverged around 105 to 120 million years ago.

Placental Mammals (Placentalia)

Evolution

Habitat

Although most placentals, such as ungulates, rodents, carnivores, and primates, are terrestrial, seals, whales, and manatees are aquatic.

Diet

While ungulates and elephants are herbivorous, feeding on herbs and leaves of shrubs and trees, carnivores primarily consume animal flesh. Some placentals, like raccons and bears, are omnivores.

References Article last updated on 15th May 2024
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