Placental mammals (infraclass Placentalia ) are one of the three extant subdivisions of the class Mammalia, the other two being Monotremata and Marsupialia. Placentalia contains the vast majority of extant mammals, which are partly distinguished from monotremes and marsupials 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 which are then nurtured for a period inside the mother's pouch.
Anatomical features
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.
Subdivisions
Analysis of molecular data lead to the rapid changes in assessments of the phylogeny of placental orders at the close of the last millennium. A novel phylogeny and classification of placental orders appeared with Waddell, Hasegawa and Okada in 1999. "Jumping Genes" retroposon presence/absence patterns have provided corroboration of phylogenetic relationships inferred from molecular sequences, for example . It is now widely accepted that there are four major subdivisions or lineages of placental mammals: Laurasiatheria+Euarchontoglires=Boreoeutheria, Xenarthra, and Afrotheria.
The living orders of placental mammals in the three groups are:
Magnorder Atlantogenata (armadillos, sloths, anteaters, aardvark, elephant shrews, golden moles, otter shrews, tenrecs, hyraxes, elephants, and sirenians) Superorder Xenarthra (armadillos, sloths, and anteaters) Order Cingulata (armadillos) Order Pilosa (sloths and anteaters) Superorder Afrotheria (aardvark, elephant shrews, tenrecs, otter shrews, golden moles, hyraxes, elephants, manatees, and dugong) Grandorder Afroinsectiphilia Order Tubulidentata (aardvark) Mirorder Afroinsectivora (elephant shrews, golden moles, otter shrews, and tenrecs) Order Afrosoricida (tenrecs, otter shrews, and golden moles) Order Macroscelidea (elephant shrews) Grandorder Paenungulata Order Hyracoidea (hyraxes) Mirorder Tethytheria (elephants, dugongs, and manatees) Order Proboscidea (elephants) Order Sirenia (dugongs and manatees) Magnorder Boreoeutheria Superorder Euarchontoglires (treeshrews, colugos, primates, rabbits, hares, and rodents) Grandorder Gliriformes Mirorder Glires Order Lagomorpha (rabbits, hares, and pikas) Order Rodentia (rodents: mice, rats, voles, squirrels, beavers, etc.) Grandorder Euarchonta Order Scandentia (treeshrews) Mirorder Primatomorpha Order Dermoptera (colugos) Order Primates (primates: humans, monkeys, apes, lemurs, lorises, etc.) Superorder Laurasiatheria (hedgehogs, shrews, moles, whales, bats, dogs, cats, seals, and hoofed mammals) Order Eulipotyphla (hedgehogs, gymnures, shrews, moles, and solenodons) Order Chiroptera (bats) Grandorder Ferungulata Mirorder Euungulata 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.) Order Pholidota (pangolins) Order Carnivora (carnivorans: dogs, cats, bears, seals, mongooses, etc.)