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.
They have a wide opening at the bottom of the pelvis, facilitating easy birth of a relatively large baby.
A bony protrusion called the malleolus is found at the bottom of the calf bone or fibula.
In the upper ankle joint, the rearmost bones of the foot fit into a socket composed of the ends of the tibia and fibula.
Unlike marsupials, they have given away their epipubic bones, which extend from the pubis, to enable free expansion of the abdomen during pregnancy.
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.
Order: Carnivora (carnivorans, such as dogs, cats, bears, seals, mongooses, raccoons, and skunks)
Evolution
The earliest true placental mammals were believed to have originated from stem-group members of the clade Eutheria around 90 million years ago during the Late Cretaceous Period. However, the earliest undisputed placental fossils belong to the Paleocene Epoch (66 million years ago), following the Cretaceous–Paleogene extinction event.
After this event, the rapid growth of placentals indicates that the group had already originated and started diversifying around the Late Cretaceous Period. The Xenartha and Afrotheria lineages probably originated around 90 million years ago, whereas Boreoeutheria started diversifying around 70 to 80 million years ago.
The modern placental orders originated in the Paleogene Period (66 to 23 million years ago) when extinction washed off most dinosaurs and other large tetrapods. The crown orders, like modern primates, rodents, and carnivores, underwent adaptive radiation and filled the ecological niches left behind by the extinct animals.
With time, placentals grew in size and started exploring niches the non-avian dinosaurs had never exploited. For instance, bats adopted an aerial lifestyle, while whales had taken to freshwater lakes and rivers and switched to the oceans.
In Africa, the lineage Afrotheria went through extensive adaptive radiation and gave rise to elephants, elephant shrews, tenrecs, golden moles, aardvarks, and manatees. Similarly, in South America, Xenarthra diversified into modern sloths, anteaters, armadillos, and the extinct ground sloths and glyptodonts. The lineage of Boreoeutheria gradually diverged into primates, rodents, carnivorous mammals, perissodactyls, and artiodactyls.
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.
