Placentals (Placentalia)

Infraclass of therians

Food, Animal source foods, Chordates (Chordata), Vertebrates (Vertebrata), Mammals (Mammalia), Therians (Theria)

Consumption area(s): Earth

Introduction

Placentals constitute one of the three branches of the class Mammalia, alongside Monotremata and Marsupialia. This group encompasses the vast majority of living mammals, primarily recognized by the fact that their embryos develop extensively within the mother’s uterus before birth.

The term Placentalia is somewhat misleading, since marsupials also form a temporary placenta to sustain their embryos. However, this connection lasts for a much shorter time, and marsupials deliver their offspring at an earlier developmental stage, after which the young continue to mature inside the maternal marsupium.

Description of placentals (Placentalia)

Placental mammals can be anatomically identified through several distinct traits that set them apart from other mammalian lineages. One notable feature a sufficiently wide opening at the bottom of the pelvis, which allows the passage of a relatively large newborn compared to the size of the mother.

Unlike other mammals, placentals lack epipubic bones, the forward-projecting structures of the pelvis found in marsupials and monotremes. In non-placental species, these bones stabilize the body during movement, but in placentals their absence permits the abdomen to expand significantly throughout gestation, accommodating fetal growth.

Another diagnostic feature lies in the hind limb structure: the posterior foot bones articulate within a socket created by the tibia and fibula, producing a true mortise-and-tenon joint at the upper ankle, enhancing stability and flexibility. Additionally, placentals exhibit a distinct malleolus—a bony projection at the lower end of the fibula—that further contributes to refined limb mechanics.

Unlike most other vertebrates, placentals do not have a cloaca. Instead, they feature separate openings: the urethra discharge through the vulva or penis, while the rectum ends in an anus, ensuring a clear functional separation between excretory and reproductive systems.

Finally, a defining neurological trait is the presence of a corpus callosum, a bridge connecting the two cerebral hemispheres, which enables advanced communication and coordination between both sides of the brain—a feature absent in monotremes and marsupials.

Classification of placentals (Placentalia)

Placentals are divided into several orders, which include:

• Afrosorcida (golden moles, tenrecs)
• Artiodactyls (Artiodactyla) (cattle, deer, dolphins, giraffes, goats, hippopotamuses, pigs, whales)
• Carnivorans (Carnivora) (dogs, bears, cats, seals, weasels)
• Chiroptera (bats)
• Cingulata (armadillos)
• Dermoptera (flying lemurs)
• Eulipotyphla (gymnures, hedgehogs, moles, shrews, solenodons)
• Hyracoidea (hyraxes)
• Lagomorphs (Lagomorpha) (hares, pikas, rabbits)
• Macrosecelidea (elephant shrews)
• Pholidota (pangolins)
• Pilosa (anteaters, sloths)
• Perissodactyla (horses, rhinoceroses, tapirs)
• Primates (lemurs, monkeys)
• Proboscidea (elephants)
• Rodents (Rodentia) (capybaras, beavers, dormice, mice, porcupines, rats, squirrels)
• Scandentia (pen-tailed tree shrews, treeshrews)
• Sirenians (Sirenia) (dugong, manatees)
• Soricomorpha (shrews, moles, solenodons)
• Tubulidentata (aardvark)