Placentalia

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fossil Daeodonan extinct placental mammal.

The placentals (Placentalia) are an infraclass of mammals. They are characterized by the fact that the young are retained in the maternal uterus for a long time where they are fed by an allantoic placenta. More than 5,100 species are currently known.

Placentals originated during the Cretaceous between 135-120 million years, so they must have lived with the dinosaurs for a time, after the mass extinction of the Cretaceous-Paleogene, they had a great diversification and today they constitute the dominant terrestrial vertebrates.

Placentals belong to Eutheria which is a clade that includes placentals and all mammals more closely related to placentals than to marsupials (Metatheria). The clades were proposed by Thomas Huxley in 1880 to encompass broader groups of placentals and marsupials, but the two terms did not become synonymous. The oldest eutherian fossil is Juramaia, a small insectivorous mammal that lived during the Upper Jurassic 160 million years ago, can be interpreted as an ancestor of the placentals.

Features

Placentals are viviparous mammals in which the embryo develops inside the maternal uterus for a long time, where it is fed by an allantoic placenta. There is no marsupial pouch in them, nor are there any epipubic bones.

In the skull there is usually a separate optic foramen and the palatal foramen are absent; the angle of the mandible does not retract inward. The tympanic bone may be annular or form a blister on the alisphenoid. The dental formula is 3·1·4·3 in both jaws, or a reduction thereof. They present two different dentitions throughout their lives, one in the infant phase (milk teeth) and another in the adult.

The brain has large cerebral hemispheres connected by a corpus callosum.

There is no sewer.

Primitive insectivores probably possessed most of these characteristics, as well as others common to all primitive mammals, such as small size, short legs with five-toed plantigrade feet, an elongated face, and a tubular skull that encloses a small brain.

The descendants of these primitive mammals underwent changes in these characteristics; such changes occurred in at least some members of all subsequent evolutionary lines.

Size

Many placentals increased in size, which seems to be related to having a large brain that can store a lot of information throughout the animal's life, and allows for long life and slow reproduction (K strategy). This introduces new problems such as obtaining a sufficient amount of food.

Legs

The legs were elongated and specialized for various types of locomotion, almost always raising the heel off the ground and going from plantigrade to digitigrade gait. In parallel, the number of fingers was reduced. On the other hand, various evolutionary lines became aquatic, with transformation of the forelimbs into flippers, often accompanied by polyphalangia (increased number of phalanges).

Teeth

The number of teeth has been reduced several times, and has even disappeared (xenarthrans), and their shape has specialized, generally adding cusps and fusing them to form longitudinal or transverse crushing ridges in herbivores or cutting blades in carnivores.

Brain

The brain of primitive mammals was similar to that of reptiles; in the most advanced forms there is an increased development of the non-olfactory regions of the cortex, an increase in the frontal lobes and other changes related to more elaborate behavior and better memory.

Evolutionary history

The oldest fossils date from the Late Cretaceous, during this period the placentals were insectivorous. However, the most recent common ancestor among placentals is thought to have lived between 120-135 million years ago.

At the end of the Cretaceous and during the Paleocene the original placental trunk was branching rapidly; some can already be included among the carnivores, primates and ungulates, but they are very different from the present forms of these groups and all of them could have derived from an insectivorous ancestor.

In the Eocene most of the current orders were already well established. The main period of expansion of mammals occurred, then, at the beginning of the Cenozoic and, currently, the group can be considered to be in decline; except for bats, rodents, lagomorphs, and cetartiodactyls, the remaining orders have few living species.