August 15, 2022
Mammals (Mammalia) are a class of vertebrate animals characterized by the presence of fur (except for some marine mammals), a middle ear with three bones, a neocortex and mammary glands, whose female representatives feed their juveniles from a specialized cutaneous-glandular secretion called milk (they are then said to be nursing). Mammals include 6,495 known species in 2018 which, according to scientific classifications, are distributed in 29 orders, 153 families and nearly 1,200 genera. In terms of cladistics, which reflects evolutionary history, mammals are the only living representatives of the synapsids, a group that includes famous non-mammalian representatives like Dimetrodon. Together with the sauropsids (“reptiles” and birds), they constitute the amniote clade, within the tetrapod superclass. Synapsids split into several groups (traditionally and incorrectly called "mammalian reptiles" or by the term pelycosaurs, and now known as "stem mammals" or "proto-mammals"), before giving rise to therapsids, a major group derived from the sphenacodonts, during the beginning of the middle Permian. The oldest known mammals are Lower Jurassic fossils and descended from the cynodonts, an advanced group of therapsids. Their range is planetary, they have conquered a large part of the ecological niches of the macrofauna and have remained one of the dominant terrestrial taxa since the Eocene, after the Cretaceous-Paleogene extinction. From an evolutionary and ecological point of view, the first mammals had a terrestrial, insectivorous, and generally nocturnal way of life. This taxon has diversified greatly over its evolutionary history, to the point that one of its main orders (the bats) has acquired flapping flight. A certain number of lineages have evolved towards a partial (seals, polar bear, beaver, hippopotamus, otter, amphibious vole, platypus, etc.) or total (cetaceans, sirenians, etc.) aquatic lifestyle, while retaining breathing from their tetrapod ancestor. pulmonary. Similarly, echolocation is very present in certain orders (chiroptera, cetaceans) whereas it is rare in the rest of the animal kingdom. Many wild mammals, despite protected species status, are on the IUCN Red List (especially apex predators) – some are subject to restoration or reintroduction plans. Similarly, certain breeds of certain species bred by man until the 19th century (for animal traction, meat, milk, wool or as pack animals) have disappeared or have greatly regressed in favor of a few selected breeds. for their productivity. Some species have become invasive, in particular after deliberate or accidental introduction into new biotopes in relation to human activities, while no predator stems the growth of these new colonies.