Mammal


A mammal is a vertebrate animal of the class Mammalia. Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three middle ear bones. These characteristics distinguish them from reptiles and birds, from which their ancestors diverged in the Carboniferous Period over 300 million years ago. Around 6,640 extant species of mammals have been described and divided into 27 orders. The study of mammals is called mammalogy.
The largest orders of mammals, by number of species, are the rodents, bats, and eulipotyphlans. The next three are the primates, the even-toed ungulates, and the Carnivora.
Mammals are the only living members of Synapsida; this clade, together with Sauropsida, constitutes the larger Amniota clade. Early synapsids are referred to as "pelycosaurs." The more advanced therapsids became dominant during the Guadalupian. Mammals originated from cynodonts, an advanced group of therapsids, during the Late Triassic to Early Jurassic. Mammals achieved their modern diversity in the Paleogene and Neogene periods of the Cenozoic era, after the extinction of non-avian dinosaurs, and have been the dominant terrestrial animal group from 66 million years ago to the present.
The basic mammalian body type is quadrupedal, with most mammals using four limbs for terrestrial locomotion; but in some, the limbs are adapted for life at sea, in the air, in trees or underground. The bipeds have adapted to move using only the two lower limbs, while the rear limbs of cetaceans and the sea cows are mere internal vestiges. Mammals range in size from the bumblebee bat to the blue whale—possibly the largest animal to have ever lived. Maximum lifespan varies from two years for the shrew to 211 years for the bowhead whale. All modern mammals give birth to live young, except the five species of monotremes, which lay eggs. The most species-rich group is the viviparous placental mammals, so named for the temporary organ used by offspring to draw nutrition from the mother during gestation.
Most mammals are intelligent, with some possessing large brains, self-awareness, and tool use. Mammals can communicate and vocalise in several ways, including the production of ultrasound, scent marking, alarm signals, singing, echolocation; and, in the case of humans, complex language. Mammals can organise themselves into fission–fusion societies, harems, and hierarchies—but can also be solitary and territorial. Most mammals are polygynous, but some can be monogamous or polyandrous.
Domestication of many types of mammals by humans played a major role in the Neolithic Revolution, and resulted in farming replacing hunting and gathering as the primary source of food for humans. This led to a major restructuring of human societies from nomadic to sedentary, with more co-operation among larger and larger groups, and ultimately the development of the first civilisations. Domesticated mammals provided, and continue to provide, power for transport and agriculture, as well as food, fur, and leather. Mammals are also hunted and raced for sport, kept as pets and working animals of various types, and are used as model organisms in science. Mammals have been depicted in art since Paleolithic times, and appear in literature, film, mythology, and religion. Decline in numbers and extinction of many mammals is primarily driven by human poaching and habitat destruction, primarily deforestation.

Classification

Mammal classification has been through several revisions since Carl Linnaeus initially defined the class, and at present, no classification system is universally accepted. McKenna & Bell and Wilson & Reeder provide useful recent compendiums. Simpson provides systematics of mammal origins and relationships that had been taught universally until the end of the 20th century.
However, since 1945, a large amount of new and more detailed information has gradually been found: The paleontological record has been recalibrated, and the intervening years have seen much debate and progress concerning the theoretical underpinnings of systematisation itself, partly through the new concept of cladistics. Though fieldwork and lab work progressively outdated Simpson's classification, it remains the closest thing to an official classification of mammals, despite its known issues.
Most mammals, including the six most species-rich orders, belong to the placental group. The three largest orders in numbers of species are Rodentia: mice, rats, porcupines, beavers, capybaras, and other gnawing mammals; Chiroptera: bats; and Eulipotyphla: shrews, moles, and solenodons. The next three biggest orders, depending on the biological classification scheme used, are the primates: apes, monkeys, and lemurs; Cetartiodactyla: whales and even-toed ungulates; and Carnivora which includes cats, dogs, weasels, bears, seals, and allies. According to Mammal Species of the World, 5,416 species were identified in 2006. These were grouped into 1,229 genera, 153 families and 29 orders. In 2008, the International Union for Conservation of Nature completed a five-year Global Mammal Assessment for its IUCN Red List, which counted 5,488 species. According to research published in the Journal of Mammalogy in 2018, the number of recognised mammal species is 6,495, including 96 recently extinct.

Definitions

The word "mammal" is modern, from the scientific name Mammalia coined by Carl Linnaeus in 1758, derived from the Latin mamma. In an influential 1988 paper, Timothy Rowe defined Mammalia phylogenetically as the crown group of mammals, the clade consisting of the most recent common ancestor of living monotremes and therians and all descendants of that ancestor. Since this ancestor lived in the Jurassic Period, Rowe's definition excludes all animals from the earlier Triassic, despite the fact that Triassic fossils in the Haramiyida have been referred to the Mammalia since the mid-19th century. If Mammalia is considered as the crown group, its origin can be roughly dated as the first known appearance of animals more closely related to some extant mammals than to others. Ambondro is more closely related to monotremes than to therian mammals while Amphilestes and Amphitherium are more closely related to the therians; as fossils of all three genera are dated about in the Middle Jurassic, this is a reasonable estimate for the appearance of the crown group.
T. S. Kemp has provided a more traditional definition: "Synapsids that possess a dentary–squamosal jaw articulation and occlusion between upper and lower molars with a transverse component to the movement" or, equivalently in Kemp's view, the clade originating with the last common ancestor of Sinoconodon and living mammals. The earliest-known synapsid satisfying Kemp's definitions is Tikitherium, dated, so the appearance of mammals in this broader sense can be given this Late Triassic date. However, this animal may have actually evolved during the Neogene.

Molecular classification of placentals

As of the early 21st century, molecular studies based on DNA analysis have suggested new relationships among mammal families. Most of these findings have been independently validated by retrotransposon presence/absence data. Classification systems based on molecular studies reveal three major groups or lineages of placentals—Afrotheria, Xenarthra and Boreoeutheria—which diverged in the Cretaceous. The relationships between these three lineages is contentious, and all three possible hypotheses have been proposed with respect to which group is basal. These hypotheses are Atlantogenata, Epitheria and Exafroplacentalia. Boreoeutheria in turn contains two major lineages—Euarchontoglires and Laurasiatheria.
Estimates for the divergence times between these three placental groups range from 105 to 120 million years ago, depending on the type of DNA used and varying interpretations of paleogeographic data.
Mammal phylogeny according to Álvarez-Carretero et al., 2022:

Evolution

Origins

, a clade that contains mammals and their extinct relatives, originated during the Pennsylvanian subperiod, when they split from the reptile lineage. Crown group mammals evolved from earlier mammaliaforms during the Early Jurassic. The cladogram takes Mammalia to be the crown group.

Evolution from older amniotes

The first fully terrestrial vertebrates were amniotes. Like their amphibious early tetrapod predecessors, they had lungs and limbs. Amniotic eggs, however, have internal membranes that allow the developing embryo to breathe but keep water in. Hence, amniotes can lay eggs on dry land, while amphibians generally need to lay their eggs in water.
The first amniotes apparently arose in the Pennsylvanian subperiod of the Carboniferous. They descended from earlier reptiliomorph amphibious tetrapods, which lived on land that was already inhabited by insects and other invertebrates as well as ferns, mosses and other plants. Within a few million years, two important amniote lineages became distinct: the synapsids, which would later include the common ancestor of the mammals; and the sauropsids, which now include turtles, lizards, snakes, crocodilians and dinosaurs. Synapsids have a single hole low on each side of the skull. Primitive synapsids included the largest and fiercest animals of the early Permian such as Dimetrodon. Nonmammalian synapsids were traditionally—and incorrectly—called "mammal-like reptiles" or pelycosaurs; we now know they were neither reptiles nor part of reptile lineage.
Therapsids, a group of synapsids, evolved in the Middle Permian, about 265 million years ago, and became the dominant land vertebrates. They differ from basal eupelycosaurs in several features of the skull and jaws, including: larger skulls and incisors which are equal in size in therapsids, but not for eupelycosaurs. The therapsid lineage leading to mammals went through a series of stages, beginning with animals that were very similar to their early synapsid ancestors and ending with probainognathian cynodonts, some of which could easily be mistaken for mammals. Those stages were characterised by:
  • The gradual development of a bony secondary palate.
  • Abrupt acquisition of endothermy among Mammaliamorpha, thus prior to the origin of mammals by 30–50 millions of years .
  • Progression towards an erect limb posture, which would increase the animals' stamina by avoiding Carrier's constraint. But this process was slow and erratic: for example, all herbivorous nonmammaliaform therapsids retained sprawling limbs ; Permian carnivorous therapsids had sprawling forelimbs, and some late Permian ones also had semisprawling hindlimbs. In fact, modern monotremes still have semisprawling limbs.
  • The dentary gradually became the main bone of the lower jaw which, by the Triassic, progressed towards the fully mammalian jaw and middle ear.