A mammal (from
The largest orders of mammals, by number of species, are the
Mammals are the only living members of
The basic mammalian body type is
Most mammals are
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 civilizations. Domesticated mammals provided, and continue to provide, power for transport and agriculture, as well as food (meat and dairy products), fur, and leather. Mammals are also hunted and raced for sport, 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.
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 (1997) and Wilson & Reeder (2005) provide useful recent compendiums. Simpson (1945) 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
Most mammals, including the six most species-rich
The word "
T.S. Kemp has provided a more traditional definition: "
In 1997, the mammals were comprehensively revised by
In the following list,
- Subclass Prototheria: monotremes: echidnas and the platypus
- Subclass Theriiformes: live-bearing mammals and their prehistoric relatives
- Infraclass †Allotheria: multituberculates
- Infraclass †Eutriconodonta: eutriconodonts
- Infraclass Holotheria: modern live-bearing mammals and their prehistoric relatives
- Superlegion †Kuehneotheria
- Supercohort Theria: live-bearing mammals
- Cohort Marsupialia: marsupials
- Magnorder Australidelphia: Australian marsupials and the monito del monte
- Magnorder Ameridelphia: New World marsupials. Now considered paraphyletic, with shrew opossums being closer to australidelphians.
- Cohort Placentalia: placentals
- Magnorder Xenarthra: xenarthrans
- Magnorder Epitheria: epitheres
- Superorder †Leptictida
- Superorder Preptotheria
- Grandorder Anagalida: lagomorphs, rodents and elephant shrews
- Grandorder creodontsand relatives
- Grandorder Lipotyphla: insectivorans
- Grandorder Archonta: bats, primates, colugos and treeshrews (now considered paraphyletic, with bats being closer to other groups)
- Grandorder Ungulata: ungulates
- Order Tubulidentata incertae sedis: aardvark
- Mirorder artiodactyls(even-toed ungulates)
- Mirorder †Meridiungulata: South American ungulates
- Superlegion †
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 placental mammals—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 (basal Boreoeutheria), Epitheria (basal Xenarthra) and Exafroplacentalia (basal Afrotheria). 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 (such as
The cladogram above is based on Tarver et al.. (2016)
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
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 characterized 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 (some late forms may have had semierect hind 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 (the lower consisting only of the dentary) and middle ear (which is constructed by the bones that were previously used to construct the jaws of reptiles).
The Permian–Triassic extinction event about 252 million years ago, which was a prolonged event due to the accumulation of several extinction pulses, ended the dominance of carnivorous therapsids. In the early Triassic, most medium to large land carnivore niches were taken over by archosaurs which, over an extended period (35 million years), came to include the crocodylomorphs, the pterosaurs and the dinosaurs; however, large cynodonts like Trucidocynodon and traversodontids still occupied large sized carnivorous and herbivorous niches respectively. By the Jurassic, the dinosaurs had come to dominate the large terrestrial herbivore niches as well.
The first mammals (in Kemp's sense) appeared in the Late Triassic epoch (about 225 million years ago), 40 million years after the first therapsids. They expanded out of their nocturnal
The oldest known fossil among the Eutheria ("true beasts") is the small shrewlike Juramaia sinensis, or "Jurassic mother from China", dated to 160 million years ago in the late Jurassic. A later eutherian relative, Eomaia, dated to 125 million years ago in the early Cretaceous, possessed some features in common with the marsupials but not with the placentals, evidence that these features were present in the last common ancestor of the two groups but were later lost in the placental lineage. In particular, the epipubic bones extend forwards from the pelvis. These are not found in any modern placental, but they are found in marsupials, monotremes, other nontherian mammals and Ukhaatherium, an early Cretaceous animal in the eutherian order Asioryctitheria. This also applies to the multituberculates. They are apparently an ancestral feature, which subsequently disappeared in the placental lineage. These epipubic bones seem to function by stiffening the muscles during locomotion, reducing the amount of space being presented, which placentals require to contain their fetus during gestation periods. A narrow pelvic outlet indicates that the young were very small at birth and therefore pregnancy was short, as in modern marsupials. This suggests that the placenta was a later development.
One of the earliest known monotremes was
Earliest appearances of features
Hadrocodium, whose fossils date from approximately 195 million years ago, in the early Jurassic, provides the first clear evidence of a jaw joint formed solely by the squamosal and dentary bones; there is no space in the jaw for the articular, a bone involved in the jaws of all early synapsids.
The earliest clear evidence of hair or fur is in fossils of
The evolution of erect limbs in mammals is incomplete—living and fossil monotremes have sprawling limbs. The parasagittal (nonsprawling) limb posture appeared sometime in the late Jurassic or early Cretaceous; it is found in the eutherian Eomaia and the metatherian Sinodelphys, both dated to 125 million years ago.
It has been suggested that the original function of lactation (milk production) was to keep eggs moist. Much of the argument is based on monotremes, the egg-laying mammals. In human females, mammary glands become fully developed during puberty, regardless of pregnancy.
Rise of the mammals
Therian mammals took over the medium- to large-sized ecological niches in the Cenozoic, after the Cretaceous–Paleogene extinction event approximately 66 million years ago emptied ecological space once filled by non-avian dinosaurs and other groups of reptiles, as well as various other mammal groups, and underwent an exponential increase in body size (megafauna). Then mammals diversified very quickly; both birds and mammals show an exponential rise in diversity. For example, the earliest known bat dates from about 50 million years ago, only 16 million years after the extinction of the non-avian dinosaurs.
Molecular phylogenetic studies initially suggested that most placental orders diverged about 100 to 85 million years ago and that modern families appeared in the period from the late
The earliest known ancestor of primates is
Living mammal species can be identified by the presence of sweat glands, including those that are specialized to produce milk to nourish their young. In classifying fossils, however, other features must be used, since soft tissue glands and many other features are not visible in fossils.
Many traits shared by all living mammals appeared among the earliest members of the group:
- Middle ear – In crown-group mammals, sound is carried from the eardrum by a chain of three bones, the malleus, the incus and the stapes. Ancestrally, the malleus and the incus are derived from the articular and the quadrate bones that constituted the jaw joint of early therapsids.
- Tooth replacement – Teeth can be replaced once (diphyodonty) or (as in toothed whales and murid rodents) not at all (monophyodonty). Elephants, manatees, and kangaroos continually grow new teeth throughout their life (polyphyodonty).
- Prismatic enamel – The enamel coating on the surface of a tooth consists of prisms, solid, rod-like structures extending from the dentin to the tooth's surface.
- Occipital condyles – Two knobs at the base of the skull fit into the topmost neck vertebra; most other tetrapods, in contrast, have only one such knob.
For the most part, these characteristics were not present in the Triassic ancestors of the mammals. Nearly all mammaliaforms possess an epipubic bone, the exception being modern placentals.
On average, male mammals are larger than females, with males being at least 10% larger than females in over 45% of investigated species. Most mammalian orders also exhibit male-biased
The majority of mammals have seven cervical vertebrae (bones in the neck). The exceptions are the manatee and the two-toed sloth, which have six, and the three-toed sloth which has nine. All mammalian brains possess a neocortex, a brain region unique to mammals. Placental brains have a corpus callosum, unlike monotremes and marsupials.
The mammalian heart has four chambers, two upper atria, the receiving chambers, and two lower ventricles, the discharging chambers. The heart has four valves, which separate its chambers and ensures blood flows in the correct direction through the heart (preventing backflow). After gas exchange in the pulmonary capillaries (blood vessels in the lungs), oxygen-rich blood returns to the left atrium via one of the four pulmonary veins. Blood flows nearly continuously back into the atrium, which acts as the receiving chamber, and from here through an opening into the left ventricle. Most blood flows passively into the heart while both the atria and ventricles are relaxed, but toward the end of the ventricular relaxation period, the left atrium will contract, pumping blood into the ventricle. The heart also requires nutrients and oxygen found in blood like other muscles, and is supplied via coronary arteries.