According to ''Mammal Species of the World'', which is updated through periodic editions, 5676 species were known in 2005, distributed in 1,229 genera, 153 families and 29 orders. In 2008 the IUCN completed a 5-year, 17,000 scientist Global Mammal Assessment for its IUCN Red List, which counted 5488 accepted species at the end of that period. The class is divided into two subclasses (not counting fossils): the Prototheria (order of Monotremata) and the Theria, the latter containing the infraclasses Metatheria (including marsupials) and Eutheria (the placentals). The classification of mammals between the relatively stable class and family levels having changed often, different treatments of subclass, infraclass and order appear in contemporaneous literature, especially for Marsupialia.
Except for the five species of monotremes (which lay eggs), all living mammals give birth to live young. Most mammals, including the six most species-rich orders, belong to the placental group. The three largest orders, in descending order, are Rodentia (mice, rats, porcupines, beavers, capybaras, and other gnawing mammals), Chiroptera (bats), and Soricomorpha (shrews, moles and solenodons). The next three largest orders, depending on classification scheme, include the Primates, to which the human species belongs, the Cetartiodactyla (including the even-toed hoofed mammals and the whales) and the Carnivora (dogs, cats, weasels, bears, seals, and their relatives).
The early synapsid mammalian ancestors, a group which included pelycosaurs such as ''Dimetrodon'', diverged from the amniote line that would lead to reptiles at the end of the Carboniferous period. Preceded by many diverse groups of non-mammalian synapsids (sometimes referred to as mammal-like reptiles), the first true mammals appeared 220 million years ago in the Triassic period. Modern mammalian orders appeared in the Palaeocene and Eocene epochs of the Palaeogene period. Phylogenetically, the clade Mammalia is defined as all descendants of the most recent common ancestor of monotremes (e.g., echidnas and platypuses) and therian mammals (marsupials and placentals). This means that some extinct groups of "mammals" are not members of the crown group Mammalia, even though most of them have all the characteristics that traditionally would have classified them as mammals. These "mammals" are now usually placed in the unranked clade Mammaliaformes.
Mammals also have a double occipital condyle: they have two knobs at the base of the skull which fit into the topmost neck vertebra, and other vertebrates have a single occipital condyle. Paleontologists use only the jaw joint and middle ear as criteria for identifying fossil mammals, since it would be confusing if they found a fossil that had one feature, but not the other.
George Gaylord Simpson's "Principles of Classification and a Classification of Mammals" (AMNH ''Bulletin'' v. 85, 1945) was the original source for the taxonomy listed here. Simpson laid out a systematics of mammal origins and relationships that was universally taught until the end of the 20th century. Since Simpson's classification, the paleontological record has been recalibrated, and the intervening years have seen much debate and progress concerning the theoretical underpinnings of systematization itself, partly through the new concept of cladistics. Though field work gradually made Simpson's classification outdated, it remained the closest thing to an official classification of mammals.
Class Mammalia
The McKenna/Bell hierarchical listing of all of the terms used for mammal groups above the species includes extinct mammals as well as modern groups, and introduces some fine distinctions such as legions and sublegions (ranks which fall between classes and orders) that are likely to be glossed over by the nonprofessionals.
The published re-classification forms both a comprehensive and authoritative record of approved names and classifications and a list of invalid names.
Extinct groups are represented by a dagger (†).
Class Mammalia
Following molecular DNA sequence analyses, the first divergence was that of the Afrotheria 110–100 million years ago. The Afrotheria proceeded to evolve and diversify in the isolation of the African-Arabian continent. The Xenarthra, isolated in South America, diverged from the Boreoeutheria approximately 100–95 million years ago. According to an alternative view, the Xenarthra has the Afrotheria as closest allies, forming the Atlantogenata as sistergroup to Boreoeutheria. The Boreoeutheria split into the Laurasiatheria and Euarchontoglires between 95 and 85 mya; both of these groups evolved on the northern continent of Laurasia. After tens of millions of years of relative isolation, Africa-Arabia collided with Eurasia, exchanging Afrotheria and Boreoeutheria. The formation of the Isthmus of Panama linked South America and North America, which facilitated the exchange of mammal species in the Great American Interchange. The traditional view that no placental mammals reached Australasia until about 5 million years ago when bats and murine rodents arrived has been challenged by recent evidence and may need to be reassessed. These molecular results are still controversial because they are not reflected by morphological data, and thus not accepted by many systematists. Further there is some indication from retrotransposon presence/absence data that the traditional Epitheria hypothesis, suggesting Xenarthra as the first divergence, might be true. With the old order Insectivora shown to be polyphylectic and more properly subdivided (as Afrosoricida, Erinaceomorpha, and Soricomorpha), the following classification for placental mammals contains 21 orders:
The first amniotes apparently arose in the late Carboniferous. They descended from earlier reptiliomorph amphibians, which lived on land already inhabited by insects and other invertebrates, and by ferns, mosses and other plants. Within a few million years two important amniote lineages became distinct: the synapsids, which include mammals; and the sauropsids, which include lizards, snakes, crocodilians, dinosaurs and birds. Synapsids have a single hole (temporal fenestra) low on each side of the skull.
One synapsid group, the pelycosaurs, were the most common land vertebrates of the early Permian and included the largest land animals of the time.
Therapsids descended from pelycosaurs in the middle Permian, about 260M years ago, and took over their position as the dominant land vertebrates. They differ from pelycosaurs in several features of the skull and jaws, including: larger temporal fenestrae and incisors which are equal in size. The therapsids went through a series of stages, beginning with animals which were very like their pelycosaur ancestors and ending with the Triassic cynodonts, some of which could easily be mistaken for mammals. Those stages were characterized by: gradual development of a bony secondary palate.
The first true mammals appeared in the Late Triassic (ca. 200 million years ago), over 70 million years after the first therapsids and approximately 30 million years after the first mammaliaformes. ''Hadrocodium'' appears to be in the middle of the transition to true mammal status — it had a mammalian jaw joint (formed by the dentary and squamosal bones), but there is some debate about whether its middle ear was fully mammalian. The majority of the mammal species that existed in the Mesozoic Era were characterized by Multituberculates.
The earliest known monotreme is ''Teinolophos'', which lived about 123M years ago in Australia. Monotremes have some features which may be inherited from the original amniotes:
The oldest known marsupial is ''Sinodelphys'', found in 125M-year old early Cretaceous shale in China's northeastern Liaoning Province. The fossil is nearly complete and includes tufts of fur and imprints of soft tissues. The living Eutheria ("true beasts") are all placentals. But the earliest known eutherian, ''Eomaia'', found in China and dated to 125M years ago, obtained some features which are more like those of marsupials, which suggested it was perhaps a transitional fossil that eventually gave rise to the placental lineage (the surviving metatherians):
Mammals and near-mammals expanded out of their nocturnal insectivore niche from the mid Jurassic onwards – for example ''Castorocauda'' had adaptations for swimming, digging and catching fish.
Recent molecular phylogenetic studies suggest that most placental orders diverged about 100M to 85M years ago, but that modern families first appeared in the late Eocene and early Miocene But paleontologists object that no placental fossils have been found from before the end of the Cretaceous.
During the Cenozoic several groups of mammals appeared which were much larger than their nearest modern equivalents – but none was even close to the size of the largest dinosaurs with similar feeding habits.
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 (egg-laying mammals).
The earliest clear evidence of hair or fur is in fossils of ''Castorocauda'', from 164M years ago in the mid Jurassic. From 1955 onwards some scientists have interpreted the foramina (passages) in the maxillae (upper jaws) and premaxillae (small bones in front of the maxillae) of cynodonts as channels which supplied blood vessels and nerves to vibrissae (whiskers), and suggested that this was evidence of hair or fur. But foramina do not necessarily show that an animal had vibrissae – for example the modern lizard ''Tupinambis'' has foramina which are almost identical to those found in the non-mammalian cynodont Thrinaxodon. The evolution of erect limbs in mammals is incomplete — living and fossil monotremes have sprawling limbs. In fact some scientists think that the parasagittal (non-sprawling) limb posture is a synapomorphy (distinguishing characteristic) of the Boreosphenida, a group which contains the Theria and therefore includes the last common ancestor of modern marsupial and placentals – and therefore that all earlier mammals had sprawling limbs. ''Sinodelphys'' (the earliest known marsupial) and ''Eomaia'' (the earliest known eutherian) lived about 125M years ago, so erect limbs must have evolved before then.
It is currently very difficult to be confident when endothermy first appeared in the evolution of mammals. Modern monotremes have a lower body temperature and more variable metabolic rate than marsupials and placentals. So the main question is when a monotreme-like metabolism evolved in mammals. The evidence found so far suggests Triassic cynodonts may have had fairly high metabolic rates, but is not conclusive. In particular it is difficult to see how small animals can maintain a high and stable body temperature without fur.
Breathing is largely driven by the muscular diaphragm which divides the thorax from the abdominal cavity, forming a dome with its convexity towards the thorax. Contraction of the diaphragm flattens the dome increasing the volume of the cavity in which the lung is enclosed. Air enters through the oral and nasal cavities; it flows through the larynx, trachea and bronchi and expands the alveoli. Relaxation of the diaphragm has the opposite effect, passively recoiling during normal breathing. During exercise, the abdominal wall contracts, increasing visceral pressure on the diaphragm, thus forcing the air out more quickly and forcefully. The rib cage itself also is able to expand and contract the thoracic cavity to some degree, through the action of other respiratory and accessory respiratory muscles. As a result, air is sucked into or expelled out of the lungs, always moving down its pressure gradient. This type of lung is known as a bellows lung as it resembles a blacksmith's bellows. Mammals take oxygen into their lungs, and discard carbon dioxide.
The epidermis is typically ten to thirty cells thick, its main function being to provide a waterproof layer. Its outermost cells are constantly lost; its bottommost cells are constantly dividing and pushing upward. The middle layer, the dermis, is fifteen to forty times thicker than the epidermis. The dermis is made up of many components such as bony structures and blood vessels. The hypodermis is made up of adipose tissue. Its job is to store lipids, and to provide cushioning and insulation. The thickness of this layer varies widely from species to species.
Although mammals and other animals have cilia that superficially may resemble it, no other animals except mammals have hair. It is a definitive characteristic of the class. Some mammals have very little, but nonetheless, careful examination reveals the characteristic, often in obscure parts of their bodies. None are known to have hair that naturally is blue or green in color although some cetaceans, along with the mandrills, appear to have shades of blue skin. Many mammals are indicated as having blue hair or fur, but in all known cases, it has been found to be a shade of gray. The two-toed sloth and the polar bear may seem to have green fur, but this color is caused by algae growths.
Certain glands of mammals known as mammary glands are specialized to produce milk, a liquid used by newborns as their primary source of nutrition. The monotremes branched early from other mammals and do not have the nipples seen in most mammals, but they do have mammary glands.
Viviparous mammals are classified into the subclass Theria and are divided into two infraclasses: Metatheria (of which only the Marsupialia survive), and Eutheria. Marsupialia, or marsupials, have short gestation periods and give birth to undeveloped young which are contained within a pouch-like sac (marsupium) located in front of the mothers' abdomen. Eutherians, commonly known as placentals, are mammals that give birth to complete and fully developed young. This is usually characterized by long gestation periods. The majority of mammal species are classified as eutherians.
Endothermy requires plenty of food energy, so pound for pound mammals eat more food than most reptiles. Small insectivorous mammals eat prodigious amounts for their size.
A rare exception, the naked mole rat produces little metabolic heat, so it is considered an operational poikilotherm. Birds are also endothermic, so endothermy is not a defining mammalian feature.
In intelligent mammals, such as primates, the cerebrum is larger relative to the rest of the brain. Intelligence itself is not easy to define, but indications of intelligence include the ability to learn, matched with behavioral flexibility. Rats, for example, are considered to be highly intelligent as they can learn and perform new tasks, an ability that may be important when they first colonize a fresh habitat. In some mammals, food gathering appears to be related to intelligence: a deer feeding on plants has a brain smaller than a cat, which must think to outwit its prey.
Specializations in herbivory include: Granivory "seed eating", folivory "leaf eating", fruivory "fruit eating", nectivory "nectar eating", gumivory "gum eating", and mycophagy "fungus eating".
kbd:ШэрыпӀхэр af:Soogdier als:Säugetiere am:ጡት አጥቢ ang:Sycedēor ar:ثدييات an:Mammalia ast:Mamíferu gn:Okambúva ay:Ñuñuri az:Məməlilər bn:স্তন্যপায়ী zh-min-nan:Chhī-leng tōng-bu̍t ba:Һөтимәрҙәр be:Сысуны be-x-old:Сысуны bar:Spofiecha bo:འུ་འཐུང་སྲོག་ཆགས། bs:Sisari br:Bronneged bg:Бозайници ca:Mamífer cv:Сĕтпе ӳсекенсем cs:Savci co:Mammiferu cy:Mamal da:Pattedyr de:Säugetiere et:Imetajad el:Θηλαστικό es:Mammalia eo:Mamuloj eu:Ugaztun fa:پستانداران hif:Mammal fo:Súgdjór fr:Mammifère fy:Sûchdieren ga:Mamach gv:Sheeintagh gd:Mamalan gl:Mamífero xal:Үсн теҗәлтнр ko:포유류 hi:स्तनधारी hsb:Cycaki hr:Sisavci io:Mamifero id:Binatang menyusui ia:Mammal is:Spendýr it:Mammalia he:יונקים jv:Mamalia kn:ಸಸ್ತನಿ ka:ძუძუმწოვრები kk:Сүтқоректілер kw:Bronnvil sw:Mamalia kg:Nyama ya mabele ht:Mamifè ku:Guhandar koi:Нимӧтчиссез la:Mammalia lv:Zīdītāji lb:Mamendéieren lt:Žinduoliai lij:Mammalia li:Zoogdiere ln:Nyama ya mabɛ́lɛ jbo:mabru lmo:Mamifer hu:Emlősök mk:Цицачи ml:സസ്തനി mt:Mammiferu mr:सस्तन प्राणी ms:Mamalia mn:Хөхтөн my:နို့တိုက်သတ္တဝါ nah:Mammalia nl:Zoogdieren ja:哺乳類 frr:Tetjdiarten no:Pattedyr nn:Pattedyr nrm:Mammiféthe nov:Mammalia oc:Mammalia mhr:Шӧр пукшышо om:Hosiftotta (mammals) pnb:میمل ps:تی لرونکی nds:Söögdeerten pl:Ssaki pt:Mamíferos ro:Mamifer rm:Mamifer qu:Ñuñuq rue:Ссавцї ru:Млекопитающие sah:Кыыл sa:सस्तनः sco:Mammal stq:Suugedierte sq:Gjitarët scn:Mammìfiri simple:Mammal sk:Cicavce sl:Sesalci sr:Сисари sh:Sisavac su:Mamalia fi:Nisäkkäät sv:Däggdjur tl:Mamalya ta:பாலூட்டி tt:Имезүчеләр te:క్షీరదాలు tet:Mamíferu th:สัตว์เลี้ยงลูกด้วยน้ำนม tg:Ширхӯр tr:Memeliler udm:Пӧйшур uk:Ссавці ur:پستانیہ vec:Mamìfari vi:Lớp Thú fiu-vro:Imetäjä wa:Biesse ås tetes vls:Zoogbêestn war:Mamalya yi:זויגער yo:Afọmúbọ́mọ zh-yue:哺乳動物 zea:Zoogdieren bat-smg:Žėndoulē zh:哺乳动物
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