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Mammals (class Mammalia /məˈmeɪli.ə/) are a clade of endothermic amniotes. Among the features that distinguish them from the other amniotes, the reptiles and the birds, are hair, three middle ear bones, mammary glands in females, and a neocortex (a region of the brain). The mammalian brain regulates body temperature and the circulatory system, including the four-chambered heart. The mammals include the largest animals on the planet, the rorqual whales, as well as some of the most intelligent, such as elephants, some primates and some cetaceans. The basic body type is a four-legged land-borne animal, but some mammals are adapted for life at sea, in the air, in the trees, or on two legs. The largest group of mammals, the placentals, have a placenta which feeds the offspring during pregnancy. Mammals range in size from the 30–40 mm (1.2–1.6 in) bumblebee bat to the 33-meter (108 ft) blue whale.

The word "mammal" is modern, from the scientific name Mammalia coined by Carl Linnaeus in 1758, derived from the Latin mamma ("teat, pap"). All female mammals nurse their young with milk, which is secreted from special glands, the mammary glands. According to Mammal Species of the World, 5,702 species were known in 2006. These were grouped in 1,229 genera, 153 families and 29 orders.[1] In 2008 the IUCN completed a five-year, 1,700-scientist Global Mammal Assessment for its IUCN Red List, which counted 5,488 accepted species at the end of that period.[2] In some classifications, the mammals are divided into two subclasses (not counting fossils): the Prototheria (order of Monotremata) and the Theria, the latter composed of the infraclasses Metatheria and Eutheria. The marsupials comprise the crown group of the Metatheria and therefore include all living metatherians as well as many extinct ones; the placentals likewise constitute the crown group of the Eutheria.

Except for the five species of monotremes (egg-laying mammals), all modern 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 the classification scheme used, are the primates, to which the human species belongs, the Cetartiodactyla (including the even-toed hoofed mammals and the whales), and the Carnivora (cats, dogs, weasels, bears, seals, and their relatives).[1] While the classification of mammals at the family level has been relatively stable, different treatments at higher levels—subclass, infraclass, and order—appear in contemporaneous literature, especially for the marsupials. Much recent change has reflected the results of cladistic analysis and molecular genetics. Results from molecular genetics, for example, have led to the adoption of new groups such as the Afrotheria and the abandonment of traditional groups such as the Insectivora.

The early synapsid mammalian ancestors were sphenacodont pelycosaurs, a group that also included Dimetrodon. At the end of the Carboniferous period, this group diverged from the sauropsid line that led to today's reptiles and birds. Preceded by many diverse groups of non-mammalian synapsids (sometimes referred to as mammal-like reptiles), the first mammals appeared in the early Mesozoic era. The modern mammalian orders arose in the Paleogene and Neogene periods of the Cenozoic era.

Contents [hide]

1 Varying definitions, varying dates

2 Distinguishing features

3 Classification

3.1 McKenna/Bell classification

3.2 Molecular classification of placentals

4 Evolutionary history

4.1 Evolution from amniotes in the Paleozoic

4.2 The mammals appear

4.3 Rise to dominance in the Cenozoic

4.4 Earliest appearances of features

5 Anatomy and morphology

5.1 Skeletal system

5.2 Respiratory system

5.3 Nervous system

5.4 Integumentary system

5.5 Reproductive system

6 Physiology

6.1 Endothermy

6.2 Intelligence

6.3 Social structure

6.4 Locomotion

6.4.1 Terrestrial

6.4.2 Arboreal

6.4.3 Aquatic

6.4.4 Aerial

6.5 Feeding

7 See also

8 References

9 Further reading

10 External links

Varying definitions, varying dates[edit]

In an influential 1988 paper, Timothy Rowe defined Mammalia phylogenetically as the crown group mammals, the clade consisting of the most recent common ancestor of living monotremes (echidnas and platypuses) and therian mammals (marsupials and placentals) and all descendants of that ancestor.[3] A broader phylogenetic definition was provided in a 2004 book by Kielan-Jaworowska, Cifelli, and Luo, who defined Mammalia as the clade originating with the most recent common ancestor, not only of the monotremes and the therians, but also of Sinoconodon, the morganucodonts, and the docodonts.[4] The morganucodonts and the docodonts, included by Rowe in the unranked clade Mammaliaformes, had a widespread distribution in the northern continents and had many of the characteristics that traditionally would have classified them as mammals. In particular, some docodonts were furry. Finally, T. S. Kemp, along with many other paleontologists, defines Mammalia based on skeletal characteristics rather than ancestral relations; Adelobasileus is included on this basis, though this animal satisfies neither Rowe's definition nor that of Kielan-Jaworowska et al.[5]

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 167 million years ago in the Middle Jurassic, this is a reasonable estimate for the appearance of the crown group.[6] Adelobasileus is dated 225 Ma as is Tikitherium, the earlist known animal satisfying the Kielan-Jaworowska et al. criterion, so the appearance of mammals in a broader sense can be given this Late Triassic date.[7][8][9] In any case, the temporal range of the group extends to the present day.

Distinguishing features[edit]

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:

Jaw joint - The dentary (the lower jaw bone which carries the teeth) and the squamosal (another small skull bone) meet to form the joint. In most gnathostomes, including early therapsids, the joint consists of the articular (a small bone at the back of the lower jaw) and the quadrate (a small bone at the back of the upper jaw).

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 are replaced once or (as in toothed whales and murid rodents) not at all, rather than being replaced continually throughout life.[10]

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 tetrapods, in contrast, have only one such knob.

For the most part, these characteristics were not present in the Triassic ancestors of the mammals.

For palaeontologists who define Mammalia phylogenetically, no limit can be set on the features used to distinguish the group. Any feature may be relevant to a fossil's phylogenetic position. Palaeontologists defining Mammalia in terms of traits, on the other hand, need only consider those features that appear in the definition. The dentary-squamosal jaw joint is generally included.

Classification[edit]

Main article: Mammal classification

The orders Rodentia (blue), Chiroptera (red), and Soricomorpha (yellow) together comprise over 70% of mammal species.

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.

McKenna/Bell classification[edit]

In 1997, the mammals were comprehensively revised by Malcolm C. McKenna and Susan K. Bell, which has resulted in the McKenna/Bell classification. Their 1997 book, Classification of Mammals above the Species Level,[11] is the most comprehensive work to date on the systematics, relationships, and occurrences of all mammal taxa, living and extinct, down through the rank of genus, though recent molecular genetic data challenge several of the higher level groupings. The authors worked together as paleontologists at the American Museum of Natural History, New York. McKenna inherited the project from Simpson and, with Bell, constructed a completely updated hierarchical system, covering living and extinct taxa that reflects the historical genealogy of Mammalia.

The McKenna/Bell hierarchical listing of many 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.

Extinct groups are represented by a dagger (†).

Class Mammalia

Subclass Prototheria: monotremes: echidnas and the platypus

Subclass Theriiformes: live-bearing mammals and their prehistoric relatives.

Infraclass †Allotheria: multituberculates

Infraclass †Triconodonta: triconodonts

Infraclass Holotheria: modern live-bearing mammals and their prehistoric relatives.

Supercohort Theria: live-bearing mammals

Cohort Marsupialia: marsupials

Magnorder Australidelphia: Australian marsupials and the monito del monte

Magnorder Ameridelphia: New World marsupials.

Cohort Placentalia: placentals

Magnorder Xenarthra: xenarthrans

Magnorder Epitheria: epitheres

Grandorder Anagalida: lagomorphs, rodents, and elephant shrews

Grandorder Ferae: carnivorans, pangolins, †creodonts, and relatives

Grandorder Lipotyphla: insectivorans

Grandorder Archonta: bats, primates, colugos, and treeshrews

Grandorder Ungulata: ungulates

Order Tubulidentata incertae sedis: aardvark

Mirorder Eparctocyona: †condylarths, whales, and artiodactyls (even-toed ungulates)

Mirorder †Meridiungulata: South American ungulates

Mirorder Altungulata: perissodactyls (odd-toed ungulates), elephants, manatees, and hyraxes

Molecular classification of placentals[edit]

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Molecular studies based on DNA analysis have suggested new relationships among mammal families over the last few years. Most of these findings have been independently validated by retrotransposon presence/absence data. The most recent classification systems based on molecular studies have proposed four groups or lineages of placental mammals. Molecular clocks suggest that these clades diverged from early common ancestors in the Cretaceous, but fossils have not yet been found to corroborate this hypothesis. These molecular findings are consistent with mammal zoogeography:

Following molecular DNA sequence analyses, the first divergence was that of the Afrotheria 110–100 million years ago (mya). 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 sister group 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 therefore 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:

Clade Atlantogenata

Group I: Afrotheria

Clade Afroinsectiphilia

Order Macroscelidea: elephant shrews (Africa)

Order Afrosoricida: tenrecs and golden moles (Africa)

Order Tubulidentata: aardvark (Africa south of the Sahara)

Clade Paenungulata

Order Hyracoidea: hyraxes or dassies (Africa, Arabia)

Order Proboscidea: elephants (Africa, Southeast Asia)

Order Sirenia: dugong and manatees (cosmopolitan tropical)

Group II: Xenarthra

Order Pilosa: sloths and anteaters (neotropical)

Order Cingulata: armadillos (Americas)

Clade Boreoeutheria

Group III: Euarchontoglires (Supraprimates)

Superorder Euarchonta

Order Scandentia: treeshrews (Southeast Asia).

Order Dermoptera: flying lemurs or colugos (Southeast Asia)

Order Primates: lemurs, bushbabies, monkeys, apes, humans (cosmopolitan)

Superorder Glires

Order Lagomorpha: pikas, rabbits, hares (Eurasia, Africa, Americas)

Order Rodentia: rodents (cosmopolitan)

Group IV: Laurasiatheria

Order Erinaceomorpha: hedgehogs

Order Soricomorpha: moles, shrews, solenodons

Clade Ferungulata

Clade Cetartiodactyla

Order Cetacea: whales, dolphins and porpoises

Order Artiodactyla: even-toed ungulates, including pigs, hippopotamus, camels, giraffe, deer, antelope, cattle, sheep, goats

Clade Pegasoferae

Order Chiroptera: bats (cosmopolitan)

Clade Zooamata

Order Perissodactyla: odd-toed ungulates, including horses, donkeys, zebras, tapirs, and rhinoceroses

Clade Ferae

Order Pholidota: pangolins or scaly anteaters (Africa, South Asia)

Order Carnivora: carnivores (cosmopolitan), including cats and dogs

Evolutionary history[edit]

For more details on this topic, see Evolution of mammals.

Cladogram following,[12] which takes Mammalia to be the crown group.