Fossil range: Late Triassic – Recent, 220–0 Ma
|Clockwise from the upper left: giraffe, golden crown fruit bat, lion, hedgehog|
Mammals (formally Mammalia) are a class of vertebrate, air-breathing animals whose females are characterized by the possession of mammary glands while both males and females are characterized by sweat glands, hair and/or fur, three middle ear bones used in hearing, and a neocortex region in the brain.
Mammals are divided into three main infraclass taxa depending how they are born. These taxa are: monotremes, marsupials and placentals. Except for the five species of monotremes (which lay eggs), all mammal species give birth to live young. Most mammals also possess specialized teeth, and the largest group of mammals, the placentals, use a placenta during gestation. The mammalian brain regulates endothermic and circulatory systems, including a four-chambered heart.
There are approximately 5,400 species of mammals, distributed in about 1,200 genera, 153 families, and 29 orders (though this varies by classification scheme). Mammals range in size from the 30–40 millimeter (1- to 1.5-inch) Bumblebee Bat to the 33-meter (108-foot) Blue Whale.
Mammals are divided into two subclasses: the Prototheria, which includes the oviparous monotremes, and the Theria, which includes the placentals and live-bearing marsupials. Most mammals, including the six largest 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 include the Carnivora (dogs, cats, weasels, bears, seals, and their relatives), the Cetartiodactyla (including the even-toed hoofed mammals and the whales) and the Primates to which the human species belongs. The relative size of these latter three orders differs according to the classification scheme and definitions used by various authors.
Phylogenetically, 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.
The mammalian line of descent diverged from an amniote line at the end of the Carboniferous period. One line of amniotes would lead to reptiles, while the other would lead to synapsids. According to cladistics, mammals are a sub-group of synapsids. Although they were preceded by many diverse groups of non-mammalian synapsids (sometimes misleadingly referred to as mammal-like reptiles), the first true mammals appeared in the Triassic period. Modern mammalian orders appeared in the Palaeocene and Eocene epochs of the Palaeogene period.
Living mammal species can be identified by the presence of sweat glands, including those that are specialized to produce milk. However, other features are required when classifying fossils, since soft tissue glands and some other features are not visible in fossils. Paleontologists use a distinguishing feature that is shared by all living mammals (including monotremes), but is not present in any of the early Triassic synapsids: mammals use two bones for hearing that were used for eating by their ancestors. The earliest synapsids had a jaw joint composed 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). Most reptiles including lizards, crocodilians, dinosaurs (and their descendants the birds) use this system, as did non-mammalian synapsids such as therapsids. Mammals have a different jaw joint, however, composed only of the dentary (the lower jaw bone which carries the teeth) and the squamosal (another small skull bone). In mammals the quadrate and articular bones have become the incus and malleus bones in the middle ear.
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.
The majority of mammals have seven cervical vertebrae (bones in the neck); this includes bats, giraffes, whales, and humans. The few exceptions include the manatee and the two-toed sloth, which have only six cervical vertebrae, and the three-toed sloth with nine cervical vertebrae.
The lungs of mammals have a spongy texture and are honeycombed with epithelium having a much larger surface area in total than the outer surface area of the lung itself. The lungs of humans are typical of this type of lung.
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.
All mammalian brains possess a neocortex, a brain region that is unique to mammals.
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.
Most mammals give birth to live young (vivipary), but a few, namely the monotremes, lay eggs. The platypus and the echidna present a particular sex determination system that is different from other vertebrates.
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.
Nearly all mammals are endothermic ("warm-blooded"). Most mammals also have hair to help keep them warm. Like birds, mammals can forage or hunt in cold weather and climates where reptiles and large insects cannot.
Endothermy requires plenty of food energy, so pound for pound mammals eat more food than reptiles. Small insectivorous mammals eat prodigious amounts for their size.
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, who must think to outwit its prey.
Mammals evolved from four-legged ancestors. They use their limbs to walk, climb, swim, and fly. Some land mammals have toes that produce claws and hooves for climbing and running. Aquatic mammals such as whales and dolphins have flippers which evolved from legs.
Sloths travel slowly along branches rather than swinging energetically.
Buoyed by their aquatic environment, whales have evolved into the largest mammals and indeed the largest animals ever.
To maintain a high constant body temperature is energy expensive – mammals therefore need a nutritious and plentiful diet. While the earliest mammals were probably predators, different species have since adapted to meet their dietary requirements in a variety of ways. Some eat other animals – this is a carnivorous diet (and includes insectivorous diets). Other mammals, called herbivores, eat plants. A herbivorous diet includes sub-types such as fruit-eating and grass-eating. An omnivore eats both prey and plants. Carnivorous mammals have a simple digestive tract, because the proteins, lipids, and minerals found in meat require little in the way of specialized digestion. Plants, on the other hand, contain complex carbohydrates, such as cellulose. The digestive tract of an herbivore is therefore host to bacteria that ferment these substances, and make them available for digestion. The bacteria are either housed in the multi-chambered stomach or in a large cecum. The size of an animal is also a factor in determining diet type. Since small mammals have a high ratio of heat-losing surface area to heat-generating volume, they tend to have high-energy requirements and a high metabolic rate. Mammals that weigh less than about 18 oz (500 g) are mostly insectivorous because they cannot tolerate the slow, complex digestive process of a herbivore. Larger animals on the other hand generate more heat and less of this heat is lost. They can therefore tolerate either a slower collection process (those that prey on larger vertebrates) or a slower digestive process (herbivores). Furthermore, mammals that weigh more than 18 oz (500 g) usually cannot collect enough insects during their waking hours to sustain themselves. The only large insectivorous mammals are those that feed on huge colonies of insects (ants or termites).
Specializations in herbivory include: Granivory "seed eating", folivory "leaf eating", fruivory "fruit eating", nectivory "nectar eating", gumivory "gum eating", and mycophagy "fungus eating".
Synapsida, the group which contains mammals and their extinct relatives, originated during the Pennsylvanian epoch, when they split from the lineage that led to reptiles and birds. Non-mammalian synapsids were once called "mammal-like reptiles", although they are usually no longer considered reptiles. Mammals evolved from non-mammalian synapsids during the Early Jurassic.
The first fully terrestrial vertebrates were amniotes. Like amphibians, they have lungs and limbs. Amniotes' eggs, however, have internal membranes which 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 late Carboniferous. They descended from earlier tetrapods, 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.
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:
The Permian–Triassic extinction event ended the dominance of the therapsids, and in the Early Triassic all the medium to large land animal niches were taken over by early archosaurs, which were the ancestors of crocodilians, pterosaurs, dinosaurs and birds. After this "Triassic Takeover" the cynodonts and their descendants could only survive as small, mainly nocturnal insectivores. This may actually have accelerated the evolution of mammals – for example, the surviving cynodonts and their descendants had to evolve towards warm-bloodedness and the development of the middle ear, as their small size would have caused them to lose heat pretty quickly, and their nocturnal behaviour would not have favoured the usage of the eyes for sight.
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.
Unlike other mammals, female monotremes do not have nipples and feed their young by "sweating" milk from patches on their bellies.
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 give rise to the placental lineage (the surviving metatherians):
The traditional view is that: mammals only took over the medium- to large-sized ecological niches in the Cenozoic, after the extinction of the dinosaurs; but then they diversified very quickly; for example the earliest known bat dates from about 50M years ago, only 15M years after the extinction of the dinosaurs.
On the other hand 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.
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.
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.
A somewhat standardized classification system has been adopted by most current mammalogy classroom textbooks. The following taxonomy of extant and recently extinct mammals is from Vaughan et al. (2000).
In 1997, the mammals were comprehensively revised by Malcolm C. McKenna and Susan K. Bell, which has resulted in the "McKenna/Bell classification".
McKenna and Bell, Classification of Mammals: Above the species level, (1997) 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. The new McKenna/Bell classification was quickly accepted by paleontologists. The authors work 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 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 cross (†).
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 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. 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:
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Mammals are warm blooded. They are vertebrates, or have backbones. They have fur or hair. Mammals are the only organisms to produce milk. Actually, the word "mammal" comes from the Latin word "mamma" which means "breast".