Mammal

Taxobox
name = Mammals
fossil_range = Late Triassic or Jurassic– Recent


image_caption = Clockwise from the upper left: giraffe, golden crown fruit bat, lion, hedgehog
regnum = Animalia
phylum = Chordata
subphylum = Vertebrata
infraphylum = Gnathostomata
superclassis = Tetrapoda
classis = Mammalia
classis_authority = Linnaeus, 1758
subdivision_ranks = Clades
subdivision = *Subclass †Allotheria*
*Subclass Australosphenida
*Subclass Theria
**Infraclass †Trituberculata
**Infraclass Metatheria
**Infraclass Eutheria

Mammals (class Mammalia) are a class of vertebrate animals characterized by the presence of sweat glands, including sweat glands modified for milk production, hair, three middle ear bones used in hearing, and a neocortex region in the brain.

All mammals (except for the five species of monotremes) give birth to live young instead of laying eggs. 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, ranging in size from the convert|30|–|40|mm|in|adj=on Bumblebee Bat to the convert|33|m|ft|adj=on Blue Whale, distributed in about 1,200 genera, 153 families, and 29 orders,MSW3] though this varies by classification scheme.

Most mammals belong to the placental group. The four largest orders within the placental mammals are Rodentia (mice, rats, and other small, gnawing mammals), Chiroptera (bats), Carnivora (dogs, cats, bears, and other mammals that primarily eat meat), and Cetartiodactyla (including numerous herbivore species, such as deer, sheep, goats, and buffalos, plus whales). The human species is also a placental mammal, a member of the order Primates.

Phylogenetically, Mammalia is defined as all descendants of the most recent common ancestor of monotremes (e.g., echidnas and platypuses) and therein 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 the sauropsid line at the end of the Carboniferous period. The sauropsids would evolve into modern-day reptiles and birds, while the synapsid branch led to mammals. The first true mammals appeared in the Jurassic period. Modern mammalian orders appeared in the Palaeocene and Eocene epochs of the Palaeogene period.

Distinguishing features

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 and non-mammalian synapsids use this system including lizards, crocodilians, dinosaurs (and their descendants the birds), and therapsids (mammal-like "reptiles"). 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. Note: "non-mammalian synapsids" above implies that mammals are a sub-group of synapsids, and that is exactly what cladistics says they are.

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, as it would be confusing if they found a fossil that had one feature, but not the other.

Anatomy and morphology

keletal system

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.

Respiratory system

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 at the bottom of the thorax. Contraction of the diaphragm pulls the bottom of the cavity in which the lung is enclosed downward. Air enters through the oral and nasal cavities; it flows through the larynx and into the trachea, which branches out into bronchi. Relaxation of the diaphragm has the opposite effect, passively recoiling during normal breathing. During exercise, the diaphragm contracts, forcing the air out more quickly and forcefully. The rib cage itself also is able to expand and contract 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.

Circulatory system

The mammalian heart has four chambers: the right atrium, right ventricle, left atrium, and left ventricle. Atria are for receiving blood; ventricles are for pumping blood to the lungs and body. The ventricles are larger than the atria and their walls are thick, because muscular walls are needed to forcefully pump the blood from the heart to the body and lungs. Deoxygenated blood from the body enters the right atrium, which pumps it to the right ventricle. The right ventricle pumps blood to the lungs, where carbon dioxide diffuses out, and oxygen diffuses in. From the lungs, oxygenated blood enters the left atrium, where it is pumped to the left ventricle (the largest and strongest of the 4 chambers), which pumps it out to the rest of the body, including the heart's own blood supply.

Nervous system

All mammalian brains possess a neocortex, a brain region that is unique to mammals.

Integumentary system

Mammals have integumentary systems made up of three layers: the outermost epidermis, the dermis, and the hypodermis. This characteristic is not unique to mammals, since it is found in all vertebrates.

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 order. Some mammals have very little, albeit in obscure parts of their bodies, but nonetheless, careful examination reveals the characteristic. 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.

Reproductive system

Most mammals give birth to live young (vivipary), but a few, such as the monotremes lay eggs and at least one of them, the platypus, presents a particular sex determination system that in some ways resembles that of birds. Live birth also occurs in some non-mammalian species, such as guppies, snakes, and hammerhead sharks; thus it is not a distinguishing characteristic of mammals.

Mammals have sweat glands, a defining feature present only in mammals. Some of these glands produce milk (in what are called mammary glands), a liquid used by newborns as their primary source of nutrition. The monotremes branched from other mammals early on, and do not have the nipples seen in most mammals, but they do have mammary glands.

Physiology

Endothermy

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.

A rare exception, the naked mole rat is ectothermic ("cold-blooded"). Birds are also endothermic, so endothermy is not a defining mammalian feature.

Intelligence

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 relatively smaller than a cat, who must think to outwit its prey.cite book
editor = Don E. Wilson & David Burnie
title = Animal: The Definitive Visual Guide to the World's Wildlife
pages = 86–89
publisher = DK Publishing
edition = 1st edition
year = 2001
id = ISBN 978-0789477644 ]

ocial structure

The dependence of the young mammal on its mother for nourishment has made possible a period of training. Such training permits the nongenetic transfer of information between generations. The ability of young mammals to learn from the experience of their elders has allowed a behavioral plasticity unknown in any other group of organisms and has been a primary reason for the evolutionary success of mammals. The possibility of training is one of the factors that has made increased brain complexity a selective advantage. Increased associational potential and memory extend the possibility of learning from experience, and the individual can make adaptive behavioral responses to environmental change. Individual response to short-term change is far more efficient than genetic response.

Some types of mammals are solitary except for brief periods when the female is in estrus. Others, however, form social groups. Such groups may be reproductive or defensive, or they may serve both functions. In those cases that have been studied in detail, a more or less strict hierarchy of dominance prevails. Within the social group, the hierarchy may be maintained through physical combat between individuals, but in many cases stereotyped patterns of behaviour evolve to displace actual combat, thereby conserving energy while maintaining the social structure.

A pronounced difference between sexes (sexual dimorphism) is frequently extreme in social mammals. In large part this is because dominant males tend to be those that are largest or best-armed. Dominant males also tend to have priority in mating or may even have exclusive responsibility for mating within a “harem.” Rapid evolution of secondary sexual characteristics, including size, can take place in a species with such a social structure.

A complex behavior termed “play” frequently occurs between siblings, between members of an age class, or between parent and offspring. Play extends the period of maternal training and is especially important in social species, providing an opportunity to learn behaviour appropriate to the maintenance of dominance.

Locomotion

"See also Animal locomotion"

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 fins which evolved from legs.

Terrestrial

"See also Terrestrial locomotion"

Specialization in habitat preference has been accompanied by locomotor adaptations. Terrestrial mammals have a number of modes of progression. The primitive mammalian stock walked plantigrade—that is, with the digits, bones of the midfoot, and parts of the ankle and wrist in contact with the ground. The limbs of ambulatory mammals are typically mobile, capable of considerable rotation.

Mammals modified for running are termed cursorial. The stance of cursorial species may be digitigrade (the complete digits contacting the ground, as in dogs) or unguligrade (only tips of digits contacting the ground, as in horses). In advanced groups limb movement is forward and backward in a single plane.

Saltatory (leaping) locomotion, sometimes called “ricochetal,” has arisen in several unrelated groups (some marsupials, lagomorphs, and several independent lineages of rodents). This mode of locomotion is typically found in mammals living in open habitats. Jumping mammals typically have elongate, plantigrade hind feet, reduced forelimbs, and long tails. Convergent evolution within a given adaptive mode has contributed to the ecological similarity of regional mammalian faunas.

Mammals of several orders have attained great size (elephants, hippopotamuses, and rhinoceroses) and have converged on specializations for a ponderous mode of locomotion referred to as “graviportal.” These animals have no digit reduction and deploy the digits in a circle around the axis of the limb for maximum support, like the pedestal of a column."mammal." Encyclopædia Britannica. Standard Edition. Chicago: Encyclopædia Britannica, 2007.]

Arboreal

"See also Scansorial locomotion"

Well-adapted arboreal mammals frequently are plantigrade, five-toed, and equipped with highly mobile limbs. Some species, including many New World monkeys, have a prehensile tail, which is used like a fifth hand. Brachiation, or “arm walking,” in which the animal hangs from branches and moves by a series of long swings, is an adaptation seen in gibbons. The primitive opposable anthropoid thumb is reduced as a specialization for this method of locomotion. Tarsiers are highly arboreal primates that have expanded pads on the digits to improve grasping, whereas many other arboreal mammals have claws or well-developed nails.

Sloths travel slowly along branches rather than swinging energetically.

Aquatic

Four mammalian groups are fully aquatic. Sirenians (dugongs and manatees) eat plants. Cetaceans (whales, dolphins, and porpoises) and pinnipeds (seals and walruses) eat krill or fish. The sea otter eats a variety of invertebrates and fish. Some semiaquatic mammals are very similar to their close land-borne relatives (otters, muskrats, and water shrews, for example). Other mammals have undergone profound adaptation for swimming and life at sea. Walruses and seals give birth to and nurse their young on land, but cetaceans are completely helpless out of water. They depend on water for mechanical support and thermal insulation. Buoyed by their aquatic environment, whales have evolved into the largest mammals and indeed the largest animals ever.

Aerial

"See also Aerial locomotion"

Bats are the only truly flying mammals. Only with active flight have the resources of the aerial habitat been successfully exploited. Mammals belonging to other groups (colugos, marsupials, rodents) are adapted for gliding. A gliding habit is frequently accompanied by scansorial (climbing) locomotion. Many nongliders, such as tree squirrels, are also scansorial.

Feeding

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 animal prey- this is a carnivorous diet (and includes insectivorous diets). Other mammals, called herbivores, eat plants. An herbivorous diet includes sub-types such as fruit-eating and grass-eating. An omnivore eats boths 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 multichambered 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 18oz (500g) 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 18oz (500g) 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).

Evolutionary history

Clade
label1="Mammaliaformes"
1=Clade
1="Adelobasileus"
label2=void
2=Clade
1="Sinocodon"
label2=void
2=clade
1="Morganucodon"
label2=void
2=Clade
1="Docodonta"
label2=void
2=Clade
1=––"Hadrocodium"
2=––"Mammalia"
The evolution of mammals from synapsids, also known as mammal-like "reptiles" was a gradual process which took approximately 70 million years, from the mid-Permian to the mid-Jurassic, and by the mid-Triassic there were many species that looked like mammals. Note that synapsids are not reptiles at all, but belong to a distinct lineage of tetrapods.

Evolution

The first fully terrestrial vertebrates were amniotes. Like the amphibians they evolved from, they had legs and lungs. Amniotes' eggs, however, had internal membranes which allowed the developing embryo to breathe but kept water in. This allowed amniotes to 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 amphibians, which were numerous at the time, and lived on land already inhabited by insects, other invertebrates, ferns, mosses, and other plants. Within a few million years two important amniote lineages became distinct: the synapsids, from which mammals are descended; and the sauropsids, from which lizards, snakes, crocodilians, dinosaurs and birds are descended. [cite web | url = http://www.palaeos.org/Amniota | title = Amniota - Palaeos] 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. [cite web | url=http://www.palaeos.com/Vertebrates/Units/Unit390/000.html | title=Synapsida overview - Palaeos]

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; incisors which are equal in size. [cite web | url=http://www.palaeos.com/Vertebrates/Units/400Therapsida/100.html | title=Therapsida - Palaeos] 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:
*gradual development of a bony secondary palate. [cite book | last=Kermack | first D.M. | last2=Kermack | first2 K.A. | title=The evolution of mammalian characters | publisher=Croom Helm | date=1984 | ISBN=079915349]
*the dentary gradually becomes the main bone of the lower jaw.
*progress 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 therapsids retained sprawling limbs (some late forms may have had semi-erect hind limbs); Permian carnivorous therapsids had sprawling forelimbs, and some late Permian ones also had semi-sprawling hindlimbs. In fact modern monotremes still have semi-sprawling limbs.
*in the Triassic, progress towards the mammalian jaw and middle ear.
*there is possible evidence of hair in Triassic therapsids, but none for Permian therapsids.
*some scientists have argued that some Triassic therapsids show signs of lactation.

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 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. [cite web | url=http://www.palaeos.com/Vertebrates/Units/410Cynodontia/410.000.html | title=Cynodontia: Overview - Palaeos] This may actually have accelerated the evolution of mammals - for example the surviving cynodonts and their descendants had to evolve towards warm-bloodedness because their small bodies would otherwise have lost heat quickly, especially as they were active mainly at night.

The first true mammals appeared in the early Jurassic, 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. [cite web | url=http://www.palaeos.com/Vertebrates/Units/Unit420/420.300.html | title=Symmetrodonta - Palaeos]

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:
*they use the same orifice to urinate, defecate and reproduce ("monotreme" means "one hole") - as lizards and birds also do.
*they lay eggs which are leathery and uncalcified, like those of lizards, turtles and crocodilians.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. [cite web | url=http://news.nationalgeographic.com/news/2003/12/1215_031215_oldestmarsupial.html | title=Oldest Marsupial Fossil Found in China | date=December 15, 2003 | publisher=National Geographic News]

The living Eutheria ("true beasts") are all placentals. But the earliest known eutherian, "Eomaia", found in China and dated to 125M years ago, has some features which are more like those of marsupials (the surviving metatherians): [cite web | url=http://www.evolutionpages.com/Eomaia%20scansoria.htm | title=Eomaia scansoria: discovery of oldest known placental mammal]
*Epipubic bones extending forwards from the pelvis, which are not found in any modern placental, but are found in marsupials, monotremes and mammaliformes such as multituberculates. In other words, they appear to be an ancestral feature which subsequently disappeared in the placental lineage.
*A narrow pelvic outlet, which 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.It is not certain when placental mammals evolved - the earliest undisputed fossils of placentals come from the early Paleocene, after the extinction of the dinosaurs. [ [http://news.nationalgeographic.com/news/2007/06/070620-mammals-dinos.html Dinosaur Extinction Spurred Rise of Modern Mammals] ]

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. [cite web | url=http://news.nationalgeographic.com/news/2006/02/0223_060223_beaver.html | title=Jurassic "Beaver" Found; Rewrites History of Mammals]

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. [ [http://www.newscientist.com/news/news.jsp?id=ns99996647 Rogue finger gene got bats airborne] ]

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 [cite journal | last=Bininda-Emonds | first=O.R.P. | last2=Cardillo | first2=M. | last3=Jones | first3=K.E. | last4='et al' | title=The delayed rise of present-day mammals | journal=Nature | issue=446 | pages=507–511 |date=2007 | url=http://scienceblogs.com/pharyngula/2007/03/dont_blame_the_dinosaurs.php] But paleontologists object that no placental fossils have been found from before the end of the Cretaceous [ [http://news.nationalgeographic.com/news/2007/06/070620-mammals-dinos.html Dinosaur Extinction Spurred Rise of Modern Mammals] ]

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.

Earliest appearances of features

"Hadrocodium", whose fossils date from the early Jurassic, provides the first clear evidence of fully mammalian jaw joints.

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): [cite journal | last=Oftedal | first=O.T. | title=The mammary gland and its origin during synapsid evolution | journal=Journal of Mammary Gland Biology and Neoplasia | volume=7 | issue=3 | pages=225–252 |date=2002 | doi=10.1023/A:1022896515287 ] [cite journal | last=Oftedal | first=O.T. | title=The origin of lactation as a water source for parchment-shelled eggs=Journal of Mammary Gland Biology and Neoplasia | volume=7 | issue=3 | pages=253–266 |date=2002 ] [ [http://nationalzoo.si.edu/ConservationAndScience/SpotlightOnScience/oftedalolav20030714.cfm Lactating on Eggs] ]

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. [cite journal | last=Brink | first=A.S. | title=A study on the skeleton of "Diademodon" | journal=Palaeontologia Africana | volume=3 | pages=3–39 |date=1955 ] [cite book | last=Kemp | first=T.S. | title=Mammal-like reptiles and the origin of mammals | publisher=Academic Press | date=1982 | location=London | pages=363] 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. [Bennett, A. F. and Ruben, J. A. (1986) "The metabolic and thermoregulatory status of therapsids"; pp. 207–218 in N. Hotton III, P. D. MacLean, J. J. Roth and E. C. Roth (eds), "The ecology and biology of mammal-like reptiles", Smithsonian Institution Press, Washington.] [cite journal | last=Estes | first=R. | title=Cranial anatomy of the cynodont reptile "Thrinaxodon liorhinus" | journal=Bulletin of the Museum of Comparative Zoology | pages=165–180 |date=1961 | unused_data=|issue1253 ]

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. [cite journal | last=Kielan−Jaworowska | first=Z. | last2=Hurum | first2=J.H.. | title=Limb posture in early mammals: Sprawling or parasagittal | journal=Acta Palaeontologica Polonica | volume=51 | issue=3 | pages=10237–10239 |date=2006 | url=http://www.app.pan.pl/acta51/app51-393.pdf | format=dead link|date=June 2008 – ^{[http://scholar.google.co.uk/scholar?hl=en&lr=&q=author%3AKielan%E2%88%92Jaworowska+intitle%3ALimb+posture+in+early+mammals%3A+Sprawling+or+parasagittal&as_publication=Acta+Palaeontologica+Polonica&as_ylo=&as_yhi=&btnG=Search Scholar search]} ] "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. [cite book | last=Paul | first=G.S. | title=Predatory Dinosaurs of the World | publisher=Simon and Schuster | date=1988| location=New York | pages=464] 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, and there is no certain evidence of fur before "Castorocauda", about 164M years ago.

Classification

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.

tandardized textbook classification

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).

Class Mammalia
*Subclass Prototheria: monotremes: platypuses and echidnas
*Subclass Theria: live-bearing mammals
**Infraclass Metatheria: marsupials
**Infraclass Eutheria: placentals

McKenna/Bell classification

In 1997, the mammals were comprehensively revised by Malcolm C. McKenna and Susan K. Bell, which has resulted in the "McKenna/Bell classification".

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 (†).

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

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.

*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 (cosmopolitan), humans
***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)

ee also

* List of mammals
* List of prehistoric mammals
* Lists of mammals by region
* Mammal classification
* Mammals discovered in the 2000s

References

Bibliography

*Bergsten, Johannes. February 2005. "A review of long-branch attraction". "Cladistics" 21:163–193. ( [http://www.usp.br/mz/forum/pdf/Bergsten_2005_long_branch_attraction.pdf pdf version] )

* Brown, W.M. (2001). [http://www.rci.rutgers.edu/~wmbrown/BROWN.TREE.pdf Natural selection of mammalian brain components.] "Trends in Ecology and Evolution, 16," 471-473.

* [http://www.geocities.com/jaffacity/Mammalia_Palaestina.html Khalaf-von Jaffa, Norman Ali Bassam Ali Taher (2006). Mammalia Palaestina: The Mammals of Palestine.] Gazelle: The Palestinian Biological Bulletin. Number 55, July 2006. pp. 1–46.

*McKenna, Malcolm C., and Bell, Susan K. 1997. "Classification of Mammals Above the Species Level." Columbia University Press, New York, 631 pp. ISBN 0-231-11013-8

*Nowak, Ronald M. 1999. "Walker's Mammals of the World", 6th edition. Johns Hopkins University Press, 1936 pp. ISBN 0-8018-5789-9

*Simpson, George Gaylord. 1945. "The principles of classification and a classification of mammals". "Bulletin of the American Museum of Natural History", 85:1–350.

*William J. Murphy, Eduardo Eizirik, Mark S. Springer et al., "Resolution of the Early Placental Mammal Radiation Using Bayesian Phylogenetics",Science, Vol 294, Issue 5550, 2348–2351 , 14 December 2001.

*Springer, Mark S., Michael J. Stanhope, Ole Madsen, and Wilfried W. de Jong. 2004. "Molecules consolidate the placental mammal tree". "Trends in Ecology and Evolution," 19:430–438. ( [http://www.zi.ku.dk/evolbiology/courses/ME04/7_9/springer200-phyl.pdf PDF version] )

*Vaughan, Terry A., James M. Ryan, and Nicholas J. Capzaplewski. 2000. "Mammalogy: Fourth Edition". Saunders College Publishing, 565 pp. ISBN 0-03-025034-X (Brooks Cole, 1999)

*Jan Ole Kriegs, Gennady Churakov, Martin Kiefmann, Ursula Jordan, Juergen Brosius, Juergen Schmitz. (2006) Retroposed Elements as Archives for the Evolutionary History of Placental Mammals. PLoS Biol 4(4): e91. [http://biology.plosjournals.org/perlserv/?request=get-document&doi=10.1371/journal.pbio.0040091]

*David MacDonald, Sasha Norris. 2006. "The Encyclopedia of Mammals", 3rd edition. Printed in China, 930 pp. ISBN 0-681-45659-0.

External links

* [http://www.globaltwitcher.com/taxa_order.asp?classid=3 GlobalTwitcher.com - All species in the world with distribution maps and images]
* [http://www.nceas.ucsb.edu/~alroy/nafmsd.html North American Fossil Mammal Systematics Database]
* [http://paleocene-mammals.de/ Paleocene Mammals] , a site covering the rise of the mammals
* [http://www.enchantedlearning.com/subjects/mammals/Evolution.shtml Evolution of Mammals] , a brief introduction to early mammals
* [http://tellapallet.com/tree_of_life.htm Tree of Life poster] - Shows mammals' evolutionary relation to other organisms
* [http://home.arcor.de/ktdykes/mesomamm.htm The Evolution of Mesozoic Mammals, a Rough Sketch] , an informal introduction
* [http://www.carnegiemnh.org/research/news.html Carnegie Museum of Natural History] , some discoveries of early mammal fossils
* [http://www.geocities.com/mammal_taxonomy/index.html Mammal Taxonomy] , database of mammals of the world, updated each month
* [http://brainmaps.org/index.php?p=datasets-species High-Resolution Images of various Mammalian Brains]
* [http://www.learnanimals.com/mammals.php Mammal Species] , collection of information sheets about various mammal species
* [http://biology.plosjournals.org/perlserv/?request=get-document&doi=10.1371/journal.pbio.0040111 Summary of molecular support for Epitheria]
* [http://nlbif.eti.uva.nl/bis/marine_mammals.php Marine Mammals of the World] —An overview of all marine mammals, including descriptions, multimedia and a key
* [http://www.fmnh.helsinki.fi/users/haaramo/Metazoa/Deuterostoma/Chordata/Synapsida/Basal_Mammalia/Mammaliaformes_2.htm#Australophenida Mikko's Phylogeny Archive]
* [http://www.european-mammals.org/php/mapmaker.php European Mammal Atlas EMMA] from Societas Europaea Mammalogica
* [http://www.mammalogy.org MAMMALOGY .org] The American Society of Mammalogists was established in 1919 for the purpose of promoting the study of mammals, and this website includes a mammal image library