- Mammal is the title of a track from They Might Be Giants' 1992 release
Apollo 18.
The mammals are the class of vertebrate animals primarily
characterized by the presence of mammary glands in the female which produce milk for the nourishment of young;
the presence of hair or fur; and which have endothermic or "warm blooded" bodies. The brain regulates endothermic and circulatory
systems, including a four-chambered heart. Mammals embrace more than 5,000 genera,
distributed in 425 families and up to 46 orders (the number depends on the classification scheme adopted).
Phylogenetically, the Mammalia are defined as the last common ancestor of monotremes (e.g., echidnas) and therian mammals (e.g., hedgehogs), and all of this last common
ancestor's descendants.
Characteristics
While most mammals give birth to live young, there are a few mammals—the monotremes—that lay eggs. Live birth also occurs in a variety of
non-mammalian species; thus it is not a distinguishing characteristic of mammals. Birds are
also endothermic.
While monotremes do not have nipples, they do have mammary glands, meaning that they meet all conditions for inclusion in the
class Mammalia. It should be noted that the current trend in taxonomy is to emphasize common ancestry; the diagnostic characteristics are useful for identifying this ancestry,
but if, for example, a cetacean were found that had no hair at all, it would still
be classed as a mammal.
Mammals have three bones in each ear and one (the
dentary) on each side of the lower jaw; all other vertebrates with ears have one bone (the
stapes) in the ear and at least three on each side of the jaw. A group of therapsids called cynodonts had three bones in the jaw, but the
main jaw joint was the dentary and the other bones conducted sound.
Mammals have integumentary systems made up of three layers: the outermost epidermis, the dermis, and the hypodermis.
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.
Mammals belong among the amniotes, and in particular to a group called the
synapsids, distinguished by the shape of their skulls. Within this group they
developed from the therapsids, and more specifically the eucynodonts, 220 million years ago during the Triassic period. During the Mesozoic period they diversified into
the three main groups found today (monotremes, marsupials, and placentals). They remained small and shrew-like throughout the era, but rapidly developed into larger forms following the Cretaceous-Tertiary extinction
event 65 million years ago.
The names "Prototheria", "Metatheria" and "Eutheria" expressed the theory that Placentalia were descendants of Marsupialia, which were in
turn descendants of Monotremata, but this theory has been refuted. However,
Eutheria and Metatheria are often
used in paleontology, especially with regards to mammals of the Mesozoic.
Most mammals are terrestrial, but a number are secondarily aquatic, including whales which are the largest of all animals. One
order, the bats, have developed flight.
Classification
George Gaylord Simpson's classic 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 1945 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 (q.v.). Though field work gradually made
Simpson's classification outdated, it remained the closest thing to an official classification of mammals.
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 extict 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 (categories which fall
between classes and orders) that are likely to be glossed over by the layman.
The published re-classification forms both a comprehensive and authoritative record of approved names and classifications and
a list of invalid names.
Click on the highlighted link for a table comparing the traditional and the new McKenna/Bell
classifications of mammals
Class Mammalia
- Infraclass Ornithodelphia
- Order Monotremata
- Family Tachyglossidae: echidnas
- Family Ornithorhynchidae: platypus
- Family Kollikodontidae: ancient, extinct monotreme somewhat similar to
platypus
- Infraclass Metatheria
- Order Didelphimorphia
- Order Paucituberculata
- Family Caenolestidae: shrew or rat opossums
- Order Microbiotheria
- Family Microbiotheriidae: Monito del Monte
- Order Dasyuromorphia
- Family Thylacinidae: Thylacine
- Family Dasyuridae: marsupial carnivores
- Family Myrmecobiidae: Numbat
- Order Notoryctemorphia
- Family Notoryctidae: marsupial moles
- Superorder Syndactyla
- Order Peramelemorphia
- Family Peramelidae: bandicoots
and bilbies
- Family Peroryctidae: rainforest bandicoots
- Order Diprotodontia
- Family Phascolarctidae: Koala
- Family Vombatidae: wombats
- Family Phalangeridae: cuscuses, brushtail possums
- Family Burramyidae: pygmy possums
- Family Tarsipedidae: Honey Possum
- Family Petauridae: striped possums, wrist-ringed gliders
- Family Pseudocheiridae: ringtailed possums and allies
- Family Potoroidae: potoroos,
bettongs
- Family Acrobatidae: Feathertail Glider
- Family Hypsiprymnodontidae: Musky Rat Kangaroo
- Family Macropodidae: kangaroos, wallabies, and allies
- Infraclass Eutheria
- Order Xenarthra
- Family Bradypodidae: three-toed sloths
- Family Megalonychidae: two-toed sloths
- Family Choloepinae
- Family Dasypodidae: armadillos
- Family Myrmecophagidae: true anteaters
- Order Pholidota
- Family Manidae: pangolins
- Glires
- Order Rodentia
- Suborder Sciurognathi
- Family Aplodontidae: Mountain
Beaver
- Family Sciuridae: squirrels, marmots
- Family Castoridae: beavers
- Family Geomyidae: pocket gophers
- Family Hetromyidae: pocket
mice, kangaroo rats
- Family Dipodidae: jerboas, birch mice, jumping mice
- Family Muridae: rats and mice)
- Family Anomaluridae: scaly-tailed squirrels
- Family Pedetidae: Springhaas
- Family Ctenodactylidae:
gundis
- Family Myoxidae: dormice
- Suborder Hystricognathi
- Family Bathyergidae: mole
rats
- Family Hystricidae: Old World porcupines
- Family Petromuridae: Dassie
Rat
- Family Thryonomyidae: cane
rats
- Family Erethizontidae: New World porcupines
- Family Chinchillidae: chinchillas, viscachias
- Family Dinomyidae: Paracana
- Family Caviidae: cavies, guinea pigs, Patagonian Hare
- Family Hydrochaeridae:
Capybara
- Family Dasyproctidae: agoutis, acouchis
- Family Agoutidae: pacas
- Family Ctenomyidae:
tuco-tucos
- Family Octodontidae: viscacha rats, coruro
- Family Abrocomidae: chinchilla
rats
- Family Echimyidae: spiny
rats
- Family Capromyidae: hutias
- Family Myocastoridae:
Nutria
- Order Lagomorpha
- Family Ochotonidae: pikas
- Family Leporidae: rabbits, hares
- Order Macroscelidea
- Family Macroscelididae: elephant shrews
- Superorder Archonta
- Order Primates
- Suborder Strepsirhini
- Family Cheirogaleidae: dwarf and mouse lemurs
- Family Lemuridae: lemurs
- Family Megaladapidae: sportive lemurs
- Family Indridae: indrid lemurs, sifakas
- Family Daubentoniidae: Aye-aye
- Family Loridae: lorises, Potto
- Family Galagonidae: galagos
- Suborder Haplorhini
- Family Tarsiidae: tarsiers
- Family Cebidae: marmosets, tamarins, capuchins and squirrel monkeys
- Family Atelidae: owl, howler and spider monkeys, titis, sakis and ukaris
- Family Cercopithecidae: Old World monkeys
- Family Hylobatidae: gibbons, siamang)
- Family Hominidae: Gorilla, chimpanzees, Orangutan, Human
- Order Scandentia
- Family Tupaiidae: tree shrews
- Order Chiroptera
- Suborder Macrochiroptera
- Family Pteropodidae: Old
World fruit bats
- Suborder Microchiroptera
- Family Rhinopomatidae:
mouse-tailed bats
- Family Craseonycteridae: Kitti's Hog-nosed Bat
- Family Emballonuridae:
sac-winged or sheath-tailed bats
- Family Nycteridae: slit-faced
bats
- Family Megadermatidae:
false vampire bats
- Family Rhinolophidae: horseshoe bats
- Family Noctilionidae: fishing bats
- Family Mormoopidae:
leaf-chinned bats
- Family Phyllostomidae: New World leaf-nosed bats
- Family Natalidae: funnel-eared
bats
- Family Furipteridae: smoky
bats
- Family Thyropteridae:
disk-winged bats
- Family Myzopodidae:
Sucker-footed Bat
- Family Vespertilionidae: common bats
- Family Mystacinidae:
short-tailed bats
- Family Molossidae: free-tailed bats
- Order Dermoptera
- Family Cynocephalidae: colugos
- Order Insectivora
- Family Solenodontidae: solenodons
- Family Nesophontidae:
nesophontids
- Family Tenrecidae: tenrecs
- Family Chrysochloridae: golden moles
- Family Erinaceidae: hedgehogs,
gymnures
- Family Soricidae: shrews
- Family Talpidae: moles and desmans
- Order Carnivora
- Suborder Fissipedia
- Superfamily Caniformia
- Family Canidae: wolves, Coyote, foxes, jackals
- Family Procyonidae: raccoons, kinkajou, Red Panda
- Family Mustelidae: weasels, otters
- Family Mephitidae: skunks
- Family Ursidae: bears, Giant Panda
- Superfamily Feliformia
- Family Felidae: cats
- Family Herpestidae: mongooses
- Family Hyaenidae: hyenas, Aardwolf
- Family Viverridae: civets, genets
- Suborder Pinnipedia
- Superfamily Otarioidae
- Superfamily Phocoidae
- Superorder Ungulata
- Order Tubulidentata
- Family Orycteropodidae: Aardvark
- Order Hydracoidea
- Family Procaviidae: hyraxes
- Order Proboscidea
- Family Elephantidae: elephants
- Order Sirenia
- Family Dugongidae: Dugong, Stellar Sea Cow
- Family Trichechidae: manatees
- Order Perissodactyla
- Family Equidae: horses, asses, zebras
- Family Tapiridae: tapirs
- Family Rhinocerotidae: rhinceroses
- Order Artiodactyla
- Suborder Suiformes
- Suborder Tylopoda
- Suborder Ruminantia
- Order Cetacea
- Suborder Mysticeti
- Suborder Odontoceti
References
- Wilson, D. E., and D. M. Reeder (eds). 1993. Mammal Species of the World. Smithsonian Institution
Press, 1206 pp.
See also
External link
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