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Mammal

2007 Schools Wikipedia Selection. Related subjects: Mammals

   iMammals

                    Fossil range: Late Triassic - Recent

   Lion (Panthera leo)
   Lion (Panthera leo)
                Scientific classification

   Kingdom:   Animalia
   Phylum:    Chordata
   Subphylum: Vertebrata
   (unranked) Synapsida
   (unranked) Mammaliaformes
   Class:     Mammalia
              Linnaeus, 1758

                                   Orders

     * Multituberculata (extinct)
     * Palaeoryctoides (extinct)
     * Triconodonta (extinct)
     * Subclass Australosphenida
          + Ausktribosphenida
          + Monotremata
     * Subclass Eutheria (excludes extinct ancestors)
          + Afrosoricida
          + Anagaloidea (extinct)
          + Arctostylopida (extinct)
          + Artiodactyla
          + Carnivora
          + Cetacea
          + Chiroptera
          + Cimolesta (extinct)
          + Cingulata
          + Creodonta (extinct)
          + Condylarthra (extinct)
          + Dermoptera
          + Desmostylia (extinct)
          + Dinocerata (extinct)
          + Embrithopoda (extinct)
          + Hyracoidea
          + Insectivora
          + Lagomorpha
          + Leptictida (extinct)
          + Litopterna (extinct)
          + Macroscelidea
          + Mesonychia (extinct)
          + Notoungulata (extinct)
          + Perissodactyla
          + Pholidota
          + Pilosa
          + Plesiadapiformes (extinct)
          + Primates
          + Proboscidea
          + Pyrotheria (extinct)
          + Rodentia
          + Scandentia
          + Sirenia
          + Taeniodonta (extinct)
          + Tubulidentata
     * Subclass Marsupialia
          + Dasyuromorphia
          + Didelphimorphia
          + Diprotodontia
          + Microbiotheria
          + Notoryctemorphia
          + Paucituberculata
          + Peramelemorphia

   The mammals are the class of vertebrate animals characterized by the
   production of milk in females for the nourishment of young, from
   mammary glands present on most species and specialized skin glands in
   monotremes that seep or ooze milk; the presence of hair or fur;
   specialized teeth; three minute bones within the ear; the presence of a
   neocortex region in the brain; and endothermic or " warm-blooded"
   bodies. The brain regulates endothermic and circulatory systems,
   including a four-chambered heart. Mammals encompass some 5,500 species
   (including humans), distributed in about 1,200 genera, 152 families and
   up to 46 orders, though this varies with the classification scheme.

   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).

Mammal anatomy

Skeletal system

   The vast majority of mammals have seven cervical vertebrae (bones in
   the neck), including bats, giraffes, whales, and humans. The few
   exceptions include the manatee and the two-toed sloth, which each have
   only six cervical vertebrae, and the three-toed sloth with nine
   cervical vertebrae.

Respiratory system

   See the section about mammalian lungs in the lung article.

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.

Head and brain

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

Skin

   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.

   No mammals are known to have hair that is naturally blue or green in
   colour. 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 grey. The
   two-toed sloth can seem to have green fur, but this colour is caused by
   algae growths.

Reproduction

   Most mammals give birth to live young, but a few (the monotremes) lay
   eggs. Live birth also occurs in some non-mammalian species, such as
   guppies and hammerhead sharks; thus it is not a distinguishing
   characteristic of mammals. Although all mammals are endothermic, so are
   birds, and so this too is not a defining feature.

   A characteristic of mammals is that they have mammary glands, a
   defining feature present only in mammals. The monotremes branched from
   other mammals early on, and do not have nipples, but they do have
   mammary glands. Most mammals are terrestrial, but some are aquatic,
   including sirenia ( manatees and dugongs) and the cetaceans (dolphins
   and whales). Whales are the largest of all animals. There are
   semi-aquatic species such as seals which come to land to breed but
   spend most of the time in water.

Flight

   True flight has been observed only once in mammals, the bats; mammals
   such as flying squirrels and flying lemurs are more accurately
   classified as gliding mammals.

Origins

   Mammals belong among the amniotes, and in particular to a group called
   the synapsids, which are distinguished by the shape of their skulls,
   having a single hole on each side where jaw muscles attach, called a
   temporal fenestra. In comparison, dinosaurs, birds, and most reptiles
   are diapsids, with two temporal fenestrae on each side of the skull;
   and turtles, with no temporal fenestra, are anapsids.

   From early synapsids came the first mammal precursors, therapsids, and
   more specifically the eucynodonts, 220 million years ago ( mya) during
   the Triassic period.

   From the earliest synapsids (such as Archaeothyris), their temporal
   fenestra expanded as synapsids evolved. In cynodonts, the temporal
   fenestra is much larger than the pelycosaurs and the primitive
   therapsids. From cynodonts to mammals, the temporal fenestra has been
   modified, now no longer a hole. The erect posture (unlike reptiles and
   pelycosaurs whose posture was sprawling) evolved in the Middle Permian
   by therapsids. The secondary palate also evolved by therapsids at the
   same time (the therocephalians had both of these traits). Mammalian
   hair also evolved in the Middle Permian, probably evolved from scales.
   Pre-mammalian ears began evolving in the late Permian to early Triassic
   to their current state, as three tiny bones ( incus, malleus, and
   stapes) inside the skull; accompanied by the transformation of the
   lower jaw into a single bone. Other animals, including reptiles and
   pre-mammalian synapsids and therapsids, have several bones in the lower
   jaw, some of which are used for hearing; and a single ear-bone in the
   skull, the stapes. This transition is evidence of mammalian evolution
   from reptilian beginnings: from a single ear bone, and several lower
   jaw bones (for example the sailback pelycosaur, Dimetrodon) to
   progressively smaller "hearing jaw bones" (for example the cynodont,
   Probainognathus), and finally (possibly with Morganucodon, but
   definitely with Hadrocodium), true mammals with three ear bones in the
   skull and a single lower jaw bone. Hence pelycosaurs and cynodonts are
   sometimes called " mammal-like reptiles", but this is strictly
   incorrect as these two are not reptiles but synapsids.

   During the Mesozoic Era , mammals diversified into four main groups:
   multituberculates (Allotherium), monotremes, marsupials, and
   placentals. Multituberculates went extinct during the Oligocene, about
   30 million years ago, but the three other mammal groups are all
   represented today. Most early mammals remained small and shrew-like
   throughout the Mesozoic, but rapidly developed into larger more diverse
   forms following the Cretaceous-Tertiary extinction event 65 mya.

   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.

   A series of vertebrate animal lineages is listed below. All of these
   groups predate mammals, and are close relatives.
     * Jawless fish: Cambrian period to mid Ordovician periods
     * Bony fish: mid-Ordovician period to late Devonian period
     * Amphibians: late Devonian period to early Carboniferous period
     * Reptiliomorpha: Early Carboniferous period
     * Pelycosaurs (synapsids, or "mammal-like reptiles"): late
       Carboniferous period to very early Triassic period
     * Therapsids: Early Permian-Cretaceous (includes dicynodonts,
       dinocephalia, etc.)
     * Cynodonts: Late Permian-Middle Cretaceous (non-mammalian)
     * Mammaliformes: Mid-Triassic to Early Oligocene (includes
       non-therian mammals)

   Mammals appear in the mid-Jurassic period, and persist to the present
   (as Monotremes, Metatheria, and Eutheria)

In the Mesozoic

   Evolutionary biology has long held that most early mammals were tiny
   shrew-like animals that fed on insects. However, in January 2005, the
   journal Nature reported the discovery of two 130 million year old
   fossils of Repenomamus, one more than a meter in length, the other
   having remains of a baby dinosaur in its stomach (Nature, Jan. 15, 2005
   ). And the 2004 discovery in China of a 164 million year old 50 cm long
   aquatic mammal-like fossil of a thus far unknown species, dubbed
   Castorocauda, by a team led by Dr. Ji Qiang of Nanjing University and
   the Chinese Academy of Geological Sciences, was reported in February
   2006 in the journal Science (Science, Feb. 24, 2006 ).

   The earliest mammals include:
     * Adelobasileus: mid-Triassic
     * Megazostrodon: late Triassic and early Jurassic
     * Eozostrodon: late Triassic and early Jurassic
     * Sinoconodon: early Jurassic
     * Hadrocodium: early Jurassic
     * Fruitafossor: late Jurassic

   Although mammals existed alongside the dinosaurs, mammals only began to
   dominate after the mass extinction of the dinosaurs 65 mya, in the
   Cenozoic.

In the Paleocene

   During the next 8 million years, the Paleocene period (64–58 mya),
   mammals exploded into the ecological niches left by the extinction of
   the dinosaurs. Small rodent-like mammals still dominated, but medium
   and larger-sized mammals evolved.
     * Ptilodus: multituberculate
     * Pucadelphys andinus: an opossum-like marsupial
     * Purgatorius: a primate-like mammal, placental
     * Ectoconus: an early hoofed mammal, placental

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.

Standardized 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: platypus 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".

   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 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

   Extinct groups are represented by a dagger (†).

   Class Mammalia
     * Subclass Prototheria: monotremes: platypuses and echidnas
     * 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
               o 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 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 mya. 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
               o Clade Afroinsectiphilia
                    # Order Macroscelidea: elephant shrews (Africa).
                    # Order Afrosoricida: tenrecs and golden moles
                      (Africa)
                    # Order Tubulidentata: aardvark (Africa south of the
                      Sahara).
               o 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
               o Order Xenarthra: sloths and anteaters (Neotropical) and
                 armadillos (Neotropical and Nearctic)
     * Clade Boreoeutheria
          + Group III: Euarchontoglires ( Supraprimates)
               o Superorder Euarchonta
                    # Order Scandentia: treeshrews (Southeast Asia).
                    # Order Dermoptera: flying lemurs or colugos
                      (Southeast Asia).
                    # Order Primates: lemurs, bushbabies, monkeys, apes
                      (cosmopolitan).
               o Superorder Glires
                    # Order Lagomorpha: pikas, rabbits, hares (Eurasia,
                      Africa, Americas).
                    # Order Rodentia: rodents (cosmopolitan)
          + Group IV: Laurasiatheria
               o Order Insectivora: moles, hedgehogs, shrews, solenodons
               o Order Chiroptera: bats (cosmopolitan)
               o Order Cetartiodactyla: cosmopolitan; includes former
                 orders Cetacea (whales, dolphins and porpoises) and
                 Artiodactyla (even-toed ungulates, including pigs,
                 hippopotamus, camels, giraffe, deer, antelope, cattle,
                 sheep, goats).
               o Order Perissodactyla: odd-toed ungulates, including
                 horses, donkeys, zebras, tapirs, and rhinoceroses.
               o Clade Ferae
                    # Order Pholidota: pangolins or scaly anteaters
                      (Africa, South Asia).
                    # Order Carnivora: carnivores (cosmopolitan)

Classification system used in related articles

   In light of all the options available, the following classification
   system has been adopted for use in related articles.

   Class Mammalia
     * Subclass/Order Monotremata: egg-laying mammals
          + Order Monotremata: echidnas and platypus
     * Subclass Marsupialia: marsupials
          + Order Didelphimorphia: New World opossums
          + Order Paucituberculata: shrew opossums
          + Order Microbiotheria: Monito del Monte
          + Order Dasyuromorphia: marsupial carnivores
          + Order Notoryctemorphia: marsupial mole
          + Order Peramelemorphia: bandicoots and bilbies
          + Order Diprotodontia: koalas, wombats, kangaroos, possums, etc.
     * Subclass Placentalia
          + Order Xenarthra: sloths, anteaters, armadillos
          + Superorder Glires
               o Order Rodentia: rodents
               o Order Lagomorpha: rabbits, hares, and pikas
          + Superorder Euarchonta:
               o Order Primates: primates
               o Order Scandentia: treeshrews
               o Order Dermoptera: colugos
          + Order Insectivora: shrews, moles, hedgehogs, etc.
          + Order Chiroptera: bats
          + Order Carnivora: dogs, cats, weasels, seals, etc.
          + Order Pholidota: pangolins
          + Superorder Ungulata: ungulates
               o Order Macroscelidea: elephant shrews
               o Order Tubulidentata: aardvark
               o Order Hyracoidea: hyraxes
               o Order Proboscidea: elephants
               o Order Sirenia: manatees, dugong
               o Order Perissodactyla: horses, tapirs, rhinoceroses
               o Order Artiodactyla: even-toed ungulates
               o Order Cetacea: whales

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   with only minor checks and changes (see www.wikipedia.org for details
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