mammal - Mammal anatomy, Origins, Classification

An animal characterized by having mammary glands in the female, along with several other features: a covering of hair (very sparse in some mammals); each side of the lower jaw formed from one bone (the dentary); three small bones in the middle ear (the hammer, anvil, and stirrup); seven vertebrae in the neck (only six in the manatee); and no nucleus in the red blood cells. Whales, dugongs, and manatees have lost their hind legs. Mammals are divided into placental mammals, in which the young develop in a womb (the uterus) where they are nourished from the blood of the mother and are born at an advanced stage of development; marsupials, in which the young are not nourished in a womb but are born at a very early developmental stage and develop outside the mother's body, usually in a pouch, nourished with milk from the mammary gland; and monotremes (or egg-laying mammals), in which the young hatch from an egg outside the body of the mother and are then nourished with milk. (Class: Mammalia, c.4000 species.)

iMammals Fossil range: Late Triassic - Recent
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; 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.

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.

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.

No mammals have hair that is naturally blue or green in color.

Reproduction

Most mammals give birth to live young, but a few (the monotremes) lay eggs.

A characteristic of mammals is that they have mammary glands, a defining feature present only in mammals.

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.

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

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.

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.

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.

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 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 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.

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 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. The most recent classification systems based on molecular studies have proposed four groups or lineages of placental mammals. 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 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. Order Sirenia: dugong and manatees (cosmopolitan tropical) Group II: Xenarthra Order Xenarthra: sloths and anteaters (Neotropical) and armadillos (Neotropical and Nearctic) Clade Boreoeutheria Group III: Euarchontoglires (Supraprimates) Superorder Euarchonta Order Scandentia: treeshrews (Southeast Asia). Order Rodentia: rodents (cosmopolitan) Group IV: Laurasiatheria Order Insectivora: moles, hedgehogs, shrews, solenodons Order Chiroptera: bats (cosmopolitan) Order Cetartiodactyla: cosmopolitan; 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 Order Rodentia: rodents Order Lagomorpha: rabbits, hares, and pikas Superorder Euarchonta: Order Primates: primates Order Scandentia: treeshrews Order Dermoptera: colugos Order Insectivora: shrews, moles, hedgehogs, etc. Order Pholidota: pangolins Superorder Ungulata: ungulates Order Macroscelidea: elephant shrews Order Tubulidentata: aardvark Order Hyracoidea: hyraxes Order Proboscidea: elephants Order Sirenia: manatees, dugong Order Perissodactyla: horses, tapirs, rhinoceroses Order Artiodactyla: even-toed ungulates Order Cetacea: whales