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 small bones within the ear; the presence of a neocortex region in the brain; and endothermic or "warm-blooded" bodies, and, in most cases, the existence of a placenta in the ontogeny. 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 forty-six 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).
Living mammal species can be identified by the presence of mammary glands in females which produce milk.
But other features are required when classifying fossils, since mammary glands and other features are not visible in fossils. The evolution of mammals from synapsids (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.
Paleontologists use a distinguishing feature that is shared by all living mammals (including monotremes), but is not present in any of the early Triassic mammal-like reptiles: mammals use two bones for hearing that all reptiles use for eating. The earliest reptiles 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). All non-mammalian reptiles use this system including lizards, crocodilians, dinosaurs (and their descendants the birds) and mammal-like reptiles. But mammals have a different jaw joint, composed only of the dentary (the lower jaw bone which carries the teeth) and the squamosal (another small skull bone). And in mammals the quadrate and articular bones have become the incus and malleus bones in the middle ear. Note: "non-mammalian reptiles" above implies that mammals are a sub-group of reptiles, 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. But 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.
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 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.
All mammalian brains possess a neocortex which is a brain region that is unique to mammals.
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 color. 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 and the polar bear can seem to have green fur, but this color is caused by algae growths.
Most mammals give birth to live young (vivipary), 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, so this too is not a defining feature.
Mammals 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.
The only mammals for which true flight has been observed are bats; mammals such as flying squirrels and flying lemurs are more accurately classified as gliding mammals.
Mammals belong to a group of amniotes called the synapsids that have a single hole (temporal fenestra) low on each side of the skull on each side where jaw muscles attach. In comparison, dinosaurs, birds, and most living reptiles are diapsids, with two temporal fenestrae on each side of the skull; and turtles, with no temporal fenestra, are anapsids. The synapsids diverged from the other reptile lineages very early, in the late Carboniferous, and one of the earliest examples is Archaeothyris.
The synapsid pelycosaurs were the dominant land vertebrates of the early Permian. The therapsids probably evolved from pelycosaur ancestors, and consist of a sequence of groups which became increasingly mammal-like, especially the Triassic cynodonts ("dog-teeth"). The main article on synapsids presents a probable phylogeny ("family tree") which shows how mammals evolved from early synapsids.
Some cynodonts such as the early to mid Triassic Cynognathus had erect limbs and possibly hair or fur, and may have been warm-blooded.
Several groups of mammaliformes ("almost mammals") arose from the mid Triassic onwards - these had non-mammalian jaw joints but almost certainly had hair and were warm-blooded. Megazostrodon is a good example. Some of them were very successful in their own right, for example the multituberculates appeared in the mid-Jurassic and became extinct about 100 million years later, in the Oligocene.
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 mammaliformes. Hadrocodium is an example of the transition to true mammal status - it had a mammalian jaw joint but there is some debate about whether its middle ear was fully mammalian. The triconodonts may have been the earliest true mammals known so far.
Like their predecessors and modern monotremes, the first mammals laid eggs. The earliest known fossils of marsupial and placental mammals date from the mid to late Cretaceous, about 170 million years after the first therapsids.
Lactation is the distinguishing feature of living mammals, but scientists are not sure when it evolved. Since all living mammals (including monotremes) produce milk, their last common ancestor produced milk. Some scientists have suggested that the cynodonts produced milk, but this idea is a plausible speculation with little hard evidence to support it.
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.
The earliest-known species seen to be mammal-like is the megazostrodon, which evolved toward the end of the Triassic period. It had evolved to possess fur, be warm-blooded, and was the first animal to have mammary glands, the defining feature of mammals. It is also believed to be nocturnal, for which a warm-blooded nature is necessary. However, it still bore young by laying reptile-like leathery eggs.
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 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.
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).
Throughout the Mesozoic therapsids, mammaliformes and mammals lived in the shadow of archosaurs - first the "thecodonts" and following, the dinosaurs were the dominant land vertebrates.
From the mid-Triassic onwards, mammaliformes and mammals were mostly about between shrew-sized and rat-sized. But 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).
Some orders of placental mammals appear to have arisen before the end of the Cretaceous - insectivores, rodents, ungulates, and possibly primates. But they were not able to expand into their modern ecological niches until after the extinction of the dinosaurs.
Shortly after the start of the Paleocene, mammals exploded into the ecological niches left by the extinction of the dinosaurs and also found some which the dinosaurs apparently never occupied:
* Ungulates became the major herbivores.
* Primates became arboreal insectivores and later frugivores.
Some orders of mammals only appeared after the extinction of the dinosaurs:
* Various orders of carnivorous mammals - mesonychids, then creodonts, and finally Carnivora.
* Cetaceans (whales, dolphins and their ancestors).
* Probably bats, although bat-like teeth have been found in late Cretaceous rocks.
Some Cenozoic mammals were considerably larger than their nearest modern equivalents - but none approached the size of the large dinosaurs.
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 layman.
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 (†).
* Subclass Prototheria: monotremes: echidnas and the Platypus
* Subclass Theriiformes: live-bearing mammals and their prehistoric relatives
o Infraclass †Allotheria: multituberculates
o Infraclass †Triconodonta: triconodonts
o 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
o Grandorder Anagalida: lagomorphs, rodents, and elephant shrews
o Grandorder Ferae: carnivorans, pangolins, †creodonts, and relatives
o Grandorder Lipotyphla: insectivorans
o Grandorder Archonta: bats, primates, colugos, and treeshrews
o 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 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
o 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)
o Group II: Xenarthra
+ Order Pilosa: sloths and anteaters (Neotropical)
+ Order Cingulata: armadillos (Americas)
* Clade Boreoeutheria
o 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).
+ Superorder Glires
# Order Lagomorpha: pikas, rabbits, hares (Eurasia, Africa, Americas).
# Order Rodentia: rodents (cosmopolitan)
o Group IV: Laurasiatheria
+ Order Erinaceomorpha: hedgehogs
+ Order Soricomorpha: moles, shrews, solenodons
+ Order Chiroptera: bats (cosmopolitan)
+ 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).
+ 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)
In light of all the options available, the following classification system has been adopted for use in related articles.
* Subclass/Order Prototheria: egg-laying mammals
o Order Monotremata: echidnas and platypus
* Subclass Marsupialia: marsupials
o Order Didelphimorphia: New World opossums
o Order Paucituberculata: shrew opossums
o Order Microbiotheria: Monito del Monte
o Order Notoryctemorphia: marsupial mole
o Order Dasyuromorphia: marsupial carnivores
o Order Peramelemorphia: bandicoots and bilbies
o Order Diprotodontia: koalas, wombats, kangaroos, possums, etc.
* Subclass Placentalia
o Order Afrosoricida: tenrecs and golden moles
o Superorder Xenarthra
+ Order Cingulata: armadillos
+ Order Pilosa: anteaters and sloths
o Superorder Glires
+ Order Rodentia: rodents
+ Order Lagomorpha: rabbits, hares, and pikas
o Superorder Euarchonta:
+ Order Primates: primates
+ Order Scandentia: treeshrews
+ Order Dermoptera: colugos
o Order Erinaceomorpha: hedgehogs and gymnures
o Order Soricomorpha: shrews and moles
o Order Chiroptera: bats
o Order Carnivora: dogs, cats, weasels, seals, etc.
o Order Pholidota: pangolins
o 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