Mammalia is a class of animal within the phylum Chordata. Mammal classification has been through several iterations since Carl Linnaeus initially defined the class.
Many earlier, pre-Linnaean ideas have been completely abandoned by modern taxonomists, among these are the idea that bats are related to birds or that humans represent a group outside of other living things.
George Gaylord Simpson's classic "Principles of Classification and a Classification of Mammals" (1945) taxonomy text 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. Though field work gradually made Simpson's classification outdated, it remained the closest thing to an official classification of mammals until revision from the end of the twentieth century into the twenty-first century.
No classification system is universally accepted; McKenna et al. and Wilson & Reader provide useful recent compendiums. Competing ideas about the relationships of mammal orders do persist and are currently in development. Most significantly in recent years, cladistic thinking has led to an effort to ensure that all taxonomic designations represent monophyletic groups. The field has also seen a recent surge in interest and modification due to the results of molecular phylogenetics.
Molecular studies by molecular systematists, based on DNA analysis, in the early 21st century have revealed new relationships among mammal families. Classification systems based on molecular studies reveal three major groups or lineages of placental mammals, Afrotheria, Xenarthra, and Boreoeutheria, which diverged from early common ancestors in the Cretaceous.
The relationships between these three lineages are contentious, and all three have been proposed as basal in different hypotheses.
The following taxonomy only includes living placentals (infraclass Eutheria):[citation needed]
Family Ziphiidae: (24 species) beaked whales (cosmopolitan)
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 taken from the 6th edition of Vaughan's Mammalogy. This approach emphasizes an initial split between egg-laying prototherians and live-bearing therians. The therians are further divided into the marsupial Metatheria and the "placental" Eutheria. No attempt is made in this classification to further distinguish among the orders within these subclasses and infraclasses. This system also makes no note of the position of entirely fossil groups.
In this and later taxonomies, families are merely listed under the order to which they belong. More detailed relationships among families is presented in the article of each order.
In 1997, Simpson's classification of mammals was revised by Malcolm C. McKenna and Susan K. Bell. The Classification of Mammals Above the species level, here referred to as the "McKenna/Bell classification", is a comprehensive work on the systematics, relationships, and occurrences of all mammal taxa, living and extinct, down through the rank of genus. The authors worked 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 and ranks which fall between classes and orders that are likely to be glossed over by the layman.
Luo, Kielan-Jaworowska, and Cifelli classification
Several important fossil mammal discoveries have been made that have led researchers to question many of the relationships proposed by McKenna et al. (1997). Additionally, researchers are subjecting taxonomic hypotheses to more rigorous cladistic analyses of early mammal fossils. Luo et al. (2002) summarized existing ideas and proposed new ideas of relationships among mammals at the most basal level. They argued that the term mammal should be defined based on characters (especially the dentary-squamosal jaw articulation) instead of a crown-based definition (the group that contains most recent common ancestor of monotremes and therians and all of its descendants). Their definition of Mammalia is roughly equal to the Mammaliaformes as defined by McKenna et al. and other authors. They also define their taxonomic levels as clades and do not apply Linnean hierarchies.
Unnamed clade 2 - a clade containing all living mammals and some fossil relatives. It is characterized by the loss of a postdentary trough and a widened braincase.
Crown-group Mammalia - the group that contains most recent common ancestor of monotremes and therians and all of its descendants. This group is defined by additional characters relating the occlusion of molars and the presence of a well-developed masseteric fossa.
Crown-group Theria - the group that contains most recent common ancestor of marsupials and placentals and all of its descendants. Characterized by a host of molar features, aspects of the alispenoid, and aspects of the astragalus region.
†Eutriconodonta - incertae sedis. Triconodonts appear to be a member of the crown-Mammalia clade, but their relationships within it are unknown. It is also not certain that they represent a monophyletic group. Examples include Repenomamus.
†Multituberculata - incertae sedis. Luo et al. (2002) argue that multituberculates cannot be confidently placed in a particular clade of mammals. They suggest that they represent either basal mammals or are sister to the Trechnotheria.
Simplified classification for non-specialists
The following classification is a simplified version based on current understanding suitable for non-specialists who want to understand how living genera are related to each other. It appears to be based on the classification of McKenna et al. and the revisions proposed by Luo et al., after extinct groups have been removed. The classification ignores differences in levels and thus cannot be used to estimate the respective distances between taxa. English names are preferred whenever they exist. This makes it especially suited for non-specialists who wish to gain an easy overview. For the full picture, the non-simplified versions above should be consulted (see McKenna/Bell classification and Luo, Kielan-Jaworowska, and Cifelli classification).
Monotremes (prototheria): echidnas and platypus
Platypus (ornithorhynchids)
Echidnas (tachyglossids)
Live-bearing mammals (theria)
Marsupials
Opossums (didelphids)
Shrew opossums (caenolestids)
Australodelphia: Australian marsupials and monito del Monte
Monito del Monte
Dasyuromorphs
Dasyurids: antechinuses, quolls, dunnarts, Tasmanian devil, and allies
Numbat
Peramelemorphs: bilbies and bandicoots
Bilbies (thylacomyids)
Bandicoots (peramelids)
Marsupial moles (notoryctids)
Diprotodonts
Koala
Wombats (vombatids)
Phalangerids: brushtail possums and cuscuses
Pygmy possums (burramyids)
Honey possum
Petaurids: striped and Leadbeater's possums, and yellow-bellied, suger, mahogany and squirrel glider
Ringtailed possums (pseudocheirids)
Potorids: potoroos, rat kangaroos and bettongs
Acrobatids: feathertail glider and feather-tailed possum
Musky rat-kangaroo
Macropods: kangaroos, wallabies and allies
Placentals
Atlantic placentals (atlantogenatans)
Afroplacentals (afrotherians)
Afroinsectiphilians: elephant shrews, tenrecs, otter shrews, golden moles, and aardvark
Elephant shrews (macroscelidids)
Afrosoricids: tenrecs and golden moles
Tenrecids: tenrecs and otter shrews
Golden moles (chrysochlorids)
Aardvark
Paenungulates: hyraxes, elephants, dugongs and manatees
Hyraxes or dassies (procaviids)
Elephants (elephantids)
Sirenians: dugong and manatees
Dugong
Manatees (trichechids)
Xenarthrans
Pilosans: sloths and anteaters
Anteaters (vermilinguans)
Silky anteater
Myrmecophagids: giant anteater and tamanduas
Sloths (folivorans)
Three-toed sloths (bradypodids)
Two-toed sloths (megalonychids)
Armadillos (dasypodids)
Northern placentals (boreoeutherians)
Supraprimates (euarchontoglires)
Euarchontans: treeshrews, colugos and primates
Treeshrews (scandentians)
Tupaiids: all treeshrews except pen-tailed
Pen-tailed treeshrew
Colugos or flying lemurs (cynocephalids)
Primates
Strepsirrhines: lemur- and loris-like primates
Lemur-like primates (lemuriforms)
Cheirogaleids: dwarf lemurs and mouse-lemurs
Aye-aye
True lemurs (lemurids)
Sportive lemurs (lepilemurids)
Indriids: woolly lemurs and allies
Loris-like primates (lorisiforms)
Lorisids: lorises, pottos and allies
Galagos (galagids)
Haplorhines: tarsiers, monkeys and apes
Tarsiers (tarsiids)
Anthropoid primates
New World monkeys (platyrrhines)
Callitrichids: marmosets and tamarins
Cebids: capuchins and squirrel monkeys
Aotids: night or owl monkeys
Pitheciids: titis, sakis and uakaris
Atelids: howler, spider, woolly spider, and woolly monkeys
Catarrhines
Old World monkeys (cercopithecids)
Hominoid primates
Gibbons (hylobatids)
Great apes (hominids): incl. Humans
Glires: pikas, rabbits, hares, and rodents
Lagomorphs: pikas, rabbits and hares
Leporids: rabbits and hares
Pikas (ochotonids)
Rodents
Anomalure-like rodents (anomaluromorphs): Scaly-tailed squirrels and springhares
Scaly-tailed squirrels or anomalures (anomalurids)
Springhares (pedetids)
Beaver-like rodents (castorimorphs)
Beavers (castorids)
Gopher-like rodents (geomyoid rodents)
Pocket or true gophers (geomyids)
Heteromyids: kangaroo rats and kangaroo mice
Porcupine-like rodents (hystricomorphs)
Laotian rock rat
Gundis (ctenodactylids)
Hystricognaths
African mole rats (bathyergids)
Old World porcupines (hystricids)
Dassie rat
Cane rats (thryonomyids)
Cavy-like rodents (caviomorphs)
Chinchilla rats (abrocomids)
Hutias (capromyids)
Cavies (caviids): incl. Guinea pigs and capybara
Chinchillids: chinchillas and viscachas
Tuco-tucos (ctenomyids)
Agoutis (dasyproctids)
Pacas (cuniculids)
Pacarana
Spiny rats (echymyids)
New World porcupines (erethizontids)
Myocastorids: nutria and coypu
Octodonts (octodontids): Andean rock-rats, degus and viscacha-rats
Mouse-like rodents (myomorphs)
Dipodids: jerboas and jumping mice
Muroid rodents
Mouse-like hamsters (calomyscids)
Cricetids: hamsters, New World rats and mice, voles
Murids: true mice and rats, gerbils, spiny mice, crested rat
Nesomyids: climbing mice, rock mice, white-tailed rat, Malagasy rats and mice
Spiny dormice (platacanthomyids)
Spalacids: mole rats, bamboo rats, and zokors
Squirrel-like rodents (sciuromorphs)
Mountain beaver
Dormice (glirids)
Squirrels (sciurids): incl. chipmunks, prairie dogs, and marmots
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