Mammalia is a class of animal within the phylum Chordata. Mammal classification has been through several iterations since Carl Linnaeus initially defined the class. No classification system is universally accepted; McKenna & Bell (1997) and Wilson & Reader (2005) provide useful recent compendiums. Many earlier ideas from Linnaeus et al. 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. 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.

George Gaylord Simpson's classic "Principles of Classification and a Classification of Mammals" (Simpson, 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 Though field work gradually made Simpson's classification outdated, it remained the closest thing to an official classification of mammals. See List of placental mammals and List of monotremes and marsupials for more detailed information on mammal genera and species.

Molecular classification of placentals

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 Boreotheria. 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):

Atlantogenata

Afrotheria

Class Afroinsectiphilia
- Order Macroscelidea
  - Family Macroscelididae: (20 species), sengis or elephant shrews (Africa)
- Order Afrosoricida
  - Family Tenrecidae: (31 species), tenrecs (Madagascar)
  - Family Potamogalidae: (3 species), otter-shrews (West and Central Africa)
  - Family Chrysochloridae: (21 species), golden moles (Africa south of the Sahara)
- Order Tubulidentata
  - Family Orycteropodidae: (1 species), aardvark (Africa south of the Sahara)
Class Paenungulata
- Order Proboscidea
  - Family Elephantidae: (3 species), elephants (Africa, Southeast Asia)
- Order Hyracoidea
  - Family Procaviidae: (4 species), hyraxes, dassies (Africa, Arabia)
- Order Sirenia
  - Family Dugongidae: (1 species), dugong (East Africa, Red Sea, North Australia)
  - Family Trichechidae: (3 species), manatees (tropical Atlantic coasts and adjacent rivers)

Xenarthra

Order Cingulata
- Family Chlamyphoridae: (14 species), armadillos (Neotropical)
- Family Dasypodidae: (7 species), long-nosed armadillos (Neotropical and Nearctic)
Order Pilosa (=Dasypoda)
- Suborder Vermilingua (anteaters)
  - Family Cyclopedidae: (1 species), silky anteater (Neotropical)
  - Family Myrmecophagidae: (3 species), anteaters (Neotropical)
- Suborder Folivora (sloths)
  - Family Choloepodidae: (2 species), two-toed sloths (Neotropical)
  - Family Bradypodidae: (4 species), three-toed sloths (Neotropical)

Boreoeutheria

Euarchontoglires

Masoala fork-marked lemur (Cheirogaleus) Phaner furcifer

Superorder Euarchonta
- Order Scandentia
  - Family Ptilocercidae (1 species), pen-tailed treeshrews (Southeast Asia)
  - Family Tupaiidae: (19 species), treeshrews (Southeast Asia)
- Mirorder Primatomorpha
  - Order Dermoptera
    - Family Cynocephalidae: (2 species), flying lemurs or colugos (Southeast Asia)
  - Order Primates: lemurs, bushbabies, monkeys, apes (cosmopolitan)
    
    Aye aye (Daubentonid)
    - Family Cheirogaleidae: (32 species), dwarf lemurs (Madagascar)
    - Family Lemuridae: (22 species), lemurs (Madagascar)
    - Family Lepilemuridae: (26 species), sportive lemurs (Madagascar)
    - Family Indriidae: (19 species), indri and sifakas (Madagascar)
    - Family Daubentoniidae: (1 species), aye-aye (Madagascar area)
    - Family Lorisidae: (9 species), lorises and potto (Africa and Southeast Asia)
    - Family Galagidae: (19 species), galagos (Africa)
      
      Capuchin monkey (Cebid)
    - Family Tarsiidae: (9 species), tarsiers (Southeast Asia)
    - Family Callitrichidae: (41 species), marmosets and tamarins (South America)
    - Family Cebidae: (14 species), New World monkeys (South America)
    - Family Cercopithecidae: (137 species), Old World monkeys (Africa and Eurasia)
    - Family Hylobatidae: (14 species), gibbons (Southeast Asia)
    - Family Hominidae: (8 species), great apes (worldwide)
Superorder Glires
- Order Lagomorpha: pikas, rabbits, hares (Eurasia, Africa, Americas)
  
  Arctic hare (Leporid)
  - Family Leporidae: (60 species), rabbits and hares (Eurasia, Africa, Americas)
  - Family Ochotonidae: (30 species), pikas (Holarctic)
- Order Rodentia: rodents (cosmopolitan)
  - Suborder Castorimorpha
    - Family Castoridae: (2 species) beavers (Holarctic)
    - Family Geomyidae: (about 35 species) pocket gophers (North America)
    - Family Heteromyidae: (about 59 species) kangaroo rats and kangaroo mice (North America)
  - Suborder Myomorpha
    - Family Dipodidae: (33 species) jerboas (Africa, Eurasia, North America)
    - Family Zapodidae: (11 species) jumping mice (North America, Asia)
    - Family Sicistidae: (19 species) birch mice (Eurasia)
    - Family Platacanthomyidae: (3 species) spiny dormouse (Southeast Asia)
    - Family Spalacidae: (37 species) zokors, root rats, blind mole rats (Africa, Eurasia)
    - Family Calomyscidae: (8 species) mouse-like hamsters (Asia)
    - Family Nesomyidae: (68 species) old endemic African muroids (Africa, Madagascar)
    - Family Cricetidae: (about 580 species) hamsters, voles, and New World rats and mice (Holarctic, South America)
    - Family Muridae: (about 1,383 species) Old World rats and mice and gerbils (Africa, Eurasia, Australia)
  - Suborder Anomaluromorpha
    - Family Anomaluridae: (6 species) scaly-tailed flying squirrels (Africa)
    - Family Pedetidae: (2 species) springhares or springhaas (Africa)
  - Suborder Hystricomorpha
    - Family Ctenodactylidae: (5 species) gundis (Africa, Asia)
    - Family Diatomyidae: (1 species) Laotian rock rat (Southeast Asia)
    - Family Hystricidae: (11 Species) Old World porcupines (Africa, Asia)
    - Family Bathyergidae: (about 21 species) African mole-rats (Africa)
    - Family Petromuridae: (1 species) rock dassies (Africa)
    - Family Thryonomyidae: (2 species) cane rats (Africa)
    - Family Erethizontidae: (19 species) New World porcupines (New World)
    - Family Chinchillidae: (3 species) chinchillas and viscachas (South America)
    - Family Dinomyidae: (1 species) pacarana (South America)
    - Family Caviidae: (18 species) cavies and capybara (South America)
    - Family Dasyproctidae: (13 species) agoutis and acouchis (South America)
    - Family Cuniculidae: (about 3 species) paca (South America)
    - Family Ctenomyidae: (about 60 species) tuco-tucos (South America)
    - Family Octodontidae: (14 species) degus (South America)
    - Family Abrocomidae: (9 species) chinchilla-rats (South America)
    - Family Echimyidae: spiny rats (South America)
    - Family Capromyidae: (10 species) hutias (South America)
    - Family Heptaxodontidae: giant Hutias (recently extinct)
    - Family Myocastoridae: (57 species) nutrias (South America)
  - Suborder Sciuromorpha
    
    Mountain beaver (Aplodont)
    - Family Aplodontiidae: (1 species) mountain beaver (western North America)
    - Family Sciuridae: (about 285 species) squirrels, chipmunks, and marmots (cosmopolitan except Australia)
    - Family Gliridae: (29 species) dormice (Africa, Eurasia)

Laurasiatheria

Order Eulipotyphla
- Family Solenodontidae: (2 species) solenodons (Cuba, Hispaniola)
- Family Nesophontidae: nesophontes (West Indies shrews) [recently extinct]
- Family Soricidae: (385 species) shrews (Eurasia, Africa, North America to northern South America)
- Family Talpidae: (59 species) moles, shrew-moles, desmans (Eurasia, North America)
- Family Erinaceidae: (26 species)hedgehogs, gymnures (Eurasia, Africa)
- Family Galericidae: (8 species) moonrats (southeast Asia)
Grandorder Chiroptera
- Order Chiroptera: bats
  - Suborder Yinpterochiroptera
    - Family Pteropodidae: (about 197 species) flying foxes (Africa, Eurasia, Australia)
    - Family Hipposideridae: (84 species) trident bats, leaf-nosed bats
    - Family Rhinolophidae: (106 species) horseshoe bats (Old World)
    - Family Rhinopomatidae: (6 species) mouse-tailed bats (Africa, Southeast Asia)
    - Family Craseonycteridae: (1 species) Kitti's hog-nosed bat (Thailand)
    - Family Megadermatidae: (6 species) false vampire bats (Africa, Southeast Asia, Australia)
  - Suborder Yangochiroptera
    - Family Emballonuridae: (54 species) sac-winged bats (southern continents)
    - Family Nycteridae: (about 15 species) slit-faced bats (Africa, Southeast Asia)
    - Family Mystacinidae: (about 2 species) short-tailed bats (New Zealand)
    - Family Thyropteridae: (5 species) sucker-footed bats (South America)
    - Family Furipteridae: (2 species) smoky bats (South America)
    - Family Noctilionidae: (2 species) fishing bats (South America)
    - Family Mormoopidae: (about 11 species) leaf-chinned bats (South America)
    - Family Phyllostomidae: (192 species) leaf-nosed bats (South America)
    - Family Myzopodidae: (2 species) sucker-footed bats (Madagascar)
    - Family Natalidae: (10 species) funnel-eared bats (South America)
    - Family Molossidae: (about 110 species) free-tailed bats (cosmopolitan)
    - Family Miniopteridae: (about 40 species) long-fingered bats (Africa, Eurasia, Australia)
    - Family Cistugidae: (2 species) wing-gland bats (Southern Africa)
    - Family Vespertilionidae: (over 300 species) vesper bats (cosmopolitan)
Grandorder Ferae
- Order Pholidota
  - Family Manidae: (about 8 species) pangolins, scaly anteaters (Africa, South Asia)
- Order Carnivora: carnivorans (cosmopolitan)
  - Suborder Feliformia
    - Family Nandiniidae: (4 species) African palm civet (Central Africa)
    - Family Prionodontidae: (2 species) Asiatic linsangs (Southeast Asia)
    - Family Felidae: (41 species) cats (cosmopolitan except Australia)
    - Family Viverridae: (33 species) civets, Asiatic palm civets (Africa, Southern Europe, Southeast Asia)
    - Family Herpestidae: (34 species) mongooses (Africa, Asia, Southern Europe)
    - Family Eupleridae: (10 species) Malagasy carnivorans (Madagascar)
    - Family Hyaenidae: (4 species) hyaenas, aardwolf (Africa)
  - Suborder Caniformia
    - Family Canidae: (38 species) dogs (cosmopolitan)
    - Family Ursidae: (8 species) bears (Europe, Asia, New World)
    - Family Otariidae: (15 species) eared seals (cosmopolitan except North Atlantic)
    - Family Odobenidae: (1 species) walrus (Northern North American, Northern Europe, Northern Asia)
    - Family Phocidae: (18 species) true seals (cosmopolitan)
    - Family Ailuridae: (1 species) red panda (South-Central Asia)
    - Family Mephitidae: (12 species) skunks (Southeast Asia, New World)
    - Family Mustelidae: (about 69 species) weasels and relatives (cosmopolitan except Australia)
    - Family Procyonidae: (14 species) ringtails, olingos, kinkajou, raccoons, coatis (New World)
Grandorder Ungulata
- Order Perissodactyla: odd-toed ungulates
  - Family Equidae: (13 species) horses, zebras, donkeys (Africa, West and Central Asia)
  - Family Tapiridae: (3 species) tapirs (Central and South America, Southeast Asia)
  - Family Rhinocerotidae: (5 species) rhinoceroses (Africa, Southeast Asia)
- Order Artiodactyla: even-toed ungulates (now includes Cetaceans)
  - Suborder Suiformes
    - Family Suidae: (18 species) pigs (Africa, Eurasia)
      
      Pig and piglet
    - Family Tayassuidae: (about 3 species) peccaries (New World)
  - Suborder Tylopoda
    - Family Camelidae: (7 species) camels (South America, Asia)
  - Suborder Ruminantia
    - Family Tragulidae: (10 species) mouse-deer (Africa, Asia)
    - Family Antilocapridae: (1 species) pronghorn (North America)
    - Family Giraffidae: (2-9 species) giraffe and okapi (Africa)
      
      Giraffe
    - Family Cervidae: (26 species) deer (Holarctic, South America)
    - Family Moschidae: (7 species) musk deer (Asia)
      
      Muntjac deer
    - Family Bovidae: (143 species) cattle, antelope, sheep, etc. (Africa, Holarctic)
      
      Pair of Icelandic Sheep
  - Suborder Whippomorpha
    - Family Hippopotamidae: (2 species) hippos (Africa)
    - Infraorder Cetacea
      - Parvorder Mysticeti
        
        Family Balaenopteridae: (10 species) rorquals and grey whales (cosmopolitan)
        
        Family Balaenidae: (4 species) right and bowhead whales (polar and temperate waters)
        
        Family Eschrichtiidae: (1 species) gray whale (North Pacific and North Atlantic)
        
        Family Neobalaenidae: (1 species) pygmy right whales (southern hemisphere)
      - Parvorder Odontoceti
        
        Family Delphinidae: (about 37 species) dolphins (cosmopolitan)
        
        Family Monodontidae: (2 species) beluga and narwhal (Arctic, Atlantic, and Pacific)
        
        Family Phocoenidae: (8 species) porpoises (cosmopolitan)
        
        Family Physeteridae: (3 species) sperm whales (cosmopolitan)
        
        Family Kogiidae: (2 species) dwarf sperm whales (cosmopolitan)
        
        Family Platanistidae: (2 species) South Asian river dolphin (Southern Asia)
        
        Family Iniidae: (1-4 species) Amazon River dolphin (South America)
        
        Family Pontoporiidae: (1 species) La Plata River dolphin (South America)
        
        Family Lipotidae: baiji
        
        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.

Subclass Prototheria

Order Monotremata
- Family Tachyglossidae (echidnas)
- Family Ornithorhynchidae (platypuses)

Subclass Theria

Infraclass Metatheria (marsupials and their nearest ancestors)
- Order Didelphimorphia
  - Family Didelphidae (opossums, etc.)
- Order Paucituberculata
  - Family Caenolestidae (shrew opossums)
- Order Microbiotheria
  - Family Microbiotheriidae (monitos del monte)
- Order Dasyuromorphia (most carnivorous marsupials)
  - Family Thylacinidae (Tasmanian tigers)
  - Family Myrmecobiidae (numbats)
  - Family Dasyuridae (Tasmanian devils, quolls, dunnarts, planigale, etc.)
- Order Peramelemorphia (bandicoots, bilbies, etc.)
  - Family Peramelidae (bandicoots, echymiperas)
  - Family †Chaeropodidae (pig-footed bandicoot)
- Order Notoryctemorphia (marsupial moles)
  - Family Notoryctidae
- Order Diprotodontia
  - Family Phascolarctidae (koalas)
  - Family Vombatidae (wombats)
  - Family Phalangeridae (brushtail possums and cuscuses)
  - Family Potoroidae (bettongs, potoroos and rat kangaroos)
  - Family Macropodidae (kangaroos, wallabies, etc.)
  - Family Burramyidae (pygmy possums)
  - Family Pseudocheiridae (ringtailed possums, etc.)
  - Family Petauridae (striped possum, Leadbeater's possum, yellow-bellied glider, sugar glider, mahogany glider and squirrel glider)
  - Family Tarsipedidae (honey possum)
  - Family Acrobatidae (feathertail glider and feather-tailed possum)
  - Family Hypsiprymnodontidae (musky rat kangaroo)
Infraclass Eutheria
- Order Afrosoricida
  - Family Tenrecidae (tenrecs)
  - Family Chrysochloridae (golden moles)
- Order Macroscelidea
  - Family Macroscelididae (elephant-shrews
- Order Tubulidentata
  - Family Orycteropodidae (aardvark)
- Order Proboscidea
  - Family Elephantidae (elephants)
- Order Sirenia
  - Family Dugongidae (dugongs, sea cows)
  - Family Trichechidae (manatees)
- Order Hyracoidea
  - Family Procaviidae (hyraxes)
- Order Pilosa
  - Family Bradypodidae (three-toed tree sloths)
  - Family Megalonychidae (two-toed tree sloths)
  - Family Myrmecophagidae (tamanduas and giant anteater)
  - Family Cyclopedidae (silky anteater)
- Order Cingulata
  - Family Dasypodidae (armadillos)
- Order Dermoptera
  - Family Cynocephalidae (colugos)
- Order Scandentia
  - Family Tupaiidae (tree shrews)
  - Family Ptilocercidae (pen-tailed treeshrew
    
    Noctilio albiventris
    
    Acerodon celebensis
    
    Cynopterus brachyotis
    
    Epomophorus wahlbergi
    
    Epomophorus
    
    Hypsignathus monstrosus
- Order Primates
  - Family Cheirogaleidae (dwarf lemurs, mouse lemurs)
  - Family Lemuridae (lemurs)
  - Family Lepilemuridae (sportive lemurs)
  - Family Indriidae (wooly lemurs, sifakas)
  - Family Daubentoniidae (aye-aye)
  - Family Lorisidae (lorises)
  - Family Galagidae (bushbabies, galagos)
  - Family Tarsiidae (tarsiers)
  - Family Cebidae (marmosets, tamarins, capuchins, squirrel monkeys)
  - Family Aotidae (night monkeys)
  - Family Pitheciidae (titis, uacaris, sakis)
  - Family Atelidae (howlers, spider monkeys, wooly monkeys)
  - Family Cercopithecidae (Old World monkeys)
  - Family Hylobatidae (gibbons)
  - Family Hominidae (apes, human)
- Order Rodentia
  - Family Aplodontiidae (sewellel or mountain beaver)
  - Family Sciuridae (squirrels)
  - Family Gliridae (dormice)
  - Family Castoridae (beavers)
  - Family Heteromyidae (kangaroo rats, pocket mice)
  - Family Geomyidae (pocket gophers)
  - Family Dipodidae (jerboas, birch mice, jumping mice)
  - Family Platacanthomyidae (tree mice)
  - Family Spalacidae (zokors, bamboo rats, mole rats)
  - Family Calomyscidae (calomyscuses)
  - Family Nesomyidae (pouched rats and mice, climbing and fat mice, etc.)
  - Family Cricetidae (voles, hamsters, New World rats and mice
  - Family Muridae (rats, mice)
  - Family Anomaluridae (scaily-tailed flying squirrels)
  - Family Pedetidae (springhaas, springhares)
  - Family Ctenodactylidae (gundis)
  - Family Diatomyidae (kha-nyous or Laotian rock rat)
  - Family Bathyergidae (mole-rats)
  - Family Hystricidae (African and Asian porcupines)
  - Family Petromuridae (dassie rat)
  - Family Thryonomyidae (can rats)
  - Family Erethizontidae (bristle-spined rat and New World porcupines)
  - Family Chinchillidae (chinchillas, vizcachas)
  - Family Dinomyidae (pacarana)
  - Family Caviidae (cuis, guinea-pigs, cavies, maras, capybaras)
  - Family Dasyproctidae (agoutis, acouchis)
  - Family Cuniculidae (pacas)
  - Family Ctenomyidae (tuco-tucos)
  - Family Octodontidae (degus, rock rats, vizcacha-rats)
  - Family Abrocomidae (chinchilla rats)
  - Family Echimyidae (spiny rats, tree rats, hutias, & coypu)
  - Family †Heptaxodontidae (giant hutias and key mice)
- Order Lagomorpha
  - Family Ochotonidae (pikas)
  - Family †Prolagidae (Sardinian pika)
  - Family Leporidae (rabbits)
- Order Eulipotyphla
  - Family Erinaceidae (hedgehogs, gymnures)
  - Family †Nesophontidae (nesophontes)
  - Family Solenodontidae (solenodons, alamiquis)
  - Family Soricidae (shrews)
  - Family Talpidae (moles, desmans)
- Order Chiroptera
  - Family Pteropodidae (Old World fruit bats, flying foxes)
  - Family Rhinopomatidae (mouse-tailed bats)
  - Family Craseonycteridae (hog-nosed or bumblebee bat)
  - Family Megadermatidae (false vampire bats)
  - Family Rhinolophidae (horseshoe bats)
  - Family Emballonuridae (sac-winged bats)
  - Family Nycteridae (slit-faced bats)
  - Family Myzopodidae (sucker-footed bats)
  - Family Mystacinidae (New Zealand short-tailed bats)
  - Family Thyropteridae (disk-winged bats)
  - Family Furipteridae (smokey bat and thumbless bat)
  - Family Noctilionidae (bulldog bats)
  - Family Mormoopidae (mustached and ghost-faced bats)
  - Family Phyllostomidae (New World leaf-nosed bats)
  - Family Natalidae (funnel-eared bats)
  - Family Molossidae (free-tailed bats)
  - Family Vespertilionidae (evening bats, common bats)
  - Family Miniopteridae (bent-winged or long-fingered bats)
- Order Pholidota
  - Family Manidae (pangolins)
- Order Carnivora
  - Family Felidae (cats)
  - Family Viverridae (civets, genets)
  - Family Eupleridae (falanouc, fossa, Madagascaran mongooses)
  - Family Nandiniidae (African palm civet)
  - Family Herpestidae (mongooses)
  - Family Hyaenidae (hyaenas, aardwolf)
  - Family Canidae (wolves, foxes, jackals)
  - Family Ursidae (bears, giant panda)
  - Family Odobenidae (walrus)
  - Family Otariidae (eared seals, fur seals, sea lions)
  - Family Phocidae (earless seals)
  - Family Mustelidae (weasels, badgers, otters)
  - Family Procyonidae (raccoons, ringtails, coatis)
  - Family Ailuridae (red panda)
- Order Perissodactyla
  - Family Equidae (horses, asses, zebras)
  - Family Tapiridae (tapirs)
  - Family Rhinocerotidae (rhinoceroses)
- Order Artiodactyla
  - Family Suidae (hogs, pigs)
  - Family Tayassuidae (peccaries)
  - Family Hippopotamidae (hippopotamuses)
  - Family Camelidae (camels, vicunas, guanacos, llamas)
  - Family Tragulidae (chevrotains and mouse deer)
  - Family Moschidae (musk deer)
  - Family Cervidae (deer)
  - Family Antilocapridae (pronghorn)
  - Family Giraffidae (giraffe and okapi)
  - Family Bovidae (antelope, bison, cattle, duikers, goats, sheep, etc.)
- Order Cetacea
  - Family Balaenidae (right whales)
  - Family Balaenopteridae (rorquals)
  - Family Eschrichtiidae (gray whales
  - Family Cetotheriidae (pygmy right whale)
  - Family Delphinidae (ocean dolphins)
  - Family Monodontidae (narwhal and beluga)
  - Family Phocoenidae (porpoises)
  - Family Physeteridae (sperm whales)
  - Family Platanistidae (Ganges and Indus river dolphins)
  - Family Iniidae (baiji, franciscana, and Amazon river dolphins)
  - Family Ziphiidae (beaked whales)

McKenna/Bell classification

In 1997, the 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.

Click on the highlighted link for a table comparing the traditional and the new McKenna/Bell classifications of mammals.

Extinct groups are represented by †.

Subclass Prototheria

(monotremes)

Order Platypoda: platypuses
- Family Ornithorhynchidae: platypuses
Order Tachyglossa: echidnas (spiny anteaters)
- Family Tachyglossidae: echidnas

Subclass Theriiformes

Infraclass †Allotheria
- Order †Multituberculata: multituberculates
  - Family †Plagiaulacidae
  - Family †Bolodontidae
  - Family †Hahnodontidae
  - Family †Albionbaataridae
  - Family †Arginbaataridae
  - Family †Kogaionidae
  - Suborder †Cimolodonta
    - Family †Sloanbaataridae
    - Superfamily †Ptilodontoidea
      - Family †Cimolodontidae
      - Family †Ptilodontidae
    - Superfamily †Taeniolabidoidea
      - Family †Cimolomyidae
      - Family †Eucosmodontidae
      - Family †Taeniolabididae
  - Suborder †Gondwanatheria
    - Family †Ferugliotheriidae
    - Family †Sudamericidae
Infraclass †Triconodonta
- Family †Austrotriconodontidae
- Family †Amphilestidae
- Family †Triconodontidae
Infraclass Holotheria
- Family †Chronoperatidae
- Superlegion †Kuehneotheria
  - Family †Kuehneotheriidae
  - Family †Woutersiidae
- Superlegion Trechnotheria
  - Legion †Symmetrodonta
    - Family †Shuotheriidae
    - Order †Amphidontoidea
      - Family †Amphidontidae
    - Order †Spalacotherioidea
      - Family †Tinodontidae
      - Family †Spalacotheriidae
      - Family †Barbereniidae
  - Legion Cladotheria
    - Sublegion †Dryolestoidea
      - Order †Dryolestida
        
        Family †Dryolestidae
        
        Family †Paurodontidae
        
        Family †Donodontidae
        
        Family †Mesungulatidae
        
        Family †Reigitheriidae
        
        Family †Brandoniidae
      - Order †Amphitheriida
        Family †Amphitheriidae
    - Sublegion Zatheria
      - Family †Arguitheriidae
      - Family †Arguimuridae
      - Family †Vincelestidae
      - Infralegion †Peramura
        Family †Peramuridae
      - Infralegion Tribosphenida
        
        Family †Necrolestidae
        
        Supercohort †Aegialodontia
        Family †Aegialodontidae
        
        Supercohort Theria: therian mammals
        
        Family †Pappotheriidae
        
        Family †Holoclemensiidae
        
        Family †Kermackiidae
        
        Family †Endotheriidae
        
        Family †Picopsidae
        
        Family †Potamotelsidae
        
        Family †Plicatodontidae
        
        Order †Deltatheroida
        
        Family †Deltatheridiidae
        
        Family †Deltatheroididae
        
        Order †Asiadelphia
        Family †Asiatheriidae
        
        Cohort Marsupialia: marsupials
        
        Family †Yingabalanaridae
        
        Suborder †Archimetatheria
        
        Family †Stagodontidae
        
        Family †Pediomyidae
        
        Magnorder Australidelphia
        
        Superorder Microbiotheria
        Family Microbiotheriidae: monito del monte
        
        Superorder Eometatheria
        
        Order †Yalkaparidontia
        Family †Yalkaparidontidae
        
        Order Notoryctemorphia: marsupial moles
        Family Notoryctidae: marsupial moles
        
        Grandorder Dasyuromorphia: marsupial carnivores
        
        Family †Thylacinidae: recently extinct Tasmanian tiger and relatives
        
        Family Dasyuridae: Tasmanian devil, quolls, numbat, etc.
        
        Grandorder Syndactyli: syndactylous marsupials
        
        Order Peramelia: bandicoots
        
        Family Peramelidae
        
        Family Peroryctidae
        
        Order Diprotodontia
        
        Family †Palorchestidae
        
        Family †Wynardiidae
        
        Family †Thylacoleonidae
        
        Family Tarsipedidae: honey possum
        
        Superfamily Vombatoidea
        
        Family †Ilariidae
        
        Family †Diprotodontidae
        
        Family Vombatidae: wombats
        
        Superfamily Phalangeroidea
        
        Family Phalangeridae: phalangers
        
        Family Burramyidae: pygmy possums
        
        Family Macropodidae: rat kangaroos, kangaroos and wallabies
        
        Family Petauridae: gliders
        
        Family †Ektopodontidae
        
        Family Phascolarctidae: koala
        
        Family †Pilkipildridae
        
        Family †Miralinidae
        
        Family Acrobatidae: feather-tail glider, pen-tailed phalanger
        
        Magnorder Ameridelphia
        
        Order Didelphimorphia: opossums
        
        Family Didelphidae: opossums
        
        Family †Sparassocynidae
        
        Order Paucituberculata
        
        Superfamily Caenolestoidea
        
        Family †Sternbergiidae
        
        Family Caenolestidae: rat or shrew opossums
        
        Family †Paleothentidae
        
        Family †Abderitidae
        
        Superfamily †Polydolopoidea
        
        Family †Sillustaniidae
        
        Family †Polydolopidae
        
        Family †Prepidolopidae
        
        Family †Bonapartheriidae
        
        Superfamily †Argyrolagoidea
        
        Family †Argyrolagidae
        
        Family †Patagoniidae
        
        Family †Groeberiidae
        
        Superfamily †Caroloameghinioidea
        
        Family †Glasbiidae
        
        Family †Caroloameghiniidae
        
        Order †Sparassodonta
        
        Family †Mayulestidae
        
        Family †Hondadelphidae
        
        Family †Borhyaenidae
        
        Cohort Placentalia: placentals
        
        Order †Bibymalagasia
        
        Magnorder Xenarthra: edentates
        
        Order Cingulata: armadillos and relatives
        
        Family †Protobradidae
        
        Superfamily Dasypodoidea
        
        Family Dasypodidae: armadillos
        
        Family †Peltephilidae
        
        Superfamily †Glyptodontoidea
        
        Family †Pampatheriidae
        
        Family †Palaeopeltidae
        
        Family †Glyptodontidae: glyptodonts
        
        Order Pilosa: anteaters, sloths, and relatives
        
        Family †Entelopidae
        
        Suborder Vermilingua
        
        Family Myrmecophagidae: giant anteaters and relatives
        
        Family Cyclopedidae: pygmy anteater
        
        Suborder Phyllophaga
        
        Family †Rathymotheriidae
        
        Infraorder †Mylodonta
        
        Superfamily †Mylodontoidea
        
        Family †Scelidotheriidae
        
        Family †Mylodontidae
        
        Superfamily †Orophodontoidea
        Family †Orophodontidae
        
        Infraorder Megatheria
        
        Superfamily Megatherioidea
        
        Family †Megatheriidae: ground sloths
        
        Family Megalonychidae: two-toed sloths
        
        Superfamily Bradypodoidea
        Family Bradypodidae: three-toed sloths
        
        Magnorder Epitheria: epitheres
        
        Superorder †Leptictida
        
        Family †Gypsonictopidae
        
        Family †Kulbeckiidae
        
        Family †Didymoconidae
        
        Family †Leptictidae
        
        Superorder Preptotheria
        
        Grandorder Anagalida
        
        Family †Zambdalestidae
        
        Family †Anagalidae
        
        Family †Pseudictopidae
        
        Mirorder Macroscelidea: elephant shrews
        Family Macroscelididae: elephant shrews
        
        Mirorder Duplicidentata
        
        Order †Mimotonida
        Family †Mimotonidae
        
        Order Lagomorpha
        
        Family Ochotonidae: pikas
        
        Family Leporidae: rabbits
        
        Mirorder Simplicidentata
        
        Order †Mixodontia
        Family †Eurymylidae
        
        Order Rodentia: rodents
        
        Family †Alagomyidae
        
        Family †Laredomyidae
        
        Suborder Sciuromorpha
        
        Superfamily †Ischyromyoidea
        Family †Ischyromyidae
        
        Superfamily Aplodontoidea
        
        Family †Allomyidae
        
        Family Aplodontiidae: mountain beaver
        
        Family †Mylagaulidae
        
        Infraorder †Theridomyomorpha
        Family †Theridomyidae
        
        Infraorder Sciurida
        
        Family †Reithroparamyidae
        
        Family Sciuridae: squirrels
        
        Infraorder Castorimorpha
        
        Family †Eutypomyidae
        
        Family Castoridae: beavers
        
        Family †Rhizospalacidae
        
        Suborder Myomorpha
        
        Family †Protoptychidae
        
        Infraorder Myodonta
        
        Superfamily Dipodoidea
        
        Family †Armintomyidae
        
        Family Dipodidae: jumping mice, jerboas
        
        Superfamily Muroidea
        
        Family †Simimyidae
        
        Family Muridae: rats, mice, and relatives
        
        Infraorder Glirimorpha
        Family Myoxidae: dormice
        
        Infraorder Geomorpha
        
        Superfamily †Eomyoidea
        Family †Eomyidae
        
        Superfamily Geomyoidea
        
        Family †Florentiamyidae
        
        Family Geomyidae: pocket gophers, pocket mice, and kangaroo rats
        
        Suborder Anomaluromorpha
        
        Superfamily Pedetoidea
        
        Family †Parapedetidae
        
        Family Pedetidae: springhaas
        
        Superfamily Anomaluroidea
        
        Family †Zegdoumyidae
        
        Family Anomaluridae: scaly-tailed squirrels
        
        Suborder Sciuravida
        
        Family †Ivanantoniidae
        
        Family †Sciuravidae
        
        Family †Chapattimyidae
        
        Family †Cylindrodontidae
        
        Family Ctenodactylidae: gundis
        
        Suborder Hystricognatha
        
        Family †Tsaganomyidae
        
        Infraorder Hystricognathi
        
        Family Hystricidae: Old World porcupines
        
        Family Erethizontidae: New World porcupines
        
        Family †Myophiomyidae
        
        Family †Diamantomyidae
        
        Family †Phiomyidae
        
        Family †Kenyamyidae
        
        Family Petromuridae: rock rats
        
        Family Thryonomyidae: cane rats
        
        Parvorder Bathyergomorphi
        
        Family Bathyergidae: mole-rats
        
        Family †Bathyergoididae
        
        Parvorder Caviida
        
        Superfamily Cavioidea
        
        Family Agoutidae: agoutis and pacas
        
        Family †Eocardiidae
        
        Family Dinomyidae: pacarana
        
        Family Caviidae: cavies
        
        Family Hydrochoeridae: capybara
        
        Superfamily Octodontoidea
        
        Family Octodontidae: degus, tuco-tucos
        
        Family Echimyidae: spiny rats, nutria
        
        Family Capromyidae: hutias
        
        Family †Heptaxodontidae
        
        Superfamily Chinchilloidea
        
        Family Chinchillidae: chinchillas, viscachas
        
        Family †Neoepiblemidae
        
        Family Abrocomidae: rat chinchillas
        
        Grandorder Ferae
        
        Order Cimolesta - pangolins and relatives
        
        Family †Palaeoryctidae
        
        Suborder †Didelphodonta
        Family †Cimolestidae
        
        Suborder †Apatotheria
        Family †Apatemyidae
        
        Suborder †Taeniodonta
        Family †Stylinodontidae
        
        Suborder †Tillodonta
        Family †Tillotheriidae
        
        Suborder †Pantodonta
        
        Family †Wangliidae
        
        Superfamily †Bemalambdoidea
        
        Family †Harpyodidae
        
        Family †Bemalambdidae
        
        Superfamily †Pantolambdoidea
        
        Family †Pastoralodontidae
        
        Family †Titanoideidae
        
        Family †Pantolambdidae
        
        Family †Barylambdidae
        
        Family †Cyriacotheriidae
        
        Family †Pantolambdodontidae
        
        Superfamily †Coryphodontoidea
        Family †Coryphodontidae
        
        Suborder †Pantolesta
        
        Family †Pantolestidae
        
        Family †Paroxyclaenidae
        
        Family †Ptolemaiidae
        
        Suborder Pholidota
        
        Family †Epoicotheriidae
        
        Family †Metacheiromyidae
        
        Family Manidae: pangolins
        
        Suborder †Ernanodonta
        Family †Ernanodontidae
        
        Order †Creodonta: creodonts
        
        Family †Hyaenodontidae
        
        Family †Oxyaenidae
        
        Order Carnivora
        
        Suborder Feliformia
        
        Family †Viverravidae
        
        Family †Nimravidae
        
        Family Felidae: cats
        
        Family Viverridae: civets, Asiatic palm civets
        
        Family Herpestidae: mongooses
        
        Family Hyaenidae: hyaenas, aardwolf
        
        Family Nandiniidae: African palm civets
        
        Suborder Caniformia
        
        Family †Miacidae
        
        Infraorder Cynoidea
        Family Canidae: dogs
        
        Infraorder Arctoidea
        
        Parvorder Ursida
        
        Superfamily †Amphicyonoidea
        Family †Amphicyonidae
        
        Superfamily Ursoidea
        
        Family Ursidae: bears
        
        Family †Hemicyonidae
        
        Superfamily Phocoidea
        
        Family Otariidae: eared seals
        
        Family Phocidae: seals, walrus
        
        Parvorder Mustelida
        
        Family Mustelidae: weasels, skunks, and relatives
        
        Family Procyonidae: ringtails, olingos, kinkajou, raccoons, coatis, red panda
        
        Grandorder Lipotyphla
        Family †Adapisoriculidae
        
        Order Chrysochloridea
        Family Chrysochloridae: golden moles
        
        Order Erinaceomorpha
        
        Family †Sespedectidae
        
        Family †Amphilemuridae
        
        Family †Adapisoricidae
        
        Family †Creotarsidae
        
        Superfamily Erinaceoidea
        Family Erinaceidae: hedgehogs and relatives
        
        Superfamily Talpoidea
        
        Family †Proscalopidae
        
        Family Talpidae: moles
        
        Family †Dimylidae
        
        Order Soricomorpha
        
        Family †Otlestidae
        
        Family †Geolabididae
        
        Superfamily Soricoidea
        
        Family †Nesophontidae: recently extinct west Indian shrews
        
        Family †Micropternodontidae
        
        Family †Apternodontidae
        
        Family Solenodontidae: solenodons
        
        Family †Plesiosoricidae
        
        Family †Nyctitheriidae
        
        Family Soricidae: shrews
        
        Superfamily Tenrecoidea
        Family Tenrecidae: tenrecs
        
        Grandorder Archonta
        
        Order Chiroptera: bats
        
        Suborder Megachiroptera
        Family Pteropodidae: flying foxes
        
        Suborder Microchiroptera
        
        Family †Archaeonycteridae
        
        Family †Paleochiropterygidae
        
        Family †Hassianycterididae
        
        Family Emballonuridae: sac-winged bats
        
        Infraorder Yinochiroptera
        
        Superfamily Rhinopomatoidea
        
        Family Rhinopomatidae: mouse-tailed bats
        
        Family Craseonycteridae: bumblebee bats
        
        Superfamily Rhinolophoidea
        
        Family Megadermatidae: false vampire bats
        
        Family Nycteridae: hispid bats
        
        Family Rhinolophidae: horseshoe and Old World leaf-nosed bats
        
        Infraorder Yangochiroptera
        
        Family Mystacinidae: New Zealand short-tailed bats
        
        Superfamily Noctilionoidea
        
        Family Noctilionidae: fishing bats
        
        Family Mormoopidae: spectacled bats
        
        Family Phyllostomidae: New World leaf-nosed and vampire bats
        
        Superfamily Vespertilionoidea
        
        Family †Philisidae
        
        Family Molossidae: free-tailed bats
        
        Family Natalidae: funnel-eared bats
        
        Family Furipteridae: smoky bats
        
        Family Thyropteridae: New World sucker-footed bats
        
        Family Myzopodidae: Old World sucker-footed bats
        
        Family Vespertilionidae: common bats
        
        Order Primates: primates
        
        Family †Purgatoriidae
        
        Family †Microsyopidae
        
        Family †Micromomyidae
        
        Family †Picromomyidae
        
        Family †Plesiadapidae
        
        Family †Palaechthonidae
        
        Family †Picrodontidae
        
        Suborder Dermoptera
        
        Family †Paramomyidae
        
        Family †Plagiomenidae
        
        Family †Mixodectidae
        
        Family Galeopithecidae: colugos
        
        Suborder Euprimates
        
        Infraorder Strepsirrhini
        
        Family †Plesiopithecidae
        
        Superfamily Daubentonioidea
        Family Daubentoniidae: aye-aye
        
        Superfamily Lemuroidea
        
        Family †Adapidae
        
        Family Lemuridae: lemurs
        
        Superfamily Loroidea
        
        Family Lorisidae: lorises and galagos
        
        Family Cheirogaleidae: dwarf lemurs
        
        Superfamily Indroidea
        
        Family †Archaeolemuridae
        
        Family †Palaeopropithecidae
        
        Family Indriidae: indris and sifakas
        
        Infraorder Haplorhini
        
        Parvorder Tarsiiformes
        
        Superfamily †Carpolestoidea
        Family †Carpolestidae
        
        Superfamily Tarsioidea
        
        Family †Omomyidae
        
        Family †Microchoeridae
        
        Family †Afrotarsiidae
        
        Family Tarsiidae: tarsiers
        
        Parvorder Anthropoidea
        
        Family †Eosimiidae
        
        Family †Parapithecidae
        
        Superfamily Cercopithecoidea
        
        Family †Pliopithecidae
        
        Family Cercopithecidae: Old World monkeys including colobuses
        
        Family Hominidae: humans, greater apes, lesser apes
        
        Superfamily Callitrichoidea
        
        Family Callitrichidae: marmosets
        
        Family Atelidae: New World monkeys
        
        Order Scandentia
        Family Tupaiidae: tree shrews
        
        Grandorder Ungulata: ungulates
        
        Order Tubulidentata
        Family Orycteropodidae: aardvark
        
        Order †Dinocerata
        Family †Uintatheriidae
        
        Mirorder Eparctocyona
        
        Order †Procreodi
        
        Family †Oxyclaenidae
        
        Family †Arctocyonidae
        
        Order †Condylarthra
        
        Family †Hyopsodontidae
        
        Family †Mioclaenidae
        
        Family †Phenacodontidae
        
        Family †Periptychidae
        
        Family †Peligrotheriidae
        
        Family †Didolodontidae
        
        Order †Arctostylopida
        Family †Arctostylopidae
        
        Order Cete: whales and relatives
        
        Suborder †Acreodi
        
        Family †Triisodontidae
        
        Family †Mesonychidae: mesonychids
        
        Family †Hapalodectidae
        
        Suborder Cetacea
        
        Infraorder †Archaeoceti
        
        Family †Basilosauridae
        
        Family †Protocetidae
        
        Family †Remingtonocetidae
        
        Infraorder Autoceta
        
        Family †Agorophiidae
        
        Superfamily †Squalodontoidea
        
        Family †Squalodontidae
        
        Family †Rhabdosteidae
        
        Parvorder Mysticeti
        
        Family †Aetiocetidae
        
        Family †Mammalodontidae
        
        Family †Cetotheriidae
        
        Family Balaenopteridae: rorquals and grey whales
        
        Family Balaenidae: right and bowhead whales
        
        Parvorder Odontoceti
        
        Superfamily Physeteroidea
        Family Physeteridae: sperm whales
        
        Superfamily Hyperoodontoidea
        Family Hyperoodontidae: beaked whales
        
        Superfamily Platanistoidea
        Family Platanistidae: river dolphins
        
        Superfamily Delphinoidea
        
        Family Delphinidae: dolphins
        
        Family Pontoporiidae: La Plata River dolphin
        
        Family Lipotidae: baiiji
        
        Family Iniidae: Amazon River dolphin
        
        Family †Kentridontidae
        
        Family Monodontidae: beluga and narwhal
        
        Family †Odobenocetopsidae
        
        Family †Dalpiazinidae
        
        Family †Acrodelphinidae
        
        Family Phocoenidae: porpoises
        
        Family †Albireonidae
        
        Family †Hemisyntrachelidae
        
        Order Artiodactyla: even-toed ungulates
        
        Suborder Suiformes
        
        Family †Raoellidae
        
        Family †Choeropotamidae
        
        Superfamily Suoidea
        
        Family Suidae: pigs
        
        Family Tayassuidae: peccaries
        
        Family †Santheriidae
        
        Family Hippopotamidae: hippos
        
        Superfamily †Dichobunoidea
        
        Family †Dichobunidae
        
        Family †Cebochoeridae
        
        Family †Mixtotheriidae
        Family †Helohyidae
        
        Superfamily †Anthracotherioidea
        
        Family †Haplobunodontidae
        
        Family †Anthracotheriidae
        
        Superfamily †Anoplotherioidea
        
        Family †Dacrytheriidae
        
        Family †Anoplotheriidae
        
        Family †Cainotheriidae
        
        Superfamily †Oreodontoidea
        
        Family †Agriochoeridae
        
        Family †Oreodontidae
        
        Superfamily †Entelodontoidea
        Family †Entelodontidae
        
        Suborder Tylopoda
        
        Family †Xiphodontidae
        
        Superfamily Cameloidea
        
        Family Camelidae: camels and llamas
        
        Family †Oromerycidae
        
        Superfamily †Protoceratoidea
        Family †Protoceratidae
        
        Suborder Ruminantia
        
        Family †Amphimerycidae
        
        Family †Hypertragulidae
        
        Family Tragulidae: mouse deer
        
        Family †Leptomerycidae
        
        Family †Bachitheriidae
        
        Family †Lophiomerycidae
        
        Family †Gelocidae
        
        Superfamily Cervoidea
        
        Family Moschidae: musk deer
        
        Family Antilocapridae: pronghorn
        
        Family †Palaeomerycidae
        
        Family †Hoplitomerycidae
        
        Family Cervidae: deer
        
        Superfamily Giraffoidea
        
        Family †Climacoceratidae
        
        Family Giraffidae: giraffe and okapi
        
        Superfamily Bovoidea
        Family Bovidae: cattle, antelope, and relatives
        
        Mirorder †Meridiungulata
        
        Family †Perutheriidae
        
        Family †Amilnedwardsiidae
        
        Order †Litopterna
        
        Family †Protolipternidae
        
        Superfamily †Macrauchenioidea
        
        Family †Macraucheniidae
        
        Family †Notonychopidae
        
        Family †Adianthidae
        
        Superfamily †Proterotherioidea
        Family †Proterotheriidae
        
        Order †Notoungulata: notoungulates
        
        Suborder †Notioprogonia
        
        Family †Henricosborniidae
        
        Family †Notostylopidae
        
        Suborder †Toxodontia
        
        Family †Isotemnidae
        
        Family †Leontiniidae
        
        Family †Notohippidae
        
        Family †Toxodontidae
        
        Family †Homalodotheriidae
        
        Suborder †Typotheria
        
        Family †Archaeopithecidae
        
        Family †Oldfieldthomasiidae
        
        Family †Interatheriidae
        
        Family †Campanorcidae
        
        Family †Mesotheriidae
        
        Suborder †Hegetotheria
        
        Family †Archaeohyracidae
        
        Family †Hegetotheriidae
        
        Order †Astrapotheria
        
        Family †Eoastrapostylopidae
        
        Family †Trigonostylopidae
        
        Family †Astrapotheriidae
        
        Order †Xenungulata
        Family †Carodniidae
        
        Order †Pyrotheria
        Family †Pyrotheriidae
        
        Mirorder Altungulata
        
        Order Perissodactyla: odd-toed ungulates
        
        Suborder Hippomorpha
        
        Family Equidae: horses
        
        Family †Palaeotheriidae
        
        Suborder Ceratomorpha
        
        Infraorder †Selenida
        
        Superfamily †Brontotherioidea
        
        Family †Brontotheriidae
        
        Family †Anchilophidae
        
        Superfamily †Chalicotherioidea
        
        Family †Eomoropidae
        
        Family †Chalicotheriidae
        
        Infraorder Tapiromorpha
        
        Superfamily Rhinocerotoidea
        
        Family †Hyracodontidae
        
        Family Rhinocerotidae: rhinoceroses
        
        Superfamily Tapiroidea
        
        Family †Helaletidae
        
        Family †Isectolophidae
        
        Family †Lophiodontidae
        
        Family †Deperetellidae
        
        Family †Lophialetidae
        
        Family Tapiridae: tapirs
        
        Order Uranotheria: elephants, manatees, hyraxes, and relatives
        
        Suborder Hyracoidea
        
        Family †Pliohyracidae
        
        Family Procaviidae: hyraxes
        
        Suborder †Embrithopoda
        
        Family †Phenacolophidae
        
        Family †Arsinoitheriidae
        
        Suborder Tethytheria
        
        Infraorder Sirenia: manatees, dugong, and sea cow
        
        Family †Prorastomidae
        
        Family Dugongidae: dugongs
        
        Family Trichechidae: manatees
        
        Infraorder Behemota
        
        Parvorder †Desmostylia
        Family †Desmostylidae
        
        Parvorder Proboscidea
        
        Family †Anthracobunidae
        
        Family †Moeritheriidae
        
        Family †Numidotheriidae
        
        Family †Barytheriidae
        
        Family †Deinotheriidae
        
        Family †Palaeomastodontidae
        
        Family †Phiomiidae
        
        Family †Hemimastodontidae
        
        Superfamily ‡Mammutoidea
        Family †Mammutidae: mastodons and relatives
        
        Superfamily Elephantoidea
        
        Family †Gomphotheriidae: gomphotheres
        
        Family Elephantidae: modern elephants

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 and Bell (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 and Bell (1997) and other authors. They also define their taxonomic levels as clades and do not apply Linnean hierarchies.

Mammalia

†Sinoconodon - earliest and most basal of mammals
Unnamed clade 1 - a clade that contains all other mammals. These are characterized by determinant growth and occlusal features of the cheek teeth.
- †Morganucodontidae - morganucodontids, including †Morganucodon, †Megazostrodon, and others
- †Docodonta - docodonts, including †Haldanodon and †Castorocauda (Ji et al., 2006)
- 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.
  - †Hadrocodium
  - †Kuehneotherium
  - 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.
    - Australosphenida - a clade that contains monotremes and their fossil relatives. These fossils include †Ambondro, †Asfaltomylos, †Ausktribosphenos, and †Bishops. If correct, this clade represents an independent evolution of the tribosphenic molar in southern continents.
    - Trechnotheria - Therians, spalacotheriids and their relatives. They are characterized by features of the scapula, tibia, and humerus.
      - †Spalacotheriidae - including Akidolestes, Zhangheotherium, and Maotherium.
      - Cladotheria - Therians, dryolestids, and their relatives. They are characterized by features of the tribosphenic molar and the angular process of the dentary.
        
        †Dryolestidae
        
        †Amphitherium - incertae sedis (it may be a prototribosphenidan)
        
        Prototribosphenida - Therians and fossil relatives including †Vincelestes. Characterized by features of the cochlea including coiling.
        
        †Vincelestes
        
        Zatheria - Therians and fossil relatives including the "peramurids". Characterized by the presence of wear in the talonid of the lower molars.
        
        †"Peramuridae" - †Peramus and relatives. Known only from preserved mandibles and distinctly zatherian molars.
        
        Boreosphenida - Therians and fossil relatives including †Kielantherium. They are characterized by molar features.
        
        †Kielantherium
        
        †Deltatheroida including †Deltatheridium - incertae sedis (it may represent a metatherian)
        
        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 e 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. The classification ignores differences in levels and thus cannot be used to estimate the respective distances between taxa. It also ignores taxa that became extinct in pre-historic times. Finally, 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.

Monotremes (prototheria): echidnas and platypus
- Platypus
- 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
      - Macropodids: 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
    - Laurasian placentals (laurasiatherians)
      - Hedgehogs (erinaceids)
      - Soricomorphs: moles, shrews, solenodons
        
        Shrews (soricids)
        
        Moles (talpids)
        
        Solenodons (solenodontids)
      - Ferungulates: ungulates, cetaceans, bats, pangolins and carnivorans
        
        Cetartiodactyls: even-toed ungulates and cetaceans
        
        Camelids: camels and llamas
        
        Swine (suinans): pigs and peccaries
        
        Pigs (suids)
        
        Peccaries (tayassuids)
        
        Cetruminantians: cetaceans, hippos and ruminants
        
        Cetancodonts: cetaceans and hippos
        
        Cetaceans: Whales, dolphins and porpoises
        
        Baleen whales (mysticetes)
        
        Balaenids: right whales and bowhead whale
        
        Rorquals (balaenopterids)
        
        Gray whale
        
        Pygmy right whale
        
        Toothed whales (odontocetes)
        
        Dolphins (delphinids)
        
        Monodontids: beluga and narwhal
        
        Beluga
        
        Narwhal
        
        Porpoises (phocoenids)
        
        Sperm whale
        
        Kogiids: pygmy and dwarf sperm whale
        
        River dolphins (platanistoid whales)
        
        Iniids: Amazon and Bolivian river dolphin
        
        La Plata dolphin
        
        Platanistids: Ganges and Indus river dolphins
        
        Beaked whales (ziphids)
        
        Hippos (hippopotamids)
        
        Ruminantiamorphs: chevrotains, pronghorn, giraffes, musk deer, deer, and bovids
        
        Chevrotains (tragulids)
        
        Pecorans
        
        Pronghorn
        
        Giraffids: giraffe and okapi
        
        Musk deer (moschids)
        
        Deer (cervids)
        
        Bovids: cattle, goats, sheep and antelope
        
        Pegasoferans: bats, odd-toed ungulates, pangolins and carnivorans
        
        Bats (chiropterans)
        
        Megabats (pteropodids)
        
        Microbats (microchiropterans)
        
        Sac-winged or sheath-tailed bats (emballonurids)
        
        Rhinopomatoid bats
        
        Mouse-tailed bats (rhinopomatids)
        
        Bumblebee bat or Kitti's hog-nosed bat
        
        Rhinolophoid bats
        
        Horseshoe bats (rhinolophids)
        
        Hollow-faced or slit-faced bats (nycterids)
        
        False vampires (megadermatids)
        
        Vesper bats or evening bats (vespertilionids)
        
        Molossoid bats
        
        Free-tailed bats (molossids)
        
        Pallid bats (antrozoids)
        
        Nataloid bats
        
        Funnel-eared bats (natalids)
        
        Sucker-footed bats (myzopodids)
        
        Disc-winged bats (thyropterids)
        
        Smoky bats (furipterids)
        
        Noctilionoid bats
        
        Bulldog or fisherman bats (noctilionids)
        
        New Zealand short-tailed bats (mystacinids)
        
        Ghost-faced or moustached bats (mormoopids)
        
        Leaf-nosed bats (phyllostomids)
        
        Zooamatans: odd-toed ungulates, pangolins and carnivorans
        
        Odd-toed ungulates (perissodactyls)
        
        Horses (equids)
        
        Ceratomorphs
        
        Tapirs (tapirids)
        
        Rhinoceroses (rhinocerotids)
        
        Ferans
        
        Pangolins or scaly anteaters (manids)
        
        Carnivorans
        
        Cat-like carnivorans (feliforms)
        
        African palm civet
        
        Feloid carnivorans
        
        Asiatic linsangs (prionodontids)
        
        Cats (felids)
        
        Viverroid carnivorans
        
        Viverrids: civets and allies
        
        Herpestoid carnivorans
        
        Hyaenids: hyenas and aardwolf
        
        Malagasy carnivorans (euplerids)
        
        Herpestids: mongooses and allies
        
        Dog-like carnivorans (caniforms)
        
        Canids: dogs and allies
        
        Arctoid carnivorans
        
        Bears (ursids)
        
        Musteloid carnivorans
        
        Red panda
        
        Mephitids: skunks and stink badgers
        
        Mustelids: weasels, martens, badgers, wolverines, minks, ferrets and otters
        
        Procyonids: raccoons and allies
        
        Pinnipeds
        
        Walrus
        
        Otariids: sea lions, eared seals, fur seals
        
        True seals (phocids)

Mammal classification

Molecular classification of placentals

Atlantogenata

Afrotheria

Xenarthra

Boreoeutheria

Euarchontoglires

Laurasiatheria

Standardized textbook classification

Subclass Prototheria

Subclass Theria

McKenna/Bell classification

Subclass Prototheria

Subclass Theriiformes

Luo, Kielan-Jaworowska, and Cifelli classification

Simplified classification for non-specialists

See also