SUBCLASS Prototheria

• INFRACLASS Monotremata: Platypuses and other oviparous mammals

SUBCLASS Theria: Marsupials and placental mammals.

• INFRACLASS Metatheria
  • ORDER Marsupialia
    • FAMILY Didelphidae: Opossums (New world)
    • FAMILY Dasyuridae: Marsupial wolf, Tasmanian Devil, etc.
    • FAMILY Phalangeridae: koalas
    • FAMILY Macropodidae: kangaroos, wallabies
• INFRACLASS Eutheria: Placental mammals
  • ORDER Insectivora: Moles and shrews
  • ORDER Dermoptera: Flying lemurs
  • ORDER Chiroptera: Bats
  • ORDER Primates: Primates
    • SUBORDER Prosimii: lemurs, tarsiers, lorises
    • SUBORDER Anthropoidea: higher primates
      • INFRAORDER Platyrrhini
        • SUPERFAMILY Ceboidea
        • FAMILY Cebidae: New World monkeys
      • INFRAORDER Catarrhini
        • SUPERFAMILY Cercopithecoidea:
          • FAMILY Cercopithecidae: Old world monkeys
        • SUPERFAMILY Hominoidea
          • FAMILY Hylobatidae: gibbons
          • FAMILY Pongidae: chimpanzees, orangutans, gorillas
          • FAMILY Hominidae: Humans
            • GENUS Homo
              • SPECIES sapiens
  • ORDER Edentata: American anteaters, sloths, armadillos
  • ORDER Pholidota: Old world anteaters
  • ORDER Lagomorpha: Hares, rabbits
    • FAMILY Lepus: Hare
    • FAMILY Oryctolagus: rabbit
    • FAMILY Sylvilagus: cottontail
  • ORDER Rodentia: Rodents (most common mammals in the world)
    • FAMILY Sciuridae: squirrels
    • FAMILY Geomidae: gophers
    • FAMILY Muridae: rats and mice
  • ORDER Cetacea
    • SUBORDER Odontoceti: toothed whales, dolphins
    • SUBORDER Mysticeti: baleen whales
  • ORDER Carnivora: Terrestrial carnivores
    • FAMILY Canidae: wolves, foxes, dogs
    • FAMILY Ursidae: bears
    • FAMILY Procyonidae: raccoons
    • FAMILY Mustelidae: skunks, weasels, otters, badgers
    • FAMILY Felidae: cats
  • ORDER Pinnipedia: aquatic carnivores: seals and walrus.
    • FAMILY Odobenidae: walrus
    • FAMILY Otariidae: eared seals, walrus
    • FAMILY Phocidae: earless seals
  • ORDER Tubulidentata: Aardvarks
  • ORDER Proboscidea: Elephants, mastodons
  • ORDER Sirenia: Manatees, dugongs
  • ORDER Artiodactyla: Even-toed ungulates
    • SUBORDER Suiformes: pigs, hippos
    • SUBORDER Tylopoda: camels, llamas
    • SUBORDER Ruminantia: ruminants
      • FAMILY Cervidae: elk, deer
      • FAMILY Bovidae: bison, sheep
      • FAMILY Giraffidae: giraffes
      • FAMILY Antilocapridae: pronghorns
  • ORDER Perissodactyla: Odd-toed ungulates
    • SUBORDER Hippomorpha
      • FAMILY Equidae: horses, asses, zebras
    • SUBORDER Ceratomorpha
      • SUPERFAMILY Tapiroidea: Tapirs
      • SUPERFAMILY Rhinocerotoidea: rhinoceroses

Skull: Large braincase, usually with sagittal crest for attachement of jaw muscles. Zygomatic arch protects eyes, provides attachment for masseter muscles, and forms articulating surface for jaw. Jaw composed of single bone (dentary). Functional joint between dentary and squamosal bones. Quadrate and articular bones of reptiles form incus (quadrate) and maleus (articular) of middle ear (together with stapes, inherited directly from reptillian columella). Bony secondary palate separates oral and nasal cavities. Double occipital condyle articulates to spine as in reptiles, unlike birds (single).

Skeleton: Highly differentiated vertebral column (cervical, thoracic, lumbar, sacral, caudal). Usually seven cervical vertebrae (exceptions are sloths and manatees). First two cervical vertebrae (atlas and axis) specialized to support skull. Usually twelve pairs of ribs (sloths have more); most attach to sternum. No ribs on lumbar or posterior vertebrae. Scapula has strong spine for muscle attachment. Clavicle present in mammals whose limbs move in several planes.

Teeth: Highly specialized and characteristic. Enamel is hardest part of mammalian body. Generally divided into incisors, canines, and molars. Mammals show diphydontia, replacement of milk teeth by permanent teeth.

Brain: Proportion of brainsize to animal size generally greater than all other animals. Cerebrum ( cerebral cortex, or neopalliium) highly developed. Optic tectum reduced to corpora quadrigemina. Direct motor control from the cerebral cortex is possible in mammals via corticospinal tracts.

Senses: Eyes are typical of amniotes. Middle ear has impedence matching ossicles (see skull above). Cochlea long and coiled. Highly developed sense of smell.

Hair: Most mammals have hair at some point in their lives.

Skin glands: Great variety and number, including sweat glands, mammary glands, scent and musk glands.

Circulation: Pulmonary and systemic circulation fully separate. Heart rate varies with size, from 25 per minute in elephant to 1300 per minute in shrew.

Respiration: Highly developed lungs, reciprocal ventilation. Ribs and diaphragm used in breathing.

Reproduction: Internal fertilization. Amniotic embryo develops in uterus, nourished via placenta (in Eutherians). Eggs not yolky, but development as in reptile eggs.

Muscles: Highly differentiated. Many dermal muscles used in communication (facial expressions, hair raising) and thermoregulation (shivering, goosebumps, hair raising).