Class Mammalia

General Mammalian Characteristics:

  • Possess hair which is made of keratin. The evolution of mammalian keratin is believed to be independent of reptilian keratin. Hair provides insulation .
  • Endothermic. The majority of the heat energy is used to maintain their high body temperature.
  • 4 chambered heart.
  • Mammary glands are used to produce milk to nourish their young. Female glands are the only functional glands.
  • The diaphragm is a muscle that separates the thoracic cavity from the abdominal cavity.
  • 7 cervical vertebrae (neck bones) are present in most mammals.
  • Most are viviparous though some are oviparous. An extended gestation period (uterine development) is common in most placental mammals.
  • Teeth are imbedded in the jaw bone and come in a variety of forms.
  • Well developed brain.
  • Mammals developed from the therapsid ancestors during the Triassic period.

Major Groups of Mammals:

  • Monotremes. Characterized by the duckbilled platypus and the spiny anteaters, lay eggs and maintain some reptilian characteristics. They do not contain true mammary glands, but produce a fatty sweat (milk) from glands in the skin. The milk collects and drips down tufts of hair into the offspring's mouth. They are found in Australia and New Guinea.
  • Marsupials. These mammals contain a pouch (marsupium). Opossums,koalas,kangaroos, and other examples live in Australia as a result of the break up of the super continent Pangea. The young are born during the early stages of development. The new born crawls up to the mother's pouch, where it clings on to a nipple and hangs there until it fully develops.
  • Placental Mammals. These mammals are the most abundant and diverse of the class. The placenta, a reproductive structure, is housed in the uterus of the female. Here the offspring receives all that it needs to develop into a fully developed organism. This period of development (gestation) varies with the species of mammal. Whales gestate for 2 years, while a mouse develops in 21 days.
Chart of Placental Mammals

Eats meat, all teeth are pointed
cats and dogs , seals, walrus, weasels, and martins

wings used for flight, diet includes insects and vegetation


opposable thumb, plantigrade foot, usually one offspring at birth.

man , apes, monkeys,tarsiers,and lemurs
lacking teeth, large claws for digging
sloths, armadillos, South American anteater
2 incisors on both jaws, diastema between incisors and premolars
rats, mice, squirrels, beavers, gophers, and capybara
4 incisor teeth, canine teeth lacking, short stubby tails
rabbits, hares,pikas
flippers, opening on top of head
Great size,nose and upper lip form proboscis, upper incisors are tusks,thick skin,scant hair
Herbivorous, aquatic, no external ears, flippers, no hind appendages
Manatees and dugongs
Odd-toed,hoofed forms,gall bladder lacking, herbivorous
horses, donkeys,zebras,tapirs
Even-toed, hoofed forms, herbivorous, true horns or antlers present