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First appearing during the Mesozoic era 181 to 135 million years ago, the first mammals were descendents of doglike reptiles living in a world dominated by dinosaurs. Warm blooded and furry, mammals are a familiar, charismatic group. Distinctive characteristics of mammals include their fur, and milk producing mammary glands. All mammals have hair or fur. Even whales have a few course hairs around their mouths. Depending on its function, fur can vary in its appearance such as a sheep’s thick, curly wool; a porcupines’s sharp quills (yes, quills are hairs); or the human hair.

Mammals are able to survive in almost any type of habitat from frigid tundra to a hot, humid, tropical rainforest. They can live almost anywhere because the temperature inside their bodies stays the same whether it’s cold or warm outside (they are homeothermic). They can also make their own body heat to keep themselves warm enough (they are endothermic).

Unlike most birds or most reptiles who lay eggs, most mammals give birth to the “live young." Mammals can be distinguished because they have mammary glands (which produce milk for their young), ears are comprised of three ear bones, they are endothermic, and they have fur. Mammal brains are generally larger than other animals, and the largest animal in the world is a mammal - the blue whale.

Monotremes (order: Monotremata)

Monotremes are the only group of mammals that are oviparous (meaning they lay eggs), laying one to three eggs. Monotremes resemble other mammals in producing milk to nourish their young, in having three inner ear bones and a single bone in the lower jaw. However, monotremes are highly specialized feeders and the adults have no teeth.

Like marsupials, monotremes have lower metabolic rates than other mammals but they are still endothermic and can maintain their bodies at a constant temperature regardless of environmental temperatures. They are one of the only two groups of venomous mammals, shrews are the other group. Only three species exist today - the duckbilled platypus and two species of spiny anteaters.

Marsupials (order: Marsupialia)

Marsupials are distinguished by having live-born young that crawl into a pouch (marsupium) that is equipped with mammary glands. The young are born immature, much as the hatchlings of monotremes. Brains are typically smaller than placental brains.

While marsupials were once common through-out the world, they have been displaced by placental relatives in most places with the opossum being the only species that is found beyond Australia.

Placentals (multiple orders)

The placental mammals are a very diverse group with an enormous range of body forms and complex social interactions. The 3782 species are divided among 19 orders. They include the terrestrial insectivores, edentates, ungulates, rodents, carnivores and primates. 

Placental mammals have adapted to an amazing diversity of environments and many have highly specialized abilities - from bats that are capable of sustained flight aquatic seals to massive whales that live their entire lives in the water.

Distinguishing characteristics of placental mammals include embryonic development in uterus with the live birth of young at advanced and relatively mature stages.

(Click here to learn more about different types of placental mammals!))

A pine marten peeks out from its burrow. 

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