From Laura Klappenbach,
Your Guide to Animals / Wildlife.
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To understand mammals, it first helps to understand how they fit into the classification system of all animals. Mammals, as a group of animals, belong to the Class Mammalia. The Class Mammalia, in turn, belongs to the larger group known as the vertebrates (animals with backbones; also called the Phylum Chordata). Other classes in the Phylumn Chordata include fishes, amphibians, reptiles, and birds.

Looking within the Class Mammmalia, we can subdivide mammals into smaller groups (known as orders). Although scientists differ on the number of orders that exist within the Class Mammalia, the list below summarizes one possible approach to their classification:

• Monotremata (egg-laying marsupials)
• Marsupialia (marsupials)
• Insectivora (insectivores)
• Chiroptera (bats)
• Dermoptera (flying lemurs)
• Macroscelidea (elephant shrews)
• Scandentia (tree shrews)
• Primates (primates)
• Xenarthra (anteaters and relatives)
• Pholidota (pangolins)
• Lagomorpha (rabbits, hares, and pikas)
• Rodentia (rodents)
• Cetacea (cetaceans)
• Carnivora (carnivores)
• Pinnipedea (seals and sea lions)
• Proboscidea (elephants)
• Tubulidentata (aardvark)
• Hyracoidea (hydraxes)
• Sirenia (dugongs and manatees)
• Perissodactyla (odd-toed hoofed mammals)
• Artiodactyla (even-toed hoofed mammals)

The unique characteristic of mammals is that they all feed their young with milk produced by the female's mammary glands. Most mammals give birth to live young and almost all species have hair on their body. Some types of mammals form complex social structures. A quick summary of mammalian characteristics include:

• mammary glands
• hair
• diaphragm
• three middle-ear ossicles
• heterodont dentation
• sweat, sebaceous, and scent glands
• four-chambered heart
• large cerebral cortex

Mammals are believed to have evolved from an ancient group of reptiles called the therapsids. Therapsids lived about 225 million years ago during the Triassic Period. As they evolved, therapsids developed a lighter, more flexible skeleton and the alignment of their limbs changed over time to be directly beneath their body (unlike reptiles whose limbs are at the sides of their body).

The first mammals are thought to have been relatively small creatures (less than 10 cm long) with light, delicate skeletons. The fossil record suggests that these creatures had teeth and skulls characteristic of herbivores or predators that fed on arthropods or other vertebrates. Well-developed senses of hearing and smell suggest they may have been nocturnal creatures. another important adaptation mammals displayed was endothermy or the ability to regulate their own body temperatures.

When climate change took place in the Mesozoic Era, temperatures fluctuated and dropped. The reptiles that once dominated the land suffered in the low temperatures, while mammals were able to better compete since their body temperatures were less effected by temperature fluctuation.

The Tertiary period, which began about 70 million years ago, brought with it the dawning of the "age of mammals", a time following the extinction of many reptile species. This is when mammals diversified significantly in number and kind.