Mammal

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The mammals (scientific name Mammalia) are the class of animals primarily characterized by the presence of mammary glands in the female which produce milk for the nourishment of young, the presence of hair or fur, and have endothermic or "warm blooded" bodies. The brain regulates endothermic and circulatory systems including a four chamber heart. Humans are mammals. Phylogenetically, the Mammalia are defined as the last common ancestor of monotremes (e.g. echidna) and therian mammals (e.g. hedgehogs) and all of its descendants.

While most mammals give birth to live young, there are a few mammals - the monotremes - that lay eggs. Live birth also occurs in a variety of non-mammalian species; thus it is not a diagnostic characteristic for class Mammalia. Endothermy is also present in many non-mammals, primarily birds. While monotremes do not have nipples, they do have mammary glands, meaning that they do meet all conditions for inclusion in the class Mammalia. It should be noted that the current trend in taxonomy is to emphasize common ancestry; the diagnostic characteristics are useful for identifying this ancestry, but if, for example, a cetacean were found that had no hair at all, it would still be classed as a mammal.

Mammals have three bones in each ear and one (the dentary) on each side of the lower jaw; all other vertebrates with ears have one bone (the stapes) in the ear and at least three on each side of the jaw. A group of therapsids called cynodonts had three bones in the jaw, but the main jaw joint was the dentary and the other bones conducted sound.

Location in the evolutionary tree:
Domain Eukaryota, Kingdom Animalia, Phylum Chordata, Subphylum Vertebrata, Class Mammalia.

Therapsids are the direct ancestors of mammals. Therapsida and Mammalia, along with Pelycosauria make up the synapsids

The names "Prototheria", "Metatheria" and "Eutheria" expressed the theory that Placentalia were descendants of Marsupialia, which were in turn descendants of Monotremata, but this theory has been refuted.

Evolution

Mammals developed from therapsid amniotes during the Triassic 220 million years ago (which is about the same time that birds developed). Therapsids are distinguished from other amniotes such as reptiles by the shape of their skulls which scientists call synapsid. During the reign of the dinosaurs mammals continued to evolve forming the three different mammal types seen today; monotremes, marsupials and placentals. Mammals remained small and shrew-like during the entire Mesozoic but then began to radiate rapidly following the Cretaceous-Tertiary extinction event 65 million years ago.

More on Mammal

Dolphins

"Diviner than the dolphin is nothing yet created; for indeed they were aforetime men... but by the devising of Dionysus they exchanged the land for the sea and put on the form of fishes." - Oppian. Halieutica

Elephants

Elephants are the world's largest land mammals. They live in Africa, Asia, and zoos around the world. Only male Asian elephants have big tusks. The difference between Asian elephants and African elephants is the African elephants have bigger ears.

Horses

The prehistoric horse began as a small, leaf-browsing mammal some 55 million to 38 million years ago, appearing about the same time in Europe and America. It was not the large mammal that we know today, but rather a small beast, about 11 inches high that had four toes on its forefeet and three on it's hindfeet. Having no means of defending itself, the horse depended upon speed for protection. Horse skeletons and bones found in and near caves used as dwellings by Stone Age man indicate that the species comprised a significant portion of the inhabitants' diet.

Pandas

Pandas are nocturnal mammals, of the Ailuropodidae family - related to raccoons and bears. The rare Giant Panda - Ailuropoda melanoleuca - inhabits bamboo forests in the mountains of China (mainly Tibet).