There are about 4100 living species of mammals (class Mammalia), the smallest number of species in any of the five classes of vertebrates. Most large, land-dwelling vertebrates are mammals, and they tend to dominate terrestrial communities, as did the dinosaurs that they replaced. When you look out over an African plain, you see the big mammals, the lions, zebras, gazelles, and antelope. Your eye does not as readily pick out the many birds, lizards, and frogs that live in the grassland community with them. But the typical mammal is not all that large. Of the 4100 species of mammals, 3200 are rodents, bats, shrews, or moles.
Characteristics
-Mammals have heterodont dentition, with different
types of teeth that are highly specialized to match particular eating habits.
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-In most mammal species, females carry their young in a uterus during development, nourishing them through a placenta,
and give birth to live young.
-Mammals are endothermic, a crucial adaptation that has allowed mammals to be active at any time of the day or
night and to colonize severe environments, from deserts to ice fields.