Although mammals are the most familiar animals, only about 4,629 species belong to this class.  The Class of mammals evolved from a reptilian lineage ("mammal-like reptiles") early in the Mesozoic, but Mesozoic mammals remained small (shrew to house cat-sized).  After the mass extinction that ended the Mesozoic and all the large dinosaurs, mammals evolved much greater diversity.

        Mammals can be distinguished by at least eight characters:
 

1.  Hair or fur.  Hairs grow from hair follicles and are made of a protein, keratin; they are derived from reptilian scales.

2.  Mammary glands, which (in females) secrete milk for nutrition of infants.

3.  Zygomatic arch.  This is a remnant of the original bony skull roof after the evolution of a temporal opening.

4.  Temporo-mandibular jaw joints (TMJ) between the temporal bone of the skull and the mandible (lower jaw).

5.  Three small bones in each middle ear, the malleus (hammer), incus (anvil), and stapes (stirrup).  The malleus and incus are derived from the old reptilian jaw joint that is no longer used.

6.  No nuclei in red blood cells.  This increases the oxygen carrying ability of RBC's and is related to mammals' high constant body temperatures.  (But, high constant body temperature [homeothermy] is NOT distinguishing; birds are also homeothermic.)

7.  Diaphragm.  A sheet of skeletal muscle between the thoracic cavity (chest) and abdominal cavity; this is the principal muscle used in breathing and is also related to mammals' high respiratory rates and homeothermy.

8.  Specialized dentition.  Teeth are differentiated into incisors, canines, premolars, and molars.  They are usually further specialized in the orders and families according to diet.

Mammologists recognize three subclasses and 26 orders of living mammals (MSW).

Subclasses and Orders of mammals:

        Subcl.  Monotremes
          Order egg-laying mammals                                    platypus, echidna

        Subcl. Marsupials
          Order marsupials  {possibly 4-5 orders}                 opossum, kangaroos, koala

        Subcl. Placental mammals
           Order insectivores                                              shrews, moles
                About 400 species of mostly small mammals.
                As a group, the insectivores are difficult to distinguish because they mostly retain primitive traits of
                placental mammals, such as molars with three high sharp cusps (points).
                Some of the smallest living mammals, the shrews, belong to this order.
           Order bats (more) (more)
                Bats are easily distinguished by their wings, a skin membrane, supported by the finger bones.
                With about 850 species this is the second largest order of mammals.
           Order primates                                                   lemurs, monkeys, apes, humans
                Primates are primarily arborial herbivores.  To ease movement in treetops, primates have grasping
                opposable thumbs.  For good depth perception (important for leaping from branch to branch), primates
                have their eyes set in front of the face.  About 170 species.
           Order rodents                                                        rats, squirrels,  girbils
                With about 1700 species this is the largest order of mammals.
                All rodents have two upper and two lower incisors, no canines, and usually 3-4 premolars and molars
                per jaw; there is a space between the incisors and premolars.  Rodents gnaw hard seeds and nuts.
           Order carnivores (more) (more)                             dogs, cats (sabretooths)
                Carnivores are primarily meat-eaters (with few exceptions, pandas, bears).  They are distinguished by
                their carnassial teeth, the 4th upper premolar and 1st lower molar (P⁴/M₁) act to cut meat and connective
                tissue with a shearing action.  There are about 280 living species.
           Order elephants
                Elephants are easily distinguished by their nose and upper lip that form the trunk.
                The upper incisors form the tusks.  Two living species.
           Order odd-toed ungulates                                    horses, Rhinoceros, tapirs
                The 16 living species are a mere shadow of the former diversity of this order.  These are herbivores with
                high-crowned teeth and the side toes reduced leaving only one or three functional hooves.
           Order even-toed ungulates  (more)                         cattle, sheep, antelopes, deer, pigs
                Even-toed ungulates have the first digit (thumb, big toe) absent and side toes reduced leaving two or
                four toes with hooves on each foot.  About 175 living species.
           Order whales  (more)                               blue whale, humpback whale, sperm whale, dolphins
                Whales (about 80 species) are fully adapted to marine environments.  The hind legs are reduced to
                "pelvic rudiments" embedded in the body wall, and the fore limbs are flattened and used as "flippers."
                The upper jaw is lengthened forward while the nose is shortened so the nose appears on top of the head
                 as a "blowhole."  The tail is flattened into a pair of "flukes" that move up and down to power a whale forward.