Modern mammals have a number of features that can be used to characterize them. Some of these are unique to mammals among the living vertebrates, others characteristic of most or all living mammals but also occur in some other vertebrate groups. "Modern" and "living" are emphasized here because a number of fossil mammal-like reptiles display morphological characters that we think of as typically mammalian (and we presume non-fossilizable "mammalian" characters also were evolved in various taxa close to the ancestry of mammals). Although we have no difficulty in separating modern reptiles from modern mammals, the separation in the fossil record basically is arbitrary.
Characters Unique to Mammals Among Modern Vertebrates
Hair. This is an epidermal product formed of the protein keratin. It appears to have originated between scales, not from them. It is found in some stage of every mammalian taxon and only among mammals.
Mammary Glands. These milk glands give the class its name. All mammalian taxa possess them, and only mammalian taxa have them.
Four-chambered heart with the systemic artery formed from the left fourth aortic arch. Although the four-chambered heart is shared with birds (and a few reptiles), the major blood vessel distributing blood from the heart to the general body is formed embryologically from the left fourth aortic arch; in birds, this artery is formed from the right aortic arch, indicating an independent origin.
Each side of lower jaw formed from a single bone (the dentary). Other jawed vertebrates have more than one bone forming each side of the lower jaw.
Three auditory ossicles. In mammals, three small bones transmit vibrations across the middle ear from the tympanum to the inner ear; other vertebrates with auditory ossicles have the equivalent of only one of the three in the middle ear (the homologues of the other two are the quadrate of the skull and the articular of the lower jaw bone that form the jaw articulation in non-mammalian vertebrates).
Characters Typically Present in Mammals, but also Found Among Other Vertebrate Taxa
Homoiothermy (also found in birds). This is the ability to maintain a relatively constant body temperature by physiological means (as opposed to the widespread ability among tetrapods to do so by behavioral means). Varies from excellent to poor among mammals.
Heterodont dentition (found in most mammals and in most mammals is better developed than in other vertebrate groups). The typical mammal has teeth adapted to different functions and morphologically divisible into incisors, canines, premolars, and molars (any or all of which can be lost evolutionarily). A few mammals (e.g., porpoises, armadillos) are homodont, with little or no differentiation among their teeth.
Diphyodont dentition. Typically mammals have two generations of incisors, canines, and premolars. The first generation makes up the deciduous (lacteal, milk) teeth and the second generation plus the molars constitute the permanent teeth.
Thecodont dentition (teeth set in sockets in the jaw). Although also found in many other vertebrate groups, most tend to have acrodont (fused to the surface of the jaw) or pleurodont (set into the side of the jaw) dentition or some variant of these types.
Double occipital condyle (reptiles and birds with single occipital condyles). The occipital condyles form the connection between backbone and the skull.
Vertebral column well differentiated into cervical, thoracic, lumbar, sacral, and caudal vertebrae, with the cervical vertebrate seven in number except among sirenians and sloths.
Single nasal opening in the skull. In most other vertebrates (but not turtles, for example), the nostrils are separated from one another by bone.
Viviparity (but oviparity in the Subclass Prototheria). Birds are all oviparous, and other vertebrate groups are oviparous or ovoviviparous.
Plentiful epidermal glands. The skin of most mammals is well supplied with glands (scent, sebaceous, sweat, etc.), whereas birds and reptiles tend to have only a few, specialized epidermal glands.
Muscular diaphragm. The thoracic cavity is separated from the abdominal cavity by a muscular diaphragm that serves to increase and decrease the size of the thoracic cavity during respiration.
Epiphyses. These are centers of ossification at the ends of long bones and vertebrae that are separated from the diaphyseal (shaft) ossification by epiphyseal cartilage when young. Eventually, the epiphyses fuse with the diaphysis, at which time the growth in length of the bone is over.
Enucleate Red Blood Cells (RBC). Mature RBCs, unlike those of other vertebrates, have lost the cell nucleus (and thus are enucleate). They are round (usually), biconcave discs.
Enlarged neopallium. The brain characteristically has a better developed neopallium than do other vertebrates. The neopallium, originating as a small area at the front of the cerebral hemispheres in some reptiles, has expanded over the more primitive parts of the brain in mammals. This is gray matter that forms the cerebral cortex and appears to be responsible for the intelligence of mammals.
Pinnae. Most mammals have a flap (the pinna) that partly surrounds the auditory meatus (the external opening into the ear)and that plays a part in amplification and location of sound.
Vibrissae commonly are present. These are relatively long, stiff hairs protruding from specialized areas of the face; they are tactile in nature.
Facial Muscles. Facial muscles allow movements of much of the facial area in mammals. Such movements allow such actions as suckling and communication of various types (in humans,
smiles, frowns, etc.).
Although other features of mammals could be mentioned (and some will be further on), the above should sufficiently characterize the class.
Last Update: 16 Jan 2008
Centennial Museum and Department of Biological Sciences, The University of Texas at El Paso
