Zoology · Ch 4 — Animal Diversity-II: Phylum Chordata
Class Mammalia
Class Mammalia
Class Mammalia
Mammals are the chordate class showing, among other features, mammary glands (producing milk to nourish the young), hair, and, typically, viviparity (in the majority of living mammals, the Eutheria/placental mammals).
Nervous system and sense organs — mammals possess the most highly developed brain of any vertebrate class relative to body size, with an especially expanded and, in most mammals, deeply folded cerebral cortex (the outer layer of the cerebrum), supporting complex learning, memory and behaviour to a degree unmatched by the other vertebrate classes. The mammalian sense organs are correspondingly well developed: external, mobile pinnae (external ears) improve the localisation and collection of sound; complex, image-forming eyes are present, though visual acuity and colour vision vary considerably between mammal groups depending on lifestyle; and a well-developed olfactory (smell) system, often highly sensitive, plays an especially important role in the behaviour of many mammal groups, considerably more so than in birds, for instance.
Dentition and endoskeleton of Eutheria (the placental mammals) — unlike the generally single, uniform tooth type of reptiles, eutherian mammals typically show heterodont dentition: teeth differentiated into distinct functional types — incisors (for cutting/gnawing), canines (for gripping/tearing), premolars and molars (for grinding/chewing) — and, again unlike reptiles, most eutherians are diphyodont, developing two successive sets of teeth in a lifetime (a temporary "milk"/deciduous set replaced later by a permanent set). The eutherian endoskeleton is fully ossified and, compared with earlier vertebrate classes, shows further specialisations tied to the diversity of eutherian locomotor modes (running, climbing, swimming, flying, burrowing), while retaining the basic tetrapod limb plan (a single proximal bone, two distal bones, wrist/ankle bones, and digits) inherited from the earliest terrestrial vertebrates.