mammalia

Mammalia is a class of endothermic amniotes characterized by mammary glands, hair, and three middle ear bones. Most mammals give birth to live young, but the monotremes—the platypus and echidnas—lay eggs. Females nourish their offspring with milk produced by mammary glands.

Key anatomical features include a single dentary bone forming the lower jaw and three auditory ossicles (malleus, incus, stapes). They have high metabolic rates, a diaphragm, and extensive parental care. Hair provides insulation, camouflage, and sensory functions. Most mammals are heterodont, with a range of tooth types.

Three major lineages persist: monotremes, marsupials (metatherians), and placental mammals (eutherians). Marsupials give birth to tiny, underdeveloped young that often complete development in a pouch; placental mammals undergo longer gestations with a placenta facilitating nutrient exchange.

Diversity and ecology: Mammalia includes numerous orders such as Primates, Carnivora, Rodentia, Chiroptera, Artiodactyla, and Cetacea. They inhabit nearly every ecosystem and range in size from small bats to the blue whale.

Evolution: Mammals arose from synapsid ancestors in the Mesozoic and diversified after the extinction of the dinosaurs, acquiring fur, lactation, and adaptations for endothermy and complex brain development.

Taxonomy: The class is traditionally divided into Prototheria (monotremes) and Theria (metatheria and eutheria), with placental mammals comprising the vast majority of species.