Mammals: Origins, Diversification, and Evolutionary Trends

Mammals are unique among vertebrates in that they give birth to live young and produce milk to nourish them.

Morganucodon, a small shrew-like creature that lived during the Late Triassic period, is considered to be the earliest known mammal.

Monotremes, such as the platypus and echidna, are unique among mammals in that they lay eggs.

Marsupials, such as kangaroos and koalas, are characterized by having a pouch in which they carry their developing young, giving birth to live young, and producing milk.

Placentals, the largest group of mammals, are characterized by giving birth to live young, producing milk, and having a placenta, which provides nourishment to the developing fetus.

Rodentia, the order of rodents, is the largest order of mammals, with over 2,000 species, including rats, mice, squirrels, and beavers.

Elephants are not members of the order Carnivora. They belong to the order Proboscidea.

Rhinoceroses are not members of the order Artiodactyla. They belong to the order Perissodactyla.

Seals are not members of the order Cetacea. They belong to the order Carnivora.

Colugos are not members of the order Chiroptera. They belong to the order Dermoptera.

Hippopotamuses are not members of the order Sirenia. They belong to the order Artiodactyla.

Rhinoceroses are not members of the order Proboscidea. They belong to the order Perissodactyla.