What are the Major Groups of Mammals?
Mammals are a diverse group which has dominated terrestrial ecosystems for 65 million years, since the previous rulers, the dinosaurs, were obliterated by an incoming meteor, leaving only avians as descendants. When mammals evolved depends on what you consider a mammal. The earliest mammal-like animals, mammaliaformes, evolved during the Late Triassic, about 210 million years ago. The first true mammals evolved right at the start of the Jurassic, about 195 million years ago. Hadrocodium wui, a shrewlike animal the size of a paper clip, had the first truly mammalian jaw. It scurried around in the shadows of the dinosaurs, consuming insects. For a long time, mammals were not very diverse, all resembling small shrews. About 125 million years ago, mammals diversified, leading to the three major groups of today: monotremes (Prototheria, including the platypus, echnidna, and extinct relatives), marsupials (Metatheria, including kangaroos, koalas, and many obscure living and extinct forms), a
*Mammals obtain energy by eating plants and/or animals *The first mammals, which appeared 200 millions years ago, evolved from a now-extinct group of reptiles *Differences in ways mammals reproduce provide a means of classifying them into three groups: egg-laying mammals, pouched mammals and placental mammals *Body structures of mammals vary according to their life functions *Mammals respond to and interact with their environment in ways that help them gather food, reproduce and protect themselves. *Mammals are all chordates even though they have different structures. *The various life functions of mammals serve to maintain a stable internal and external environment.
