This page will discuss the evolution and phylogeny of the Eutherians , the Metatherians and the Prototherians . Mammals descended from a group of mammal-like reptiles known as therapsids, which diverged from reptilia perhaps as long as 220 MYA, during the Triassic period. After the demise of the dinosaurs, many niches and adaptive zones became available for mammalian occupancy. There was an explosion in mammalian biodiversity in the early Paleocene, approximately 60 MYA, during which over 100 families emerged. There are a number of distinctive traits that all modern mammals share, found no where else in the Animal Kingdom.
Mammals have keratin-based hair, used to maintain an endothermic metabolism.
They have a powerful muscle, the diaphragm, driving an extremely efficient respiratory system. Mammals also present teeth differentiated to suit the special metabolic needs of each species. In general, mammals have been found to possess large brains for their bodies, and often exhibit complex behaviors. Furthermore, all female mammals exhibit mammary glands used to supply their young with milk, which is rich in fats, sugars, proteins, and vitamins and mierals. Marsupials complete their embryonic development not in eggs, nor in a placenta, but in a specialized pouch found on the maternal abdomen called a marsupium.