Metatheria includes two sub-groups: the Ameridelphia Australidelphia Allotheria Prototheria( Ameridelphia, Australidelphia, Allotheria, Prototheria ), or opossums, and the Australidelphia Ameridelphia Prototheria Allotheria( Australidelphia, Ameridelphia, Prototheria, Allotheria ), which include the Tasmanian devil, Tasmanian tiger, bandicoot, and others.

The Mesozoic Paleozoic Cretaceous( Mesozoic, Paleozoic, Cretaceous ), or the "true age of mammals," represents 2/3 1/2 1/3 1/4( 2/3, 1/2, 1/3, 1/4 ) of mammalian history. there are about 300 200 500 400( 300, 200, 500, 400 ) genera known from this period, although most were some form of mouse dog lemur monkey( mouse, dog, lemur, monkey ). Although this has historically been attributed to the fall of dinosaurs, recent evidence suggest that patterns of plant bird bacterial( plant, bird, bacterial ) evolution had a greater influence on mammal diversity,

The end-Cretaceous extinctions also affected mammals: lost marsupials monotremes rodents( marsupials, monotremes, rodents ) and archaic type placentals monotremes cetaceans( placentals, monotremes, cetaceans ). Mammals quickly recovered, but did not really start to diversify again until the Cenozoic Paleozoic Proterozoic( Cenozoic, Paleozoic, Proterozoic ).

Marine mammals in Order Carnivora Order Sirenia and Order Cetariodactyla Order Amphibia( Order Carnivora, Order Sirenia and Order Cetariodactyla, Order Amphibia ) are amphibious, while those in Order Sirenia and Order Cetariodactyla Order Carnivora Order Sirenia and Order Cetacea( Order Sirenia and Order Cetariodactyla, Order Carnivora, Order Sirenia and Order Cetacea ) are wholly marine.