Per recent articles in the journal Science, “mammals have the greatest degree of morphological variation among vertebrate classes.” That variation includes tiny mice, enormous elephants, sea-dwelling whales, and bats flittering through the air. But, per the fossil record, that amazing variation boomed during a relatively brief period long after mammals first entered the scene and has since faded.
Mammals appeared alongside dinosaurs early in the Mesozoic Era as rat-like critters. They then boomed in variation as the Mesozoic came to a close, dinosaurs died out, and a vast ecological landscape presented itself. At that time, the rate of evolutionary change and diversification (or “adaptive radiation”) boomed for mammals.