At the end of the Cretaceous period 65 MYBP [Million Years Before Present], only two lineages of Eutherian mammals were present: insectivorous Protoeutheria and hoofed Condylarthra. [Noneutherian Marsupialia, Monotremata, and Multituberculata, a now extinct group resembling rodents, were also present]. Following the disappearance of dinosaurs at the K/T [Cretaceous / Tertiary boundary], there was a rapid evolution of new mammalian types. By the middle of the Eocene epoch (45 MYBP), most of the twenty or so present-day mammalian orders are identifiable, including forms as diverse as Chiroptera [bats] descended from Protoeutheria and Cetacea [whales] descended from Condylarthra. This rapid evolution of morphological and taxonomic diversity is an example of an adaptive radiation.
Figure after © 1966 by AS Romer; Text material © 2005 by Steven M. Carr