Paleontologists long have argued that a major evolutionary radiation occurred during the early Cenozoic, but not that all mammals, or even all eutherians, originated from a single common ancestor at that time. Nonetheless, several recent molecular analyses claim to show that because several interordinal splits occurred during the Cretaceous, a radiation of therian mammals was then underway.

These claims confuse basal splits with "radiations," employ exaggerated and unreliable molecular clock rates, and ignore the well-sampled late Cretaceous and Cenozoic North American fossil record. Evolutionary radiations only may involve changes through time in the number of species or the distribution of morphological (or other) attributes across these species.

Statistical analyses of paleofaunal data confirm that the number of mammalian species was far lower throughout the late Cretaceous than during any interval of the Cenozoic, and that a massive diversification took place during the early Paleocene, immediately after a major mass extinction. Additional measurement data illustrate similar trends through time in the distribution of body mass, the most ecologically important morphological character of mammals.

Cretaceous mammals were on average small and occupied a narrow range of body sizes; after the Cretaceous-Tertiary mass extinction, there was a rapid shift in the mean that overshadows the entire history of size increases during the rest of the Cenozoic. The fact that there was an early Cenozoic mammalian radiation is a firm statistical inference that is entirely compatible with the existence of a few modern mammal orders during the Cretaceous.