There have been three successive radiations among mammals, (i) Mesozoic radiation (ii) The Cretaceous radiations and (iii) The Tertiary radiations.
(i) The Mesozoic Radiation
During Mesozoic the radiations was comparatively unimportant because of the terrestrial dominance of the dinosaurs and other reptiles During this period the mammals were small, though differentiated with respect to the dentition, some being insectivorous, some carnivorous and some herbivorous possible fruit-or nut-eating.
(ii) Cretaceous Radiation
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During Creatceous Period, the great reptiles like dinosaurs etc. become extinct and the mammals really had their divergence opportunity. These archaic mammals branched along different lines. Some became carnivorous (Creodonts) of various sorts, swift cursorial ungulates (Condylarths) of various sorts, swift cursoiral ungulates (Condylarths), and slow-moving grotesquely armed types (amblypds). There were also sloth-like forms (taeniodonts) and probably ancestral marsupials. Only a few of them survived to Eocene.
(iii) Tertiary Radiations
This was the great Arctogaen radiation of modernized mammals, so called because their descendants still exist. The mammals of today, including man himself, are the remnant of this Tertiary radiation.