The Age of the MammalsArticle | Updated 7 years ago Mammals began to thrive after the end of the Cretaceous when all dinosaurs, except toothless birds, became extinct. Mammals evolved before the Cretaceous and during this period most were very small. This is probably because they were hunted intensely once they reached a certain size, most likely by small theropod dinosaurs like Velociraptor. Mammals living in the Cretaceous belonged to the several extinct groups, monotreme (egg-laying) and marsupial (pouched) groups, and possibly the placental group. After the Cretaceous, mammals increased rapidly in size and took over territory vacated by dinosaurs. Mammal history Mammal-like reptiles had been dominant in the Permian Period (299–252 million years ago). Then during the Triassic, 230 million years ago, dinosaurs appeared and gradually replaced these mammal-like reptiles. However one group of mammal-like reptiles, called cynodonts, survived. They became smaller, probably to avoid attention from dinosaurs, and eventually gave rise to early mammals. Rapid growthThis large bear-like but herbivorous mammal, called Titanoides, lived 60 million years ago, only a few million years after the extinction of non-avian dinosaurs. IMAGE: ROMAN UCHYTEL ‹ End of an Era View the discussion thread.