It is True.
Cenozoic animals are those which have existed in the past 65.5 million years, since the obliteration of the dinosaurs by a massive asteroid strike called the K-T extinction event.
During the latter part of the Cenozoic Era, from about 50 million years ago to the end of the last Ice Age, prehistoric mammals were significantly bigger (and stranger) than their modern counterparts.
After the K-T extinction, the primary land animals remaining were small reptiles and rodent-like mammals. These rodent-like mammals quickly diversified and came to occupy all major niches, left empty by the departure of the dinosaurs.
With a fast metabolism, close care for their young, and a relatively large brain, mammals were well-positioned to take control of terrestrial ecosystems and become the dominant Cenozoic animals.
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