Final answer:
Mammals first appeared in the Late Permian period through Cynodonts, with true diversification occurring from the Jurassic to the Cretaceous periods.
Explanation:
Mammals first appeared in the Late Permian period through their ancestor Cynodonts. The early Jurassic period saw the earliest known mammal fossils, marking the beginning of mammal evolution. However, mammals truly diversified from the Jurassic to the Cretaceous periods, with a subsequent radiation in the Cenozoic era.
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