Final answer:
The claim that mammals only appeared in the early Cenozoic after dinosaur extinction is false; mammals evolved roughly 200 million years ago in the Triassic period and began diversifying in the Mesozoic era.
Step-by-step explanation:
The statement that the first mammals are known from the early Cenozoic era and did not evolve until after most dinosaurs went extinct is false. Mammals evolved from therapsids around 200 million years ago, during the late Triassic period, which is much earlier than the Cenozoic era that began 65 million years ago. Contrary to the traditional view, the earliest known mammal fossils are from the early Jurassic period, 205 million years ago. One example of transitional mammals is the morganucodonts, which had characteristics of both reptilian and mammalian jaws. Importantly, mammals began to diversify during the Mesozoic era, which includes the Jurassic to the Cretaceous periods, and they coexisted with dinosaurs long before the mass extinction event that ended the Cretaceous and therefore the Mesozoic era.