What happened 66 years ago today?
Paleozoic, Mesozoic, and Cenozoic refer to periods in Earth's history. The Paleozoic era began 542 million years ago and ended 251 million years ago. The Mesozoic era is the age of dinosaurs and was from 245 to 66 million years ago. The Cenozoic period began 65 million years ago and continues today.
The first true mammals were insectivores that appeared about 225 million years ago in the Mesozoic Era (age of dinosaurs). The Cretaceous-Paleogene extinction event, about 66 million years ago, killed off 75% of the species of living organisms, and opened niches for mammals to proliferate and diversify.