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The Mesozoic was the geological era in which dinosaurs lived, as well as the first birds and mammals. It lasted about 186 million years, starting 252.2 mya (million years ago) with the P/Tr extinction and ending 66 mya with the K/T extinction (the one that killed dinosaurs).
Dinosaurs appeared around 231 mya (21 million years after the beginning of the Mesozoic). They evolved from early reptiles called the archosaurs (a group of which modern birds and crocodiles are still part).
The Mesozoic is the middle of the three eras that make up the Phanerozoic eon. Before the Mesozoic was the Palaeozoic era. The K/T extinction in 66 mya also marks the beginning of the Cenozoic era, the one we live in.
The three Mesozoic periods were:
- Triassic (252–201 mya)
- Jurassic (201–145 mya)
- Cretaceous (145–66 mya)