The Phanerozoic Eon is divided into the three eras. The oldest is the Paleozoic Era, which means ‘ancient life’ and occurred between 541 and 252 million years ago.

They particularly looked at the Paleozoic Era, roughly 540 to 420 million ... to the transition from the Proterozoic to the ...

The Proterozoic eon was the gathering up of oxygen in Earth's atmosphere, and the Phanerozoic eon coincides with the major diversification of animals, plants, and fungi.