Cambrian
From the Simple English Wikipedia, the free encyclopedia that anyone can change
The Cambrian epoch was the first eon of Earth’s history, from the first coalesence of the planet, about 4500 Mya, to about 542 Mya, is referred to as the Precambrian. Animals with hard-shells appeared in great numbers for the first time during the Cambrian. The continents were flooded by shallow seas.