Proterozoic is called the "Age of Stromatolites" (Abundant in Proterozoic shallow seas)

Certain minerals oxidize, or rust, in the presence of free oxygen. Proterozoic red beds are sedimentary rocks deposits that contain oxidized iron. They are the evidence that there was free oxygen in the atmosphere during the Proterozoic.

Hadean, Archeozoic, Proterozoic, Phanerozoic

a shell

The Proterozoic is a geological eon representing the time just before the proliferation of complex life on Earth. Most life forms at the time were single celled organisms.