History of the Earth
Classically, the boundary between the Proterozoic and the Paleozoic was set at the base of the Cambrian period when the first fossils of animals known as trilobites appeared. In the second half of the 20th century, a number of fossil forms have been found in Proterozoic rocks, but the boundary of the Proterozoic is still fixed at the base of the Cambrian: that is 542 mya.