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 has remained fixed at the base of the Cambrian—currently placed at 542 Ma.