Cryptozoic Eon The prefix crypto- comes from the Greek word for “hidden”, referring to the fact that very few fossil remains are found in Cryptozoic rocks. For reasons that will become apparent in the following discussion, the entire Cryptozoic Eon is commonly referred to as the pre-Cambrian. Some geologists subdivide the Cryptozoic Eon into the Archean and Proterozoic Eras. This arrangement is based primarily on the distinguishing characteristic shown in the box below. Proterozoic Era 2.5 billion to 550 million years ago Distinguishing characteristic: shallow-water marine life forms Archean Era 4.5 to 2.5 billion years ago Distinguishing characteristic: no large continents exist Phanerozoic Eon The prefix phanero- comes from the Greek word meaning “apparent” and refers to the abundant fossil remains found in these rocks. Historically, the dividing line between the Cryptozoic and Phanerozoic eons has been placed at the first appearance of abundant fossil remains about 550M years ago. As a result, the Phanerozoic Eon represents the last 550M years of Earth history. For reasons that will become apparent in the following discussion, the entire Cryptozoic Eon is commonly referred to as the pre-Cambrian.