Cryptozoic Eon

The Cryptozoic Eon is not a formally recognized term in geochronology. The term "Cryptozoic" was used historically to refer to the vast span of geologic time before the Phanerozoic Eon, which is characterized by abundant, easily visible fossil records. This earlier period, now divided into the Archean Eon and the Proterozoic Eon, spans from Earth's formation about 4 billion years ago until the beginning of the Phanerozoic Eon, approximately 541 million years ago. During this immense stretch of time, life originated and evolved, but its forms were simple and did not leave extensive fossil evidence. The Phanerozoic Eon, in contrast, is defined by the proliferation of complex life forms with visible fossil records.123