The Precambrian eon is separated from the Phanerozoic eon in the fossil record because of

The Precambrian eon is separated from the Phanerozoic eon in the fossil record because of poorer record of fossils than that of the succeeding Phanerozoic eon. This is because many Precambrian rocks have been heavily metamorphosed, obscuring their origins, while others have been destroyed by erosion, or remain deeply buried beneath Phanerozoic strata. The Phanerozoic Eon covers 541 years to the present while Precambrian Eon covers 4.6 billion years.