Why is Precambrian time harder to study using fossils than Phanerozoic time?

Precambrian time is significantly more challenging to study using fossils than the Phanerozoic Eon primarily because Precambrian sediments lack the abundant, diverse, and complex fossil remains that characterize younger rocks.12

In the Phanerozoic, the presence of widespread, rapidly evolving life-forms allows geologists to use the principle of faunal succession to correlate rock layers across different regions. Conversely, Precambrian fossils are either rare or entirely absent in many areas, preventing the use of such biostratigraphic methods for broad correlation. While stromatolites and certain microfossils exist in Precambrian strata, their utility for global correlation is relatively limited compared to the index fossils of the Phanerozoic.12

Furthermore, much of the Precambrian record is missing or has been altered by intense geological processes, such as metamorphism and deformation, which often destroy potential fossil evidence. Because of these limitations, geologists must rely on geochronology—specifically precise isotopic dating techniques like zircon dating—to correlate Precambrian rocks from continent to continent.12