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Why do fossils help geologists correlate Phanerozoic rock layers more easily than Precambrian rock layers?

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A detailed geologic time scale chart displaying Earth's history from the Hadean Eon over 4.5 billion years ago to the present, organized by eons, eras, periods, epochs, and ages with color-coded columns and corresponding dates in millions of years. — Precambrian The stratigraphic chart of geologic time.

Phanerozoic rock layers are more easily correlated than Precambrian layers because they contain abundant fossil remains of life-forms that evolved and changed over time. This allows geologists to use the principle of faunal succession, where specific fossil assemblages serve as markers for distinct intervals of geologic history.12

In contrast, Precambrian rocks—which represent more than 87 percent of Earth's history—largely lack these useful fossils. While stromatolites exist in Precambrian sediments and have been used for limited biostratigraphic division, they do not provide the same level of correlation as the diverse, rapidly evolving life-forms found in the Phanerozoic record. Because of this deficiency, geologists must rely primarily on precise isotopic dating, such as zircon dating, to correlate Precambrian rocks across different regions and continents.21