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Why were graptolites once a source of errors in dating Devonian rocks?

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Devonian paleogeography — Devonian Period Distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during the early Devonian Period. Included in the paleogeographic reconstruction are the locations of the interval's subduction zones.

The premise that graptolites were a source of error in dating Devonian rocks stems from a historical misconception regarding their extinction. For a significant period, geologists incorrectly believed that graptolites became extinct at the Silurian-Devonian boundary.1

Because of this error, researchers often miscorrelated rock units in regions—particularly across mainland Europe and Asia—where these invertebrates actually persisted into the Early Devonian, specifically up to the Emsian Stage. This misunderstanding led to serious inaccuracies in establishing the Silurian-Devonian boundary.1

Modern geochronology has corrected this by recognizing that graptolites range into the Early Devonian. Today, the base of the Devonian is precisely defined by the base of the Monograptus uniformis graptolite zone.1