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How did William Lonsdale’s coral fossils help identify the Devonian Period?

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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.

In 1837, the English geologist William Lonsdale analyzed fossil corals found in the limestones of South Devon, England. He observed that these specimens possessed characteristics intermediate between those found in the older Silurian System and the younger Carboniferous System.12

This insight was pivotal because it suggested that the rocks containing these corals represented a distinct, previously unrecognized geological interval. Based on Lonsdale’s findings, geologists Adam Sedgwick and Roderick Impey Murchison concluded that these marine strata were the chronological equivalents of the terrestrial Old Red Sandstone found elsewhere in Britain. This realization provided the foundational evidence for establishing the Devonian Period in 1839.12

While Lonsdale’s work was essential for the initial identification of the system, it is worth noting that modern geological studies have found corals to be less reliable for precise time correlation than other organisms, as they were often highly sensitive to local environmental changes.3