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How do fossils help geologists correlate separated Carboniferous rock outcrops?

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

Geologists correlate separated Carboniferous rock outcrops primarily by using the principle of faunal succession, which establishes that the relative age of fossiliferous rock is indicated by the specific types of fossils it contains. Because each interval of geologic history is associated with a unique fauna, identifying similar fossil assemblages in different locations allows geologists to determine that the strata were formed during the same time period.1

The primary tool for this process is the index fossil, which is a species that was abundant, easily recognizable, and widely distributed, but existed for only a short span of geologic time. When these fossils are found in separated outcrops, they serve as markers of synchroneity.21

In the Carboniferous Period, this method is particularly effective because the era is characterized by diverse plant and animal life, including crinoids, brachiopods, and various vascular plants that left extensive remains in the rock record. By matching these fossil sequences, geologists can link rock units across vast distances even when physical rock properties vary.31