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Why are trilobites useful fossils for dating Cambrian rock layers?
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Cambrian Period
Distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during the late Cambrian Period. Included in the paleogeographic reconstruction are the locations of the interval's subduction zones.
Trilobites are essential guide fossils for dating Cambrian rock layers primarily because they evolved rapidly, allowing geologists to establish precise biostratigraphic zonation.1
Their utility depends on their specific group:
- Polymerid trilobites: Comprising about 95 percent of all species, these bottom-dwelling organisms are highly effective for regional correlation. However, because they were often endemic to specific continental shelves, they are less useful for intercontinental dating.1
- Agnostid trilobites: These small, pelagic (open-sea) organisms are invaluable for refined intercontinental correlation. Their wide geographic distribution and rapid evolution make them particularly useful for dating the upper half of the Cambrian System.1
While other fossils like archaeocyathan sponges and brachiopods appear in Cambrian strata, their effectiveness is often limited by endemism, meaning they are restricted to specific geographic areas. Consequently, trilobites remain the principal tool for correlating Cambrian strata.1