Trilobites fossils help date rocks between the Cambrian and Permian and define the lower boundary of which interval?

• A. Cambrian Period
• B. Permian Period
• C. Ordovician Period
• D. Devonian Period

Answer: (A)

Trilobites are classic index fossils for the early Paleozoic, and their first appearance in the fossil record marks the start of widespread trilobite-bearing marine faunas. This makes them a reliable marker for the base of the Cambrian, since geologists use such distinctive, rapidly evolving and globally distributed organisms to synchronize rocks from different regions. Because trilobites mainly help identify Cambrian rocks, their presence defines the lower boundary of the Cambrian interval. Other period boundaries rely on different fossil groups (like graptolites or conodonts) rather than trilobites, so trilobites are most diagnostic for the Cambrian’s lower boundary.