Index fossils - Paleontology and Geology Glossary

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INDEX FOSSILS

Index fossils are commonly found, widely distributed fossils that are limited in time span. They help in dating other fossils found in the same sedimentary layer. For example, if you find a fossil from an unknown era near a fossil from a known time, you can assume that the two species were from about the same time.

Examples of index fossils include:

Ammonites were common during the Mesozoic Era (245 to 65 mya), They were not found after the Cretaceous period, as they went extinct during the K-T extinction (65 mya).
Brachiopods (mollusk-like marine animals) appeared during the Cambrian (540 to 500 mya); some examples still survive.
Graptolites (widespread colonial marine hemichordates) that lived from the Cambrian period (roughly 540 to 505 million years ago) to the early to mid-Carboniferous (360 to 320 million years ago).
Nanofossils are microscopic fossils (the remains of calcareous nannoplankton, coccolithophores) from various eras. Nanofossils are very abundant, widely distributed geographically, and time-specific, because of their high evolutionary rates. There are enormous numbers of useful nanofossils, including radiolarians and foraminifera. Nanofossils are the primary method of dating marine sediments.
Trilobites were common during the Paleozoic Era (540 to 245 mya); about half of the Paleozoic fossils are trilobites. They evolved at the beginning of the Paleozoic Era and went extinct during the late Permian period (248 million years ago).

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