Definition Of Index Fossils

Index fossils are the remains of animals and plants preserved within the Earth’s rock layers that date back to a certain geologic time. These fossils are very easy to identify, and they are used as a standard or guide of their short time period due to their abundant numbers and geographic distribution.

Identification states that “index fossil” is a noun, and this term is used in both geology and paleontology.


According to All About Creation, index fossils help to define the limits of the time scale in relation to the strata that it was discovered within. According to the U.S. Geological Survey (USGS), this means that they are used as guides to estimate the age of the rocks that they are discovered within.

Fun Fact

Once the rocks are dated by the index fossils, these rocks are then used to date the fossils.


Index fossils are used to date geological formations and give insight into the Earth’s environment and life forms of its time.

Time Periods

Index fossils are referenced against the Standard Geologic Column, compiled in 1830 by Sir Charles Lyell. According to the U.S. Geological Survey (USGS), it divides the four time eras–Precambrian, Paleozoic, Mesozoic and Cenozoic–into periods. From the earliest to the latest, the periods within which index fossils are classified are: Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian, Permian, Triassic, Jurassic, Tertiary and Quaternary.

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