Definition

Noun: A fossil of a species that was geographically widespread, existed for a relatively short period of geologic time, and is easily identifiable. Its presence in a rock layer is used to identify and date that specific geologic period or stratum.

Usage

An index fossil is used as a diagnostic tool in biostratigraphy and geology. Its primary function is to provide a relative date for the rock layer in which it is found, based on the known time range of the organism's existence.

Examples

• serves as a classic index fossil for the base of the Jurassic Period.*

Advanced Usage

• "To use as an index fossil": To employ a particular fossil species for stratigraphic correlation and dating.
  • This graptolite species is widely used as an index fossil for Ordovician shales.
• "Index fossil assemblage": A group of index fossils found together, providing stronger evidence for dating.
  • The rock was dated using an index fossil assemblage characteristic of the late Devonian.

Variants and Related Words

• Guide fossil: A synonym for index fossil.
• Zone fossil: A fossil used to define a specific biozone in stratigraphy.
• Biostratigraphy: (n) The branch of stratigraphy that uses fossil organisms to date and correlate rock strata.

Synonyms

• Guide fossil
• Key fossil
• Diagnostic fossil
• Zone fossil

Related Phrases

• Stratigraphic marker: A broader term for any feature (including fossils, mineral layers, or magnetic signatures) used to identify a specific layer in a sequence of rocks.
• Fossil range: The total geologic time period during which a particular fossil species existed.

Noun

1. a fossil known to have lived in a particular geologic age that can be used to date the rock layer in which it is found