Index fossil — Science definition

The remains of a species that lived for a relatively short span of geologic time but spread across a wide area, making it useful for matching the ages of rock layers on different continents.

Where it is taught

In use

An index fossil comes from a species that lived over a huge geographic area but only for a short slice of geologic time.

Fossils & Relative Dating

Questions using index fossil

Problem 1

Explain why an index fossil needs to have lived over a wide geographic area but only for a short span of geologic time.
Answer and explanation

A wide range means the same species could get fossilized in many different places, so geologists can find it in rock layers far apart; a short time span means finding that fossil narrows down the age of the layer very precisely, since the species wasn't around for very long.

Problem 2

Species X was widespread for 1 million years; Y was local for 100 million years. Which is a better potential index fossil? Explain the reasoning in your own words.
Answer and explanation

X is more useful, provided it is identifiable and occurs in the sampled layers.

A short time range narrows the possible age.

Wide distribution permits correlation across many sites.

Response criteria

  • Use the stated conditions.
  • Show the evidence or calculation.
  • Explain the conclusion and any stated limitation.

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