For each description, decide whether the organism would be ideal or not ideal as an index fossil: (1) abundant in rock layers, (2) limited to a single area, (3) widespread geographically, (4) lived for hundreds of millions of years, (5) similar in appearance to other fossils.

What you are checking for with an index fossil

Index fossils help match rock layers of the same age across different locations. To do that, the organism has to be easy to find, easy to recognize, and only exist during a short time span.

Index fossil “rules of thumb”

An organism is a good index fossil if it is:

• Abundant, so it shows up in many rock layers.
• Widespread geographically, so you can correlate layers from far-apart places.
• Distinctive, so you do not confuse it with other fossils.
• Short-lived in geologic time, meaning it existed for a relatively narrow time range.

Apply the rules to each description

1. Abundant in rock layers: Ideal, because it is easy to find in many layers.

2. Limited to a single area: Not ideal, because it cannot correlate rock layers across different regions.

3. Widespread geographically: Ideal, because it can link layers from many locations.

4. Lived for hundreds of millions of years: Not ideal, because a long time range makes it poor for pinpointing the age of a layer.

5. Similar in appearance to other fossils: Not ideal, because it is harder to identify correctly and can be mistaken for other species.

Quick memory tip

Good index fossil = wide range in space, narrow range in time, and easy to identify.