Home >> Science & Nature >> Rocks & Minerals

Characteristics of Good Index Fossils

An index fossil is formed by a plant or animal that lived during a limited geological timeframe. Its presence in a particular layer of sedimentary rock helps paleontologists and geologists determine the age of that layer and the relative ages of the layers above and below. A good index fossil needs to be abundant, easy to recognize and short-lived, and it needs to have a wide distribution.

Abundant
- Ammonites, brachiopods, graptolites, nanofossils and trilobites make good index fossils. These fossils are found in large numbers in sedimentary rock layers.
Easy to Recognize
- Index fossils have distinctive shapes. For example, ammonites have a ribbed spiral shell, while brachiopods look like clams. You can identify an index fossil just by looking at it.
Short-Lived
- Different species lived at different geological times. This rapid evolution of species lets scientists more precisely date different rock layers. For example, if you find an ammonite from a species known to be from the Triassic period, then the rock layer it came from must be Triassic.
Wide Distribution
- Marine animals make the best index fossils. Unlike land animals and plants, marine animals are found worldwide.

Rocks & Minerals