Index Fossils: How Fossils Help Date Rock Layers

Rocks do not come labeled with their age, but some of the fossils inside them work almost like date stamps. Certain species lived for a geologically short time, spread widely, and left plenty of remains. When geologists find one of these index fossils, they can tell which slice of geological time a rock layer represents, even on different continents.

The Principle of Faunal Succession

In the early 1800s, the English surveyor William Smith noticed that rock layers across Britain contained fossils in a consistent order. The same groups of fossils always appeared above or below one another, no matter where he looked. This observation became the principle of faunal succession: fossil species appear, persist for a time, and disappear in a definite, recognizable sequence.

Combined with the principle of superposition, which states that in undisturbed layers the oldest rocks are at the bottom, faunal succession allowed geologists to build the geological time scale long before scientists had any way to measure absolute ages.

What Makes a Good Index Fossil

An organism makes a useful index fossil when it meets four conditions:

A short time range. The species existed for a geologically brief period, so finding it narrows down the age of the rock.
A wide geographic range. It lived across large areas, ideally worldwide, so layers can be matched between distant regions.
Abundance. It was common enough to be found regularly.
Easy identification. Its features are distinctive enough to recognize reliably.

Free-swimming or floating marine organisms often meet these conditions well, because ocean currents spread them widely.

Classic Index Fossils

Ammonites evolved rapidly through the Mesozoic Era, and their changing shell shapes and suture patterns divide Jurassic and Cretaceous rocks into fine time zones.
Trilobites are widely used for dating Cambrian and Ordovician rocks.
Graptolites, colonial marine animals that left markings like pencil lines on shale, are key index fossils for the Ordovician and Silurian periods.
Foraminifera, single-celled organisms with tiny shells, are used extensively in oil exploration and in dating younger marine sediments.
Conodonts, tiny tooth-like fossils from extinct eel-like animals, are important for Paleozoic and Triassic rocks.

Several of these are popular with collectors too. You can read more about them in Common Fossils Collectors Find.

Relative Dating Versus Absolute Dating

Index fossils provide relative dates. They tell you that one layer is older or younger than another, or that two distant layers are about the same age, but not how many years old they are.

Absolute dates come mostly from radiometric dating, which measures the steady decay of radioactive elements in certain minerals. This method works best on igneous rocks, especially volcanic ash layers. When an ash bed lies between fossil-bearing sedimentary layers, its radiometric age brackets the age of the fossils above and below it. Over many decades, combining these two approaches has produced the detailed geological time scale used today.

Biostratigraphy in Practice

The use of fossils to correlate and date rock layers is called biostratigraphy. Geologists divide time into biozones, each defined by the presence of particular fossil species. It has practical uses well beyond research, including mapping rock layers, locating groundwater, and exploring for oil, gas, and coal. Microfossils are especially valuable here because thousands can be recovered from a small sample of drill core.

What It Means for Your Collection

If a fossil in your collection has a known locality, its rock formation and age can often be looked up, which adds context and value to the specimen. Recording that locality information is one of the most important habits a collector can develop, as explained in Caring for a Mineral Collection. For the basics of how the fossils formed in the first place, see How Fossils Form.