What is an Index Fossil?

Trilobite Index FossilWhen the allure of archeology calls your name, one of the most important ways that we learn about the past is from the many different fossils that have been uncovered over the years. While carbon matter and other clues are used to date some of the fossils found, a common tool that helps to provide useful information are index fossils. These are commonly found indicators that are distributed throughout the geological timeline, but are also limited to a specific time span.


When a fossil is found in a certain layer of sedimentary, researchers use these index fossils to assist in the dating of the fossil. This opens doors to further identifying fossils that are not associated with a specific era. Let’s say that you uncover two different species within a geological layer. One species of fossil may come from an era that you are familiar with, while the other has a history that is unknown. By using the fossil of the species you definitely have a date attached to, you can assume that the unknown fossil may have also originated during the same era.


When dealing with index fossils, you may encounter an array of common examples. During the Mesozoic Era, which is observed 245 to 65 million years ago, ammonites thrived. These fossils do not appear after the Cretaceous period; therefore it is assumed that they became extinct during the K-T extinction, which occurred 65 million years ago.


During the Cambrian Period, which covered the time period of 540 to 500 million years ago, the main index fossil was the brachiopod. The creature could best be described as a mollusk-like critter that lived in the water. This type of mollusk actually still survives today. Another index fossil connected to the Cambrian Period includes Graptolites, which are often described as a “widespread colonial marine hemichordate.” These fossils are calculated to have thrived from about 540 to 505 million years ago. These fossils also appear within the early to mid-Carboniferous period, which signifies 360 to 320 million years ago.


Nanofossils are fossils that must be seen through a microscope. These fossils present the remains of such critters like calcareous nannoplankton, and coccolithophores. Throughout the geological record, these types of fossils are rather abundant. They are distributed over wide stretches of land. They are very time-specific and showcase a high rate of evolution. Some of the more useful kinds of nanofossils are called radiolarians and foraminifera. This is one of the most common ways to date specimens found within marine terrain.


One of the most commonly known index fossil names that you may encounter is called a trilobite. This is an index fossil that is associated with the Paleozoic Era, which can be traced back 540 to 245 million years ago. About half of the index fossils found during the Paleozoic Era are trilobites. This particular specimen has evolved from the beginning of the Paleozoic Era, which became extinct by the late years of the Permian Period, which is dated 248 million years ago.