Trilobites are one of the most well-known index fossils.
Index fossils are important to the study of our Earth because they help geologists determine the relative age of rock layers and correlate them between regions. These fossils helped uncover the history of the land masses. The presence of an index fossil can pinpoint the age of rock layers because index fossils represent a distinct time period. Fossils of plants and animals that had the specific characteristics of being abundant and widely distributed around the world over a relatively short time period are good index fossils.
Unique Appearance
Index fossils must have a unique form that is easy to identify without special equipment. Geologists make maps in the field as they gather data and specimens. If the index fossil is easy to identify without the use of a microscope or laboratory equipment, the field work can progress more quickly. For example, the trilobite was a marine arthropod that had a rounded head and segmented body. While a variety of trilobites types existed, the general shape and appearance of the animal was consistent.
Worldwide Distribution
A good index fossil is one that was widely distributed. The benefit of a widely distributed fossil is that it helps scientists to correlate the age of rock layers that are on different continents. The presence of an index fossil indicates that the rock formed during a specific time, regardless of its location. Trilobites, for example, were widely present in the oceans of the early Paleozoic Era.
Abundant Numbers
For a fossil to be a good index fossil, the plant or animal must have been very abundant while it lived. Only a small percentage of plants and animals actually become fossils; therefore, it is important that there were enough living that the fossils occur in abundance. As a scientist examines a rock, it is easier to identify and refine the age of the layer if several index fossils that existed during overlapping timespans are present. Index fossils were often the dominant plants or animals of their era. Trilobites were a dominant sea animal of the Cambrian and Ordovician periods. While some trilobite species survived throughout the Paleozoic Era, they were not present in numbers vast enough to be the dominant fossil of the mid- to late part of the Era.
Short Existence
In geologic time, a "short" time may be several million years. For example, the trilobites that dominated the Cambrian and Ordovician periods mark a time span from 443 to 543 million years ago. A final important aspect of trilobites as index fossils of the Cambrian and Ordovician is that they changed rapidly in identifiable ways. Specific genera and species of trilobites existed for brief time spans of perhaps as little as a million years or less during the Cambrian, for example, a fact that allows geologists to use them to more finely date and correlate the rocks they're found in. Trilobites became less widespread after the end of the Ordovician, but continued to survive and change through the remainder of the Paleozoic.
Tags: index fossil, Cambrian Ordovician, good index, index fossils, million years, plants animals
