Purpose

In this lab you will continue to hone your fossil identification skills. You will also learn how fossils are used by geologists to determine the relative ages of rocks.

One of the first questions geologists ask when they see a rock is "How old is it?" Fossils often play a key role in answering that question. You already know that fossils were the foundation upon which the pioneers of geology built the geologic time scale. Recall that the geologic range of a fossil refers to the span of geologic time between the organism’s first appearance in the fossil record and its extinction. Most organisms’ geologic ranges are relatively long, covering multiple geologic periods. An individual fossil may not be very helpful in determining the relative age of the rock.

In order to get around this problem, geologists use whole groups of fossils found within the same rock unit. The geologic ranges of the various species may vary greatly, but all of the members of the fossil assemblage are assumed to have been living during the same period of geologic time. This is a reasonable assumption. If all the fossils were collected from the same rock unit, representing the same interval of sedimentary deposition, then all of the organisms were buried during that interval. By determining the period(s) during which the geologic ranges of the fossils overlap, one can ascertain when the sediments containing the fossils were deposited.

An example of this technique is provided in the figure below. The rock unit in question was deposited during the Permian Period, since that is the only period during which the members of the fossil assemblage coexisted.

For more information about Fossils in Ohio Visit the Ohio Department of Natural Resources Geological Survey site: http://www.dnr.state.oh.us/