Chapter 18 Fossils and Geologic Time
Chapter 18 Fossils and Geologic Time
Chapter 18 Fossils and Geologic Time
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HOW HAS GEOLOGIC TIME BEEN DIVIDED? 423<br />
Other scientists dispute the importance of a meteor impact,<br />
suggesting similar atmospheric changes result from<br />
volcanic eruptions. A third suggestion accounts for global<br />
changes in climate related to plate tectonic motions <strong>and</strong> the<br />
formation of a supercontinent known today as Pangaea.<br />
HOW HAS GEOLOGIC TIME BEEN DIVIDED?<br />
In the late eighteenth <strong>and</strong> early nineteenth centuries, engineers<br />
<strong>and</strong> geologists working in Great Britain <strong>and</strong> Europe noticed<br />
that the fossils contained in rock formations could often<br />
be used to identify them. They also noticed that layers containing<br />
certain other fossils were consistently located above<br />
or below these layers. In fact, they began to identify whole<br />
groups of fossils according to where they were found. For example,<br />
any rocks with the fossils found in Devon, in the south<br />
of Engl<strong>and</strong>, were called Devonian. These rocks were always<br />
located above those with fossils found in certain parts of<br />
Wales <strong>and</strong> below the rocks of the coal mining areas near Bristol.<br />
These observations were used to establish a geologic time<br />
scale of relative ages as shown in Figure <strong>18</strong>-4 on page 424.<br />
Some fossils are more useful than others in establishing<br />
the age of rocks. These are called index fossils. The best index<br />
fossils are easy to recognize, <strong>and</strong> are found over a large geographic<br />
area, but they existed for a brief period of time.<br />
Trilobites are a group of animals with external skeletons<br />
that lived on the bottom of the oceans for nearly 300 million<br />
years. This group of animals existed for a long time. Different<br />
species of trilobites evolved <strong>and</strong> became extinct. Each<br />
species looks distinctly different from the others. Furthermore,<br />
trilobites left good fossils all over the world. Therefore,<br />
geologists find trilobites to be one of the most useful groups<br />
of index fossils. Figure <strong>18</strong>-5 on page 425 illustrates some of<br />
the variety of trilobites used as index fossils.<br />
It is possible that humans also will be good index fossils<br />
at some time in the distant future. Humans have distinct<br />
hard parts, <strong>and</strong> we often bury our dead. We have left signs of
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