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Index fossils can provide scientists with specific information. If a fossil lived only during a specific period in history or in certain environments, the scientist can date or characterize when the rock was formed. Index fossils can help determine if types of rock that may appear identical in different locations are parts of the same rock layer or formed during different time periods.
Physical Characteristics
Index fossils that lived in a relatively short time, are extinct or have a distinct evolution line are used for making a time indicator for a rock layer. If the evolutionary process is well documented, the time period can be even more specifically determined.
Geographic Characteristics
Index fossils that lived in more than one location can help scientists to make connections between rock layers found in different locations. The fossils are best if they are fossilized in many different rock types; they are usually types of swimming or floating marine life-forms.
Discovery of Index Fossils
William Smith, a geologist from Britain active in the early 1800s, was the first person to discover index fossils. He realized that rock layer identification was not as reliable when made from rock type. Studying the fossils, he discovered that he was seeing the same fossils in the same order as well as in large geographic areas. His discoveries become known as the principle of faunal and floral succession.
Uses
Scientists can use Smith's principle to detect a sequence of events within rock layers. Fossils can provide a relatively accurate portrayal of the rock formation. For an exact age of a rock, however, a more advanced method of dating such as radiometric dating, where rocks are examined for the exact rate of radioactive decay, must be used.