Using fossils to identify strata in the geologic record has played a key role in dating and mapping geologic formations for over a hundred years. Early geologists used this method to correlate strata between widely scattered areas where continuous exposure of stratigraphy was lacking. Even before the ages of stratigraphy could be determined, geologists used fossils to say that one layer of strata was relatively older or newer than another.
The Correlation of Fossils and Strata
Many early geologists pioneered the use of fossils to map geologic formations. In 1773, English naturalist John Wood noticed that there were a "great number of shells and other productions of the sea" contained in strata on both the European mainland and Britain. This led him to the conclusion that the chalk formations that contained these fossils were both part of the same stratigraphic formation that possibly extended across the English Channel. Other developments in geologic mapping soon followed.
- In 1776, William Smith, a surveyor in England, began to create the first high-quality geologic map. He noted, "the wonderful order and regularity with which nature has disposed of these singular productions (fossils) and assigned to each its class and its particular Stratum."
- In 1815, Smith completed this map and its accompanying table of strata, considered then to be a cartographic masterpiece and a new way of mapping the world.
- At the same time, two scientists in France, Georges Cuvier and Alexander Brongniart, also created a geologic map showing the distribution of rock formations and correlating them using fossils. Cuvier wrote, "These fossils are generally the same in corresponding beds, and present tolerably marked differences of species from one group of beds to another. It is a method of recognition which up to the present has never deceived us."
After both of these discoveries were made independently, it was hard for the scientific community to dispute the usefulness of fossils in identifying, correlating and mapping stratigraphic beds through out Europe.
Index Fossils
The principle of fossil correlation states that like assemblages of fossils are the same age, and that the strata containing these fossils are also the same age. Adding to this, the oldest fossils are in the oldest strata, which are lower in the stratigraphic record, while the youngest strata and fossils are near the surface. Geologists also began to rely on certain fossils, called index fossils, to help them identify strata. Index fossils were used as an index for a particular layer or group of strata. For a fossil to be considered an index fossil it must be easily recognized, widespread in occurrence and unique to a layer or group of strata.
Some common index fossils that geologists use to correlate strata are:
- Pecten gibbus, found in strata deposited during the Quaternary Period of the Cenozoic Era (1.8 million years ago). These fossils look very much like today's sea shells.
- Perisphinctes tiziani, found in strata deposited during the Jurassic Period of the Mesozoic Era (145 to 200 million years ago). These fossils look very much like today's snail shells.
- Paradoxides pinus, found in strata deposited during the Cambrian Period of the Paleozoic Era (540 million years ago). These fossils are more commonly referred to as trilobites and look like an ancient insect. They are among the oldest fossils on the planet and one of the earliest animals.
All of these species thrived in aquatic settings and are found in sedimentary rock such as shale, limestone and sandstone – formations associated with marine environments – covering large regional areas. Trilobite fossils can be found in Cambrian shale formations throughout most of the western United States, for example, the Bright Angel Formation of the Grand Canyon and Lake Mead National Parks.
Source:
Dott, Robert H. and Prothero, Donald R. Evolution of the Earth. NY: McGraw-Hill, 1994.
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Alexandra Matiella Novak - With a PhD in Geology and expertise in science education, Alexandra is passionate about increasing the public's Earth science literacy.
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