Geologic History: Relative Dating

Fossils and Relative Dating

Studying fossils is the best way for us to understand the evolutionary relationships between species. Fossils, have left some trace of itself in rock, such as bones, shells, or footprints even though they lived more than 10,000 years ago. Only a very small fraction of the millions of species on Earth are found in the fossil record. The fossils that we do find are a valuable tool in deciphering the mysteries of geologic history.

Index fossils

Image showing the duration of the ranges of time during which certain fossilized organisms lived. The fossils are arranged in columns and offset up or down. Each fossil image includes an arrow that points up and down and is of a specific length. The arrow for the first fossil, a trilobite is rather short. The arrow for the mollusk fossil is longer than the arrow for the trilobite fossil, but the top of the trilobite arrow and the bottom of the mollusk arrow overlap the same period of time.

The diagram shows range in time during which certain organisms lived from the bottom where they first appear to the top where they become extinct. The trilobite on the far left was relatively short-lived compared to the mollusk-shell organism directly to the right of it. These time spans represent years during which the species was found on Earth not the life expectancy of a single organism. Dinosaurs were on Earth for about 180 million years before they became extinct.

Notice that fossils overlap in the times they lived. This makes it difficult to assign relative ages to rocks. In this case, scientists need a reference point to refer to. That’s where index fossils come in. An index fossil is a fossil of a species that had a relatively short life span. The trilobite on the far left is a better indicator of relative age because it only lived in a definite time period (period B); but the other species spanned many time periods. The best index fossils were short-lived, easy to identify at the species level, and had a broad geographic distribution.

Common Index Fossils

Ammonite An ammonite fossil Ammonites are by far the most useful of index fossils. They were free-swimming marine animals that lived in ancient seas. It is often very easy to link an ammonite fossil in a rock to a given period of time in geologic history.
Trilobites A trilobite fossil These are trilobites, another marine animal that lived in ancient seas. Trilobites are good index fossils because various forms of them were relatively short-lived and they are easily recognized. Trilobites were arthropods and had bodies with three segments. After they first appeared in the fossil record, they rapidly diversified and look a little different at various times in the fossil record. Because of this, you can match the unique characteristics of a trilobite fossil to a known time in history.
Brachiopod A fossil brachiopod This organism is an example of a brachiopod. The particular brachiopod shown here is about 480 million years old. It is also a good index fossil because it is easily recognized by its shape and was short-lived. Like trilobites, brachiopods evolved quickly, so unique forms are found in each major chunk of geologic time, making it easy for us to match brachiopod shape to a time in history.
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Imagine you have found some rocks with fossil in them. One rock has a fossil of the trilobite seen above, and the other rock has a fossil of the mollusk-shell organism directly beside it. Is it possible that the rocks are of the same age? Could you tell for sure which was younger than the other?
Yes, the species overlap in their spans, and so the rocks may be of the same age. The two species could have lived at the same time in geologic history. You would not be able to tell for sure which rock was older or younger than the other from the fossils alone.