Dating Fossils I: Relative Dating
This often happens in paleontology. When we use relative dating on a fossil, only know that the animal died between certain events. We cannot say exactly when it was born, or when the species first appeared on earth. We can only say when it first appeared in the fossil record.
An important tool in relative dating is the use of index fossils. Index fossils are fossils that are commonly found and have a known range in the geologic record. For instance, trilobites are an important index fossil. They first appeared during the Cambrian Period (570 to 500 million years ago) and are found until the Middle Permian Period (about 265 million years ago). They also went through a number of well-documented changes in their outward appearance, allowing paleontologists to more accurately determine the age of the sediments they are found in, and to use relative dating on other fossils found in the same stratigraphic layer.
In my article "Champsosaurs", I mentioned how paleontologists were able to determine that a champsosaur had died in a certain time period because its remains were found directly on top of the basalt layer left by a volcano. Paleontologists were able to use radiometric dating to determine the time when the volcano erupted. They then used relative dating to say that the champsosaur had to have died after the eruption, because its bones were found above the ashbed. In my next article, I will be discussing the different types of radiometric dating, and how it is useful to paleontologists.
Be sure to read Geoff Habiger's article "Relative Dating".
More information about dating fossils.
Relative Dating Activity for the classroom
Also, be sure to read Carl Zimmer's excellent article "How Old is It?", National Geographic, September 2001
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