To determine how old a fossil is, paleontologists use two basic methods. The first is called relative dating, and is the subject of this article. The second technique is called radiometric dating or radioisotope dating, and will be the subject of my next article.
By relative dating, we mean that we determine the age of a fossil by looking at its relative position in the stratigraphic record. This technique is often called stratigraphic dating. To use relative dating, we look at the sediments above and below a fossil; this allows us to say that an animal lived after one species (found in the rock below it), and before another species (found in the rock above it).
Imagine that every day you set aside the day's newspaper in a stack. If someone were to look at the newspapers, the newspaper paleontologist would soon see that the earlier newspapers were on the bottom of the stack, while the more recent ones were on the top.
Now, imagine that you gave your April 21st newspaper to your neighbour, that newspaper would not be in your stack, not because April 21st did not exist, but because it simply was not preserved. This often happens in the fossil record. For instance, the first known bats are found in the fossil record fully formed and looking just like modern bats. We know that bats did not just magically appear one day, we just do not find earlier bats in the fossil record either because they were not preserved, or we just have not been looking in the right places.
Next, imagine that the newspaper paleontologist finds a shopping list between the April 15th and April 16th newspapers. This tells our newspaper paleontologist that the list was placed in the newspapers after the 15th and before the 16th. The newspaper paleontologist cannot say for certain that the list was actually written on the 15th or 16th, but merely that it was placed on top of the April 15th newspaper before the April 16th issue was placed on top of it. It's possible that you wrote the shopping list on April 10th, did your shopping on the 11th, found the list in your pocket on the 14th, and placed it on the newspaper stack on the 15th.
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