How old are fossils?
Written for the KidsKnowIt Network by: Brandon Guymon
How do paleontologists figure out a fossil’s age?
It can be difficult to determine the age of fossils (date the fossils). Because fossil bones aren’t made from the right kind of rocks, you can’t really run scientific tests on fossils to determine their age. Scientists can use scientific tests to determine the age of rocks near the fossils. The types of tests are called radiometric dating. There are certain types of elements that are radioactive. Radioactive elements fall apart overtime and slowly change into another element that is not radioactive. We know how fast radioactive elements fall apart. We know what radioactive elements turn into after they have fallen apart. We can compare the amount of radioactive elements in a rock to the amount of specific non-radioactive elements in a rock, do some math and determine how old the rock is. For example, uranium falls apart into lead. So, if we find a rock that has uranium in it, we can compare it to the amount of lead in the rock to find out how old the rock is. If you have the right kind of rocks, this method is very accurate. The trouble with using this method to date fossils is: Radioactive elements are only found in ingenious rocks, and you can’t find fossils in igneous rocks.
So, if you can’t date the fossil directly with scientific tests, how do you date the fossil? You have to use what scientists call relative dating. The relative dating method most commonly used by paleontologists and geologists is called stratigraphy. Stratigraphic dating works like this. Rocks are formed in horizontal (flat, not up and down) beds or layers. These layers are called strata. The oldest layers are on the bottom and the youngest layers are on the top. So, the oldest fossils are on the bottom layer and the newest fossils are on the top layers. If you find a layer of the right kind of igneous rocks you can use the exact dating method to determine and exact date of that layer.
Paleontologists have found certain fossils that are different from all other fossils. These organisms lived for a relatively short amount of time, and they know when these animals lived. Paleontologists call these fossils index fossils. Index fossils can be used to determine approximately how old an unknown fossil is. For example, if you find an unknown fossil in the same layer of rock as one of the index fossils, you know your fossil is the same age as the index fossil.
Geologists and paleontologists have used a combination of dating techniques, which are radiometric dating, stratigraphic dating, and index fossils, to determine the approximate age of rocks all over the world. Once you know the approximate age of rocks, you can determine the approximate age of the fossil.