There are many ways for a paleontologist to guess how old a dinosaur bone is. One way is called stratigraphy (stra-TIG-raph-ee). This means that we study how deeply a fossil is buried underground. Layers of rock are formed over millions of years. New layers are formed on top of old layers. So one way to guess a bone’s age is to see which layer of rock it was in when it was found. The deeper the rock, the older the fossil. This isn’t always very easy. The earth is moving all the time. Old layers of rock are pushed to the surface, and new layers are formed when volcanoes erupt.
Another way to find out a fossil’s age is called radio isotope dating. This is the best way to find out how old a fossil is, but the fossil must be made out of certain kinds of rocks. Rocks like uranium-235 decay very slowly over time. This means that over many, many years, uranium-235 changes into another kind of rock called lead-207. It changes very slowly, at the same speed, no matter how old it is. The rate at which it changes is called its half-life. For example, uranium-235 has a half-life of over 700 million years. That means that it takes 700 million years for uranium-235 to turn into lead-207. So, if paleontologists find a fossil, the first thing they will do is see whether there is any uranium-235 in it. They will look at it very carefully to see whether it is turning into lead-207. If it is half-way changed, or mostly changed, or only starting to change, this will give them a good idea of how old the fossil is.
Many times paleontologists will never know exactly how old a fossil is. Usually they guess its range or span of time. A good way of guessing the range is to look at the layers of rock that were formed by volcanoes. It’s easier to guess the age of volcanic rock because it’s brand new rock from the moment that it is spit onto the earth’s surface. When a volcano spits out lava, it creates new rock. New rocks have almost no argon gas in them. As the rock gets older, the argon gas is released. The more argon gas a rock has, the older it is. Imagine that a volcano erupted 80 million years ago and spread a layer of volcanic rock over the earth. Then imagine that a Triceratops lived and died right on top of this layer. It was buried in the ground and became a fossil. And then, 10 million years later, another volcano blew a layer of rock on top of the buried skeleton. If the first layer of volcanic rock was 80 million years old and the second layer was 70 million years old, paleontologists can guess that the Triceratops skeleton was between 70 and 80 million years old!
Finally, paleontologists can use what they call index fossils. These are fossils belonging to creatures that lived during a known time period, and they are found next to other fossils. Let’s say that we find the fossilized remains of a jellyfish. Since jellyfish are one of the first kinds of life on earth and are still alive today, it will be very difficult to guess how old it is if we found it all by itself! But let’s say that we find this jellyfish next to a fossil of a Trilobite. We know that Trilobites lived during the Paleozoic era, between 2,500 and 250 million years ago. So that means that the jellyfish would have lived at the same time. Because we are using the Trilobite fossil to guess the age of the other fossil, the Trilobite is our index fossil!