Paleontology - The Origin of Tetrapods

Paleontologists look for places in the world that have rocks of the right age to answer whatever question interests them. If you're interested in the origin of tetrapods, what I am, you're looking in the Devonian. The first fish appear in the fossil record before the first limbed animals. First reptiles appear in the Fossil Record before the first mammals. You see these things after this time, but you don't see them before so you know they're somewhere in there.