EVIDENCE #18 Index Fossils When they try to date a layer of rock, they look for certain fossils in the rock. They have a list of fossils called "index fossils". They claim that if they find a certain index fossil in the rock, they know how old the rock must be because they supposedly know when those animals went extinct. But what happens when they find those animals still alive today?! It proves that they didn't go extinct and that their use of index fossils to date rocks is invalid. The whole t