Fossils (or groups of fossils) which are used by geologists and paleontologists to help correlate different events or processes in different parts of the world which happened at the same time. Species which make good index fossils are able to tolerate a wide range of ecological conditions, are geographically widespread, common, easy to find, and evolutionarily short-lived (that is, they speciated and then quickly became extinct). In general, they are also planktonic and very small.