Index fossils are used to identify geologic periods or faunal stages in rock layers. These fossils must have a wide geographic distribution and manifest obvious evolutionary trends to help geologists and others who need this geological data to better understand the time scale and to pinpoint a time period. For example, ammonites were common during the Mesozoic Era, but they were extinct by the Cretaceous period. Geologists would use ammonites to help determine this time frame if present.
