Conquer the 2026 ESS Regents Challenge – Unleash Your Inner Scholar!

Index fossils are fossils used to define and correlate rock layers of a specific geologic time period because they are widespread and abundant. They come from species that existed for only a relatively short interval and lived in many places, so finding the same fossil in different rocks signals that those rocks formed around the same time. This makes it possible to match, or correlate, layers across distant regions and assign relative ages to them. A fossil found in only one location wouldn’t help with broad correlation, so it wouldn’t serve as a reliable time marker. Fossils tied to meteor impacts or indicating a rock is igneous don’t capture this practical dating and cross-regional correlation purpose.