Index fossil

An index fossil, also called a guide fossil or zone fossil, is the remains of a species that is especially useful for telling the age of the rock that holds it. A good index fossil comes from an organism that existed for only a brief span of geologic time, lived across a wide geographic area, was abundant, and is easy to recognize. Many are hard-shelled marine invertebrates or microscopic plankton, because these were common, spread widely through the oceans and fossilize well.

Index fossils rest on the law of fossil succession: life has changed through time, so each interval of Earth history has its own distinctive set of species. If rocks in two distant places contain the same short-lived species, they must have formed at about the same time. This lets geologists match layers across regions and even continents, a practice called biostratigraphy. Index fossils give relative ages, telling which rocks are older or younger, and radiometric dates from nearby volcanic layers convert these into ages in years.

Trilobites mark different parts of the Cambrian and later Paleozoic periods, ammonites divide the Jurassic and Cretaceous into fine zones, and tiny shelled foraminifera help oil and gas geologists identify rocks in samples brought up from boreholes.