Long before radiometric dating appeared on the scene, the geological time scale was defined by the sedimentary record, and particularly by key fossils preserved within them.  Throughout the Cambrian, and to a lesser extent until the end-Permian extinction about 300 million years later, trilobite fossils served as some of the most useful of these key fossils.  Richard Fortey explains why.  Here he is holding a trilobite from the calymene genus. Richard Fortey is formerly head of arthropod paleontology at the Natural History Museum in London and is visiting professor of paleobiology at Oxford University.  He has devoted much of his research career to the study of trilobites — their systematics, evolution, and modes of life, and has named numerous trilobite species. Podcast illustrations and more about Geology Bites are on geologbites.com.