Published online by Cambridge University Press: 17 July 2017
Trilobites are the most diverse of extinct arthropod groups, being known from several thousand genera; many more are discovered each year. They range in age from near the base of the shell-bearing Cambrian to high in the Permian. Because many trilobites evolved quickly, they have been widely employed in stratigraphy; in the Cambrian they are possibly the most important stratigraphical fossils. This has been a mixed blessing because some experts studying the group have tended to place stratigraphical utility foremost in their taxonomic methods. Stratigraphical boundaries have become taxonomic boundaries. This might not matter for stratigraphy, but it does matter for the other kinds of paleobiological studies which have recently become the center of attention. How, for example, can one study extinction, unless the groups extinguished are natural, monophyletic groups? The extinction of an arbitrary phylogenetic segment at a stratigraphic boundary tells us nothing.