Crepicephalus, genus of trilobites (extinct arthropods) useful as an index fossil for Upper Cambrian rocks in North America (dating 512 to 505 million years ago); it is a relatively common fossil and occurs over a wide geographic range but within a relatively narrow time span. Crepicephalus is recognized by the prominent spines at the lateral margins of the tail section. Smaller spines occur at the margins of the head shield.