Baculites, genus of extinct cephalopods (animals related to the modern squid, octopus, and nautilus) found as fossils in Late Cretaceous marine rocks (formed from 99.6 million to 65.5 million years ago). Baculites, restricted to a narrow time range, is an excellent guide or index fossil for Late Cretaceous time and rocks. The distinctive shell begins with a tightly coiled portion that becomes straight in form, with a complex ammonite sutural pattern.