genus of extinct cephalopods (animals related to the modern squid, octopus, and nautilus) found as fossils in Late Cretaceous marine rocks (formed from 97.5 to 66.4 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.