Ammonites: extinct cephalopods with a genuinely useful dating tool

Ammonites were shelled marine cephalopods related to modern squid and nautilus, and they are recognized by their coiled, chambered shells, ranging from small enough to fit in a palm to several feet across in some species. They lived throughout the Mesozoic era and went extinct at the same event that ended the age of dinosaurs. Because ammonite species evolved and changed relatively quickly and are found across a wide geographic range, they are genuinely useful to geologists as index fossils, meaning finding a specific ammonite species in a rock layer helps date that layer with real precision, one of the more practical scientific uses of any commonly collected fossil.