Giant ammonite fossil on the Jurassic Coast in Dorset, England.

Ammonites are perhaps the most widely known fossil, possessing the typically ribbed spiral-form shell as pictured.
These creatures lived in the seas between 240 - 65 million years ago, when they became extinct along with the dinosaurs.

The name 'ammonite' (usually lower-case) originates from the Greek Ram-horned god called Ammon. Ammonites belong to a group of predators known as cephalopods, which includes their living relatives the octopus, squid, cuttlefish and nautilus. The largest one ever found was 9 feet across and nearly 225 million years old.