At least 57 species of ammonites, which were widespread and belonged to six superfamilies, were extant during the last 500,000 years of the Cretaceous, indicating that ammonites remained highly diverse until the very end of their existence.

Discover the fascinating world of ammonites—prehistoric cephalopods with iconic spiral shells. Learn their biology, evolution, and fossil significance.

Discover the fascinating world of ammonites — extinct marine cephalopods. Learn about their evolution, shell structure, fossilization, and role as key index fossils in geology.

Ammonites were shelled cephalopods that died out about 66 million years ago. Discover when they lived, why they went extinct, why they're important to science, and much more.

Scientists have identified more than 10,000 ammonite species, such as Arnioceras semocostatum pictured here, and use their shells to date other fossils.

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The coiled shell is generally the only part of the ammonite to be preserved as a fossil. As well as being aesthetically pleasing and popular with fossil collectors, they are of particular value to geologists.

Ammonoid, any of a group of extinct cephalopods (of the phylum Mollusca), forms related to the modern pearly nautilus (Nautilus), that are frequently found as fossils in marine rocks dating from the Devonian Period (began 419 million years ago) to the Cretaceous Period (ended 66 million years ago).

Fossils of ammonoid shells have been known for several centuries and in medieval Europe were thought to be petrified coiled snakes with healing or oracular powers — as a result, they were known as snake- or serpent-stones.

Ammonites exist today only as fossils. These shelled cephalopods, which thrived in Earth’s oceans for more than 300 million years, vanished in the mass extinction event that closed the Cretaceous period roughly 66 million years ago.