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Science & Nature • Paleontology

Ammonite

An ammonite is an extinct marine cephalopod, usually referring to members of the ammonoid group known from fossil shells.

By Ethan Anderson| Reviewed by Olivia Bennett, Standards & Fact-Checking Lead| Updated July 26, 2026|2 min read| Fact-checked

An ammonite is an extinct marine cephalopod, usually referring to members of the ammonoid group known from fossil shells. Ammonites were widespread and diverse in ancient oceans, and their fossils are important index fossils for dating rock layers. Most ammonites disappeared at the end of the Cretaceous period, about 66 million years ago.

Classification and biology

Ammonites were molluscs in the class Cephalopoda, related to modern squid, octopus, cuttlefish, and nautilus. They lived as marine predators and had a coiled, chambered shell used for buoyancy.

Evolution and fossil record

Ammonoid cephalopods first appeared in the Devonian, while true ammonites appeared much later, during the Jurassic. Their fossils are abundant and widespread in rocks from former seas across many parts of the world.

Extinction

Ammonites disappeared at the end of the Cretaceous, around the same time as the non-avian dinosaurs. Their extinction is commonly linked to the broader end-Cretaceous mass extinction event.

Scientific importance

Because ammonites evolved rapidly and existed over relatively short geologic intervals, paleontologists use them to correlate and date sedimentary rock layers. This makes them among the best-known index fossils.

Key facts

  • Ammonites were extinct marine cephalopods with external chambered shells.
  • Their fossils are found worldwide in former marine sediments.
  • Ammonites are important index fossils because they evolved rapidly and were widely distributed.
  • Most ammonites went extinct at the end of the Cretaceous period, around 66 million years ago.
  • Some ammonite species reached very large sizes.
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Canadian context

Ammonite fossils are common in Canadian fossil-bearing rocks, including marine sedimentary formations in western Canada, and they are used in Canadian geology and paleontology to date rock layers.

Frequently asked questions

What is an ammonite?
An ammonite is an extinct marine cephalopod known from its coiled, chambered shell. The term is often used broadly for ammonoids, a larger group that includes true ammonites.
When did ammonites live?
Ammonoids first appeared in the Devonian, but true ammonites are known mainly from the Jurassic and Cretaceous periods. The group became extinct at the end of the Cretaceous, about 66 million years ago.
Were ammonites related to nautilus?
Yes. Ammonites were cephalopods and were related to living nautilus, squid, octopus, and cuttlefish, though they were not the same as modern nautilus.
Why are ammonites important to paleontologists?
They are important index fossils because many species evolved quickly, had wide geographic ranges, and are common in sedimentary rocks of known ages.
Did ammonites have shells?
Yes. Ammonites had external shells, usually coiled and divided into chambers.
Why did ammonites go extinct?
They disappeared during the end-Cretaceous mass extinction about 66 million years ago. The sources identify the timing of extinction, but not a single exclusive cause.

References

  1. National Geographic — https://www.nationalgeographic.com/animals/facts/ammonites
    Supports: Ammonites as extinct marine predators; ammonoids terminology; broad age and diversity.
  2. British Geological Survey — https://www.bgs.ac.uk/discovering-geology/fossils-and-geological-time/ammonites/
    Supports: Jurassic and Cretaceous age range; extinction at end of Cretaceous; marine mollusc/cephalopod classification.
  3. Natural History Museum — https://www.nhm.ac.uk/discover/what-is-an-ammonite.html
    Supports: Extinct shelled cephalopods; chambered shells; distinction between ammonoids and true ammonites; index fossil value.
  4. San Diego Natural History Museum — https://www.sdnhm.org/exhibitions/fossil-mysteries/fossil-field-guide-a-z/ammonite/
    Supports: Ammonites as extinct cephalopods; marine mollusks; external shells.