Image: An ammonite

Ammonites are named after the Egyptian god Ammon, who is often depicted with rams' horns behind each ear.

Copyright © MMVII NGHT, Inc.

Fast Facts

Type:
Prehistoric
Diet:
Carnivore
Size:
Some more than 3 ft (1 m) in diameter
Group name:
School
Protection status:
Extinct
Did you know?
Female ammonites grew up to 400 percent larger than males, presumably to make room to lay eggs.
Size relative to a 6-ft (2-m) man:
Illustration: Ammonites compared with adult man

Ammonites were predatory, squidlike creatures that lived inside coil-shaped shells. Like other cephalopods, ammonites had sharp, beaklike jaws inside a ring of tentacles that extended from their shells to snare prey such as small fish and crustaceans. Some ammonites grew more than three feet (one meter) across—possible snack food for the giant mosasaur Tylosaurus.

Ammonites constantly built new shell as they grew, but only lived in the outer chamber. They scooted through the warm, shallow seas by squirting jets of water from their bodies. A thin, tubelike structure called a siphuncle reached into the interior chambers to pump and siphon air and helped them move through the water.

Ammonites first appeared about 240 million years ago, though they descended from straight-shelled cephalopods called bacrites that date back to the Devonian, about 415 million years ago. Ammonites were prolific breeders, lived in schools, and are among the most abundant fossils found today. They went extinct with the dinosaurs 65 million years ago. Scientists use the various shapes and sizes of ammonite shells that appeared and disappeared through the ages to date other fossils.

Prehistoric Features

  • Photo: Gully bones

    Digging Up Sea Monsters

    Follow the blog from the Spitsbergen Expedition as they unearth "sea monsters″ from the Upper Jurassic Period 150 million years ago.

  • Image: A bacculite

    Photo Gallery: Sea Monsters

    Get a glimpse of what prehistoric sea creatures may have looked like millions of years ago.

  • Photo: A Dimorphodon flies across the forest in a scene from the movie Flying Monsters 3D

    Flying Monsters 3D

    Could flying monsters have existed? With cutting-edge 3-D filming technology and CGI, Flying Monsters 3D recreates spectacular pterosaurs and brings these giant flying creatures to life.

  • 2565.jpg

    Bizarre Dinosaurs

    A bizarre gallery of Mesozoic monsters prompts John Updike to ask: What has evolution wrought?

Please select a test to run

Animals

The Innovators Project

See more innovators »

From the Magazine

  1. Photo: Two adult preen, Ireland

    Gannets Pictures

    Champion divers but clumsy landers, doting parents but hostile neighbors—northern gannets abound in contradictions.

  2. Photo: Silent Ural owl

    Estonia's Ural Owls

    Photographer Sven Začek provides an intimate view of this large raptor.