Ammonite is a prehistoric mollusk that lived in the oceans between about 250 million and 65 million years ago. Mollusks are soft-bodied animals with no bones. Ammonites are known from their fossilized shells. Many kinds of ammonites had a coiled shell resembling a ram’s horn. Other ammonites had uncoiled or partially uncoiled shells. Some ammonite shells feature ribs, knobs, or bumps. Ammonites range in size from about 1/2 inch (1 centimeter) to nearly 10 feet (3 meters) in diameter. Ammonites belong to a larger group of mollusks called ammonoids.
An ammonite shell is made up of many chambers, much like that of the nautilus, a living relative. As the ammonite grew, it added new chambers to its shell. Each new chamber was closed at the back with a wall called a septum. The animal lived in the outermost chamber. A tube called the siphuncle extended through all the chambers. It could adjust the animal’s buoyancy (tendency to float) by draining fluid from the chambers. The animal propelled itself by pushing out water through a soft tube of tissue called the hypernome.
Ammonites lived in the world’s oceans at the same time that dinosaurs lived on land. Both the ammonites and the dinosaurs died out about 65 million years ago at the end of the Cretaceous Period.
