Anatomy: These animals were protected by a shell (usually spiral-coiled) that contained many air filled chambers, called phragmocones; the animal lived only in the outer chamber. The opening of the shell is called the aperture. The walls of each chamber are called septa; these walls were penetrated by a ventral tubelike structure called a siphuncle that probably regulated the air pressure, allowing the ammonite to float.

Size: Ammonites ranged in size from under an inch to about 9 feet (3 m) in diameter.

When Ammonites Lived: Ammonites appeared during the Devonian and went extinct during the K-T extinction, 65 million years ago. The closest living relative of the ammonite is the chambered nautilus.

Why Are They Called Ammonites?: Ammonites were named for Amun (also spelled Ammon), an ancient Egyptian god who is pictured as having ram's horns behind each ear (which look like ammonites).

Index Fossils: Ammonite fossils are found in great quantities and are used as an index fossil. Index fossils are commonly found fossils that are limited in time span. They help in dating other fossils.