Part of Grand Gallery.

Rainbow-colored ammonite specimen. C. Chesek/© AMNH
Ammonites inhabited the planet for more than 300 million years—almost twice as long as dinosaurs—before disappearing in the mass extinction event more than 66 million years ago. 

Ammonite fossils are an incredible source of information for scientists, from dating rocks to confirming the presence of prehistoric seas.

The 2-foot-(0.6 meter)-diameter ammonite fossil on display in the Museum’s Grand Gallery is a large and particularly rare example of this once-common mollusk. The 80-million-year-old specimen features a vividly colored fossilized shell that is unique to ammonites from Alberta, Canada.

Its iridescence is the result of nacre, the substance that made up the animal’s shell, being exposed to high temperatures and pressures over millions of years, becoming fossilized and turning into a mineral known as aragonite.

Its colorful surface is produced by light reflecting off of layers within the fossil and interacting, much the way oil on water produces a rainbow sheen. Ammonites that display this characteristic are known as ammolites. Ammolites are one of only three gemstones produced by living organisms—the others being amber and pearls.

Ammonite Diversity

Ammonites thrived in the Paleozoic and Mesozoic Eras, some 400 to 66 million years ago. These ancient cephalopods, relatives of modern-day squid, octopus, cuttlefish, and the chambered nautilus, were able to swim thanks to the multi-chambered construction of their shell, which provided variable buoyancy, and probably moved through the water using jet propulsion.

Diverse, widespread, and abundant in their marine environments, ammonites left behind an extraordinary number of fossils. As many as 10,000 species may have existed, ranging from tiny organisms that measured only a fraction of an inch across to formidable animals more than 2 feet (0.6 meters) in diameter, such as the one on view in the Grand Gallery.

The most commonly found ammonite fossils are coiled like rams’ horns. In fact, ammonites are named for the Egyptian god Ammon, who was sometimes represented as a ram. But many ammonites were elongated or tubular, others snail-like, with surfaces ranging from smooth to heavily ribbed, probably for protection from predation. Since soft body tissues rarely fossilize, shells, along with jaws, are the focus of most ammonite research.

Because different species of ammonites lived during different time periods, scientists use them to determine the relative age of the rocks in which they are found. Their presence also indicates the location of ancient seas, such as the Western Interior Seaway in the middle of North America, where the Grand Gallery specimen once lived. 

Ammonites in Human Cultures

Ammonite fossils have intrigued humans for centuries. In medieval Europe, the coiled fossils were known as “snakestones” and were thought to have curative powers against snakebites and poisons.

Carved ammonite fossil “snakestone”.
Carved ammonite fossils like this one were known as “snakestones” because of their coiled shape and were thought to have curative powers.
C. Chesek/© AMNH

In North America, Crow, Sioux, and other Great Plains tribes often found such fossils, relics of the sea that once covered the continent’s western interior and used these “stones” to heal injuries and to bring good fortune in marriage, hunting, travel, and warfare.