Iridescent Ammonite Goes On Display At The Museum

by AMNH on


Rainbow on the Half Shell - This dazzling iridescent fossil of an 80-million-year-old ammonite measuring two feet in diameter was discovered near Lethbridge, Alberta, Canada, and is a particularly rare example of an ancient sea creature that went extinct at the same time as most dinosaurs. The spectacular coloration is the result of millions of years of high temperatures and pressures acting on the animal's shell to create a gemstone known as an ammolite.

©AMNH / Craig Chesek

Anyone who thinks fossils are dull should be dazzled by the rainbow hues of a new specimen now on display in the Museum's 77th Street Grand Gallery. 

The two-foot-diameter fossil, known as an ammonite, is a large and particularly rare example of a marine animal that was once one of the most common invertebrates in the oceans. Ammonites went extinct around 65 million years agothe same time as most dinosaurs.

The fossil's spectacular color is the result of nacre, the substance that made up the animal's shell, becoming fossilized and turning into a mineral known as aragonite. The colors are produced by light reflecting off of layers within the fossilized shell and interacting, much the way oil on water produces a rainbow sheen. Ammonites that display this characteristic are known as ammolites and are found only in one particular geologic formation in western Canada. Ammolite is one of only three gemstones produced by living organisms--the others being amber and pearls.

Scientists greatly value ammonites as well, whether colorful or not. 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 this one lived approximately 80 million years ago. Scientists at the American Museum of Natural History study these ancient creatures to better understand the origin and evolution of life on Earth.

Ammonites 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. Ammonites were cephalopods and related to modern-day squid, octopus, cuttlefish, and the chambered nautilus.

The ammonite on display was donated to the Museum by Korite International and Canada Fossils Ltd.

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