Antarctica’s Wealth of Ammonites

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Spiral shell of an ammonite.
Some ammonites, like this specimen from Lyme Regis, England, have spiral shells similar to nautiluses. But ammonites come in a variety of forms, from corkscrews to the paperclip-shaped Diplomoceras cylindraceum.

To round out this year’s Cephalopod Week, we’re taking a closer look at ammonites, mollusks that thrived in the world’s oceans for more than 300 million years before becoming extinct alongside non-avian dinosaurs during the Cretaceous-Paleogene extinction event 66 million years ago.

These shelled cephalopods grew up to several meters in diameter, and their shells formed in a variety of shapes from spirals to corkscrews.

And while specimens have been found almost everywhere on the planet, Antarctica is well-known for its rich ammonite fossil sites.

Among the most extraordinary ammonite species found in Antarctica is Diplomoceras cylindraceum, which could grow up to 2 meters long and is noted for its paperclip-shaped, uncoiled shell.

Ventral and lateral view of a curved Diplomoceras cylindraceum fossil,
One ammonite species found in Antarctica, Diplomoceras cylindraceum, could grow up to 2 meters long and featured a distinctive paperclip-shaped, uncoiled shell.
Courtesy of South African Museum/Wikimedia Commons

“Although it’s a totally unusual shape, it was clearly very successful at what it did, because fossils of this same species are found all over the world. It’s also one of the ammonites that survived right up until the mass extinction event at the end of the Cretaceous,” says James Witts, Lerner-Gray Postdoctoral Research Fellow in the Museum’s Division of Paleontology who researches ammonites, and how environmental changes such as mass extinction events effected these organisms over time.

According to Witts, there’s no better place to find ammonites in Antarctica than on the islands east of the Antarctic Peninsula, which includes the James Ross, Vega, Snow Hill, and Seymour Islands. Fossils here have been virtually undisturbed by the Earth’s constantly shifting tectonic plates, giving researchers a rare window into the polar life forms that inhabited the region from 85 to around 66 million years ago, including species of ammonites that have only been found on this continent.

“Even though the Cretaceous Earth was warmer than today, this was still a really extreme environment—with six months of darkness to cope with during the winter months, just like the modern Antarctic, and probably much colder temperatures compared to the rest of the world at the time,” says Witts.  

Ammonite in display case.
Jacobites anderssoni lived in Antarctica nearly 70 million years ago. This fossil ammonite can be seen in the Museum’s Ellsworth Corridor on the first floor.
A. Ruse/©AMNH

One such endemic species is Jacobites anderssoni. It had a classic spiral shell, similar to a nautilus, and lived during the Maastrichtian period, about 70 million years ago.

Visitors can see an example of Jacobites anderssoni in the Ellsworth Corridor on the Museum’s first floor. The exhibit features specimens collected during American explorer Lincoln Ellsworth’s four expeditions to the Antarctic between 1933 and 1939, including this ammonite from the late Cretaceous found on Snow Hill Island.

The Museum's Lincoln Ellsworth hall.
The Lincoln Ellsworth exhibit on the Museum’s first floor features fossil specimens collected by the American explorer during his expeditions to the polar region between 1933 and 1939.
M. Shanley/©AMNH

Visiting the Museum? “Where to Spot a Cephalopod” is a quick guide to exhibits about squids, ammonites, and more.