Giant Heteromorph Ammonite

Diplomoceras maximum

On display in the Museum of the Earth is one of the world’s largest relatively complete heteromorph ammonites. This specimen of Diplomoceras maximum—which is from the Cretaceous-aged Lopez de Bertodano Formation on Seymour Island, Antarctica—is more than 1.5 meters (4 feet) long and shaped like a giant paperclip.

Heteromorph ammonites were ammonoid cephalopods with unusual shell shape that were particularly diverse during the Cretaceous period (145-66 million years ago). Unlike most ammonoids, heteromorphs were not tightly coiled. Instead their conical shells were uncoiled in a wide variety of shapes. Many were coiled in three dimensions or in irregular patterns.

The ecology of heteromorphs ammonoids is one of paleontology’s great puzzles. Some may have floated relatively passively in the water column. Others, like Diplomoceras, may have lived close to the bottom. Recent research on the specimen on exhibit in the Museum suggests that it may have grown extremely rapidly.

Ammonoid vs Ammonite: What’s the difference?

Ammonoids, which are now extinct, were a group of externally-shelled cephalopods (relatives of modern squids and octopuses) that lived from the Devonian to the Cretaceous periods (about 370 to 66 million years ago). Like the modern chambered nautilus, ammonoid shells were divided into chambers, separated by walls of shell material called septa. Septa in modern Nautilus are simple curves, whereas septa in ammonoids were more complexly folded (see top image on this page).

Three different groups of ammonoids are recognized based on this septal folding. Goniatites, which lived during the Paleozoic era, had relatively simply folded septa, with simple “lobes” and “saddles.” Ceratites, which lived during the Permian and Triassic periods, had complex lobes but simple saddles. Ammonites, which lived during the Jurassic and Cretaceous periods, had complex lobes and saddles. All heteromorphs were ammonites.

Learn More:

• Ammonoidea. Digital Encyclopedia of Ancient Life.

• Understanding heteromorph ammonites. Deposits Magazine.

• Ancyloceratina [the taxonomic group that includes heteromorphs]. Wikipedia.

• Olivero, E.B., and W.J. Zinsmeister. 1989. Large Heteromorph Ammonites from the Upper Cretaceous of Seymour Island, Antarctica. Journal of Paleontology 63(5): 626-636. [Scientific report]