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Shelled Mollusks, related to Squid

Ammonites were a huge group of now-extinct cephalopod mollusks, relatives of the squids, octopuses, and nautilus. They are in the subclass Ammonoidea of the class Cephalopoda.

Ammonites lived from the Devonian to the end of the Cretaceous, 408 to 66 million years ago. They became extinct with the non-avian dinosaurs at the end of the Cretaceous.

They were named by Pliny the Elder (23-79 CE), who called them “Horns of Ammon,” Ammon being an Egyptian god pictured with curled rams’ horns.

Roman sculpture after Greek original of Zues-Ammon. Sculpture in the Antikensammlung München (Munich). Photo by Dan Mihai Pitea – Own work, CC BY-SA 3.0,

The fossils are their curled shells. Like the nautilus, ammonite shells have interior dividers between chambers. The dividers in most species were very complicated in their construction. The curves and shapes in them acted to increase their strength and the strength of the shell as a whole.

This made them more resistant to predators like mosasaurs, and to water pressure at greater depths. Many shells have been found with the teeth marks of mosasaurs (marine predatory reptiles).

Some ammonite species got up to 6.5 feet (2 meters) across the spiral. They came in many styles of shell, from the tight coil seen in this specimen to loose curvy open spirals.

Like the nautilus, the ammonites were able to move gas and liquid around among the chambers to allow them to float in the water column.

Ammonites were predators, and probably also were able to “jet” by holding and pushing water out of their bodies, like their modern relatives.

There is some evidence that ammonites had ink sacs like modern cephalopods, but it is far from settled. Most people who study them say they did not.

Also like the nautilus, the animals would have tentacles and a head that would stick out of the opening in the shell. The rest of the body was in the first few segments of the shell.

This Ammonite in the Museum’s collection is Devonian, about 350 million years old.