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Ammonoids are an extinct group of marine mollusks that belong to the subclass Ammnoidea and the class Cephalopoda. A popular and well-known subgroup of ammonoids are ammonites. The closest living relatives of ammonoids are also cephalopods like squids, octopods, and cuttlefish, while the modern nautilus is more distantly related.
Ammonoids had shells made of calcium carbonate just like today's snails, clams, oysters, and other shelled mollusks. Ammonoid shells varied in shape and size. Some ammonoids had tightly coiled shells (planispiral), while others had uncoiled, irregularly shaped shells (heteromorphs). Regardless of shape or size, the shell provided the ammonoid with protection and possibly camouflage.
Ammonoid shells had interior walls (septa) that created chambers inside of the shell. These chambers were connected by a narrow tube structure called a siphuncle. The ammonoid could use the siphuncle to control the amount of gas and fluid in each chamber, giving it the ability to achieve neutral buoyancy and move about in the marine environment.
Although ammonoid shells are abundant in the fossil record, there is an extremely poor record of their soft parts being preserved or fossilized. Based off of their relationships to mollusks alive today, ammonoids likely had bodies that were soft. The animal would have lived exclusively in the last chamber of its shell with numerous arms extending in a ring around its mouth, eating plankton and detritus, dead or decaying matter. Scientists study the shapes and patterns of ammonoid shells and related species, fossil and modern, to learn about the extinct animal.
Ammonoids lived around the globe and were present on earth for a very long time, about 350 million years. The entire group went extinct at the end of the Cretaceous, about 66 million years ago, along with the dinosaurs.
The abundance of ammonoids in the fossil record and their long history on earth make them good fossils to study. Geologists use ammonoid fossils as guide or index fossils, helping to date the rock layers from which the fossils were found. Paleobiologists can use fossil ammonoids to learn about patterns of extinction and glean information about the group's evolutionary history.
Learn more about ammonoids by watching the 30-minute, Smithsonian Science How webcast, Exploring Fossil Ammonoids with Paleobiologist Lucy Chang, May 30, 2019 at 11am and 2pm and interacting with the scientist through live Q&A and polls.