Exploring Fossil Ammonoids
Exploring Fossil Ammonoids Paleobiologist Dr. Lucy Chang holds an ammonite fossil over a drawer of collection specimens at the museum. (NMNH - Paleobiology Dept., Smithsonian Institution)
Exploring Fossil Ammonoids
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Ammonoids are extinct. They were a group of marine mollusks. They belonged to the subclass Ammnoidea. And they belonged to the class Cephalopoda. A subgroup of ammonoids are ammonites. They are popular. And they are well-known. What are some of the closest living relatives of ammonoids? They are cephalopods. These include squids. It includes octopods. And it includes cuttlefish. The modern nautilus is more distantly related.   

Ammonoids had shells. They were made of calcium carbonate. That is some of today's animals. These include snails. It includes clams. It includes oysters. And it includes other shelled mollusks. Ammonoid shells varied in shape. And they varied in size. Some ammonoids had tightly coiled shells. These were called planispiral. Others had uncoiled shells. They were irregularly shaped. These were called heteromorphs. The shell provided the ammonoid with protection. It may have also provided camouflage. That is regardless of shape. And it is regardless of size.

Ammonoid shells had interior walls. These were called septa. These created chambers. These were inside of the shell. These chambers were connected by a narrow tube structure. It was called a siphuncle. The ammonoid could use the siphuncle to control the amount of gas and fluid in each chamber. It gave it the ability to achieve neutral buoyancy. And it gave it the ability to move about in the marine environment.  

Ammonoid shells are abundant in the fossil record. But there is an extremely poor record of their soft parts being preserved or fossilized. Ammonoids likely had bodies that were soft. That information is based off of their relationships to mollusks alive today. The animal would have lived exclusively in the last chamber of its shell. It would have had numerous arms. These arms would extend in a ring around its mouth. I would eat plankton. It would eat detritus. It would also eat dead or decaying matter. Scientists study the shapes and patterns of ammonoid shells. They also study related species. They study both fossil and modern shells. This helps them learn about the extinct animal.  

Ammonoids lived around the globe. They were present on earth for a very long time. They were around for about 350 million years. The entire group went extinct. That happened at the end of the Cretaceous. That was about 66 million years ago. They went extinct along with the dinosaurs.  

There is an abundance of ammonoids in the fossil record. They have a long history on earth. Both of these things make them good fossils to study. Geologists use ammonoid fossils as guide or index fossils. These help them date the rock layers from which the fossils were found. Paleobiologists can use fossil ammonoids. They use them to learn about patterns of extinction. And they use them to glean information about the group's evolutionary history.

You can learn more about ammonoids. Watch the Smithsonian Science How webcast. It's called, Exploring Fossil Ammonoids. It is with Paleobiologist Lucy Chang. It's on May 30, 2019.  It is at 11am and 2pm. It will last 30-minutes. Students will have an opportunity to interact with the scientist through live Q&A and polls.

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