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 an extinct group of marine mollusks. They belonged to the subclass Ammnoidea and to the class Cephalopoda. A subgroup of ammonoids are ammonites. They are popular and well-known. What are some of the closest living relatives of ammonoids? They include cephalopods and squids. It includes octopods and cuttlefish. The modern nautilus is more distantly related.   

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

Ammonoid shells had interior walls, called septa. These created chambers 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 with numerous arms. These arms would extend in a ring around its mouth. It would eat plankton and it would eat detritus. It would also eat dead or decaying matter. Scientists study the shapes and patterns of ammonoid shells and related species. They study both fossil and modern shells to learn about the extinct animal.  

Ammonoids lived around the globe and were present on earth for a very long time. They were around for about 350 million years. The entire group went extinct 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 and 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 to learn about patterns of extinction. And they can also use them to glean information about the group's evolutionary history.

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

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CRITICAL THINKING QUESTION
What do you think scientists find most helpful about studying the ammonites?
Write your answers in the comments section below


COMMENTS (3)
  • MiaD-5
    5/29/2019 - 01:50 p.m.

    I think scientists find the shells most helpful. I think this because the shells may be able to indicate what environment the ammonoids lived in. I believe that they also give them information on their living relatives that some scientists maybe didn't know about them. This is why I think scientists find the shell most helpful.

  • AdelaideM-
    5/29/2019 - 01:57 p.m.

    I think that it is helpful because the ammonite has a shell that has a hard time to decaying.The article says that they are related to mollusks. They could study mollusks and compare them to ammonites and find out what they have in common.

  • curtisa-rit
    8/29/2019 - 05:46 p.m.

    i think that the ammonoids are really cool shell.

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