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Ammonoids were squidlike creatures that lived inside an external shell. In fact, ammonoids are relatives of the modern squid, as well as the octopus and chambered Nautilus, all of which belong to the class of animals called cephalopods.
Ammonoids appeared in the fossil record during the early part of the Devonian Period, about 419 million years ago. They died out about 66 million years ago, during the mass extinction at the end of the Cretaceous Period that killed the dinosaurs and many other kinds of land and sea animals. Their fossils are common in sedimentary rocks around the world and are fairly common in the Cretaceous rocks of western Kansas. They are also found in Pennsylvanian and Permian outcrops in the eastern part of the state.
Most ammonoids had shells that were coiled in the same plane (like a cinnamon roll). Others had straight or erratically coiled shells. The external surfaces of the shells were ornamented in a variety of ways, with different color patterns, ribs, nodes, or spines.
The size of ammonoids varied greatly throughout their long history on Earth. Most Paleozoic ammonoids were golf-ball sized or smaller. At the height of their diversity during the Cretaceous, however, many ammonoids were larger, and some with diameters up to 10 feet must have been formidable predators. The discovery of fossil ammonoids with bite marks tell us that ammonoids also were preyed upon by larger vertebrates, such as fishes, sharks, and mosasaurs.
The diversity of external shell form in ammonoids points to a wide range of adaptations to the marine environment. Some ammonoids may have spent part of their life on the ocean floor, while others spent their lives passively drifting with the currents through the water column. Others, especially those with smooth, streamlined shells, were probably energetic swimmers. The soft, squidlike animal lived in the front chamber; the other chambers, called buoyancy chambers, were used to regulate the ammonoid's position in the water column.
Because of their rapid evolution and abundance in the fossil record, ammonoids are extremely useful in correlating the ages of sedimentary rocks from different parts of the world. By matching ammonoid species contained within rock formations from different places, geologists can determine that the rocks were deposited at approximately the same time. In fact, because ammonoids evolved so quickly during the Triassic, Jurassic, and Cretaceous periods, their fossils can be used to establish zones that represent less than a million years. This is very fine resolution when compared to the 4.6 billion years of geologic time.
Although ammonoids are relatively common fossils in the Cretaceous outcrops of central and western Kansas, they are much less common in eastern Kansas, where smaller fossils occasionally are found in selected Pennsylvanian and Permian outcrops.
Stratigraphic Range: Lower Devonian to Upper Cretaceous.
Taxonomic Classification: Ammonoids belong to the Kingdom Animalia, Phylum Mollusca, Class Cephalopoda, Order Ammonoidea.
Text and photos from Windows to the Past: A Guidebook to Common Invertebrate Fossils of Kansas, Kansas Geological Survey Educational Series 16.
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