Description: 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 415 million years ago. They died out about 65 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.
The two ammonoids shown here are from Pennsylvanian rocks in southeastern Kansas. The top specimen belongs to the genus Goniatites and was collected from the Eudora Shale Member (Stanton Limestone) in Montgomery County. The lower specimen is Schistoceras missouriense, collected from the Drum Limestone, about 6.5 miles northeast of Independence, Kansas.
Most ammonoids had shells that were coiled in the same plane (like a cinnamon roll). Others had straight or erratically coiled shells. The external surface of the shells were ornamented in a variety of ways, with different color patterns, ribs, nodes, or spines. Depending on the state of preservation of individual fossils, this ornamentation is not always preserved.
Baculites is a common, straight-shelled ammonoid. These specimens are from the Pierre Shale of Logan County, Kansas. They are Upper Cretaceous in age.
Internally, ammonoid shells were divided into many chambers by a series of intricately folded walls. At times this folding was exceedingly complicated. The pattern of the folding can be seen in many specimens in which the outer shell has been removed. The junction between the wall and the outer shell produces a line called the suture, and these suture patterns are unique to each ammonoid species. Although paleontologists aren't sure why the walls were folded in such elaborate and complicated ways, the folds would have strengthened the walls, making them able to withstand increased water pressure at greater depths.
The convoluted sutures are easy to see in this fossil fragment of the genus Baculites, from the Beecher Island Shale Member of the Pierre Shale in Cheyenne County, Kansas. Note the relatively large size of this Cretaceous-aged ammonoid.
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.
Acanthoceras is a fairly large ammonoid from the Kansas Cretaceous. This specimen comes from Ellsworth County, from the upper part of the Graneros Shale.
Because ammonoids are extinct, paleontologists look to the only shelled cephalopod alive today, the Nautilus, for information about how ammonoids may have lived. Like the Nautilus, most ammonoids were probably good swimmers, moving through the water by means of a kind of jet propulsion. Ammonoids were important predators in the ancient oceans, eating fish, crabs, and other shellfish. 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.
Domatoceras (left) and Metacoceras (right) can be found in Pennsylvanian and Permian rocks in eastern Kansas (drawings by Al Kamb, KU Natural History Museum, Invertebrate Paleontology).
Stratigraphic Range: Lower Devonian to Upper Cretaceous.
Taxonomic Classfication: Ammonoids belong to the Kingdom Animalia, Phylum Mollusca, Class Cephalopoda, Order Ammonoidea.
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Text by Liz Brosius, Kansas Geological Survey. Unless noted otherwise, illustrations by Jennifer Sims, Kansas Geological Survey; photographs by John Charlton, Kansas Geological Survey.