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An ammonite is a cephalopod which once swam in shallow marine seas and became extinct at the end of the Cretaceous period (95 million years ago). The closest living relative to the ammonite is the chambered nautilus. Like the chambered nautilus, the ammonite's ability to swim was due to the unique construction of its shell. The shell contained many air filled chambers, called the phragmocone. The "walls" of each chamber, called septa, were penetrated by a tubelike structure called a siphuncle. It is believed that the air in the chambers was regulated through this "tube". The air in the chambers provided buoyancy, which allowed the ammonite to float. The ammonite lived in the last chamber, which ranged in size from a half of a whorl to one and a half whorls of the shell. The animal protruded out the end of its shell through the aperture.

In the above photo, you will notice that the living chamber, as well as the once air filled chambers have filled with minerals (calcite) and other sediments over time. Ammonite fossils can be found in a variety of geological formations around the world. Ammonites provide useful information to scientists today because they are used to determine the relative age of the rock layers in which they are found. The ammonites seen on this site were primarily collected from the Woodbine Formation in Tarrant County, Texas. A few of the ammonites collected in the Woodbine Formation can be found encased in intact septarian concretions. Septarian concretions are hard nodules which developed cracks over time. The cracks filled with a calcite deposit which resembles ridges or veins running through the concretion. If carefully prepared, these calcite ridges can be left protruding from the shell of the ammonite.

Ammonites are often identified by suture patterns which occur where the septa come in contact with the outer shell wall. In the photo above, you will see an ammonite with distinct suture patterns, which look fernlike. ( Pictures & discussion courtesy of FossilFanatics website owners )
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