An ammonite is a cephalopod that once swam in shallow marine seas and became extinct at the end of the Cretaceous period (around 65 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. Below is a photo of one of our ammonites sliced in half to show the internal shell structure.
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. To control its buoyancy (going up or down in the water column), the ammonite would use its siphuncle to regulate the water to air ratio within its chambers. 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. It protruded from the end of its shell through the aperture.
In the photo above, you will notice the living chamber and part of the phragmacone have filled with minerals (calcite) and other sediments over time. Below is a photo of one of our polished ammonites that no longer has a living chamber or shell, but the multicolored minerals are visible from its exterior.
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In the photo above, notice the fernlike pattern on the exterior of the ammonite. This pattern is called a suture pattern. The suture pattern occurs where the septa, or the walls of the phragmacone, come in contact with the outer shell wall. As the ammonite grew, it excreted new chambers. Where the chambers fused, the suture pattern formed (beneath the shell). Ammonites are identified based on their suture patterns.
The calcite ammonites on our site were primarily collected from the Woodbine Formation in Tarrant County, Texas. Some of the ammonites found in the Woodbine Formation can be found encased in intact septarian concretions.
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Septarian concretions were mud balls that rolled around on the ocean floor often picking up the remains of shelled sea life. These concretions hardened and developed cracks over time. In the Woodbine Formation, the cracks slowly filled with a calcite deposit which resembles ridges or veins running through the concretion. Below are photos showing the variety of septarian concretions.
If one of these concretions contained an ammonite, when carefully prepared, these calcite ridges could be preserved on the shell of the encased ammonite. An example of one of our prepared Woodbine septarian ammonites is shown below.
Despite being extinct, ammonites provide useful information as index fossils to scientists today. They are used to determine the relative age of the rock layers in which they are found.