The limestone ammonites began their stage of preservation when they died and were buried with sediment on the seafloor. The ammonite shell left an impression in the sediment. Then, after the ammonite decayed, it left a void or hollow area behind. This hollow area served as a natural mold that filled with sediment and minerals over time. The fossil resulting from this mold is called a cast. The cast contains the exterior details of the original ammonite. Below is an example of a limestone ammonite cast. Most of the limestone ammonites on this site are from the Grayson, Duck Creek, or Austin Chalk Formation.
The Woodbine ammonites on our site are preserved via a cell-by-cell replacement form of fossilization. The flesh of the ammonite decayed, leaving the shell behind. The cells in the shell were replaced with minerals. Specifically, the aragonite in the shells changed to a calcite chemical composition. These ammonite fossils have retained their shells and internal structure. Over time, they have been filled with calcite, other minerals, and sediment. These ammonites occur in the Woodbine Formation and Eagle Ford boundary.
We prepare the Woodbine ammonites depending on their shell condition and their mineralization. If the shell is badly eroded or pitted on the outside, the ammonite will be cut and polished. It’s cut with a diamond saw, sanded, and polished to reveal the inner chambers of the ammonite’s shell as seen below. To view our Texas cut and polished ammonites for sale visit: https://fossilfanatics.com/product-category/texas-cut-and-polished-ammonites/
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. To view the variety of Texas whole fossils available, visit: https://fossilfanatics.com/product-category/texas-whole-ammonites/
Septarian concretions were mud balls that rolled around on the ocean floor often forming around the remains of shelled sea life and gathering more as they rolled. These concretions hardened and developed cracks over time. In the Woodbine Formation, the cracks slowly filled with a calcite deposit over time. Most have ridges or veins visible from the outside of the concretion. Below are photos showing the variety of septarian concretions.
Ammonites and other fossils can be found with calcite ridges like the ammonite in the photo below. These ridges were formed when the fossils were in a concretion and the calcite minerals seeped into the cracks of the concretion down to the fossil over time. The ammonites in matrix or concretions are the least common because their concretions have often eroded or have become so soft they can’t be salvaged. Those concretions that survive have done so because the matrix itself is terribly hard and difficult to remove from the ammonite. This means hours of scribe work and micro-abrasion.
For our high-quality, shelled ammonites, we slowly strip away the surrounding matrix from the ammonite with a micro-abrasive tool. During this labor-intensive process, we may find calcite septarian veins, clams, oysters, or even serpula (marine worms) attached to the ammonite shell. With much patience and care, we do our best to preserve the ammonite as it existed years ago.
Ammonites having shell erosion, but containing a high-quality calcite core, are sanded down and polished to reveal the ammonite’s beautiful suture patterns. The suture patterns are leaflike patterns where the ammonite’s chambers join. As the ammonite grows it excretes a new chamber. Sometimes, we will strip away the last chamber from the polished ammonite using microabrasion to reveal sutures forming a three-dimensional aperture. This is a very labor-intensive process as we don’t carve the aperture.
We also have ammonites with a combination of polished internal whorls revealing beautiful suture patterns blended into the remaining shell.
