Iridescent Sphenodiscus Ammonite from Pierre Shale

Description: This is a lovely presentation of display characteristics. The center portion exhibits the rich red and green ammolite “fire” on both sides. Other colors like the subtle pink and muted greens fan outward. Suture patterns are revealed closer to the keel.

Sphenodiscus is an extinct genus of acanthoceratacean ammonite. The genus has been found from many continents and is thought to have had a large global distribution during the Maastrichtian stage of the Late Cretaceous. It was one of the last ammonoids to have evolved before the entire subclass went extinct at the end of the period during the Cretaceous–Tertiary extinction event. The shell of Sphenodiscus was streamlined and lateromedially compressed with overlapping whorls and a small umbilicus. The ventral edge of the shell tends to be sharply angled. The outer surface is generally smooth in fossil specimens, although certain species at different stages of ontogenic development may possess many small tubercles along their surfaces[

The iridescent color display of these ammonites is what makes them so stunning and collectable. The ammonite shell is comprised primarily of aragonite with a number of associated trace elements. This gem quality material is called ammolite. The percentage of ammolite in a specimen can make it very valuable. The ammolite is also graded according to color, iridescence, chromatic shift and other factors. This specimen has a thin layer of exposed ammolite on one side. The predominant colors are red and green, with brilliant iridescence.

The Cretaceous Pierre Shale formation where this ammonite originates outcrops in the United States and Canada. It’s famous ammonite fossils; include Baculites, Placenticeras, Scaphites, Hoploscaphites, and Jeletzkytes, as well as many uncoiled forms. Many of these also have much or all of the original shell, as well as the complete body chamber, still intact. Many Pierre Shale ammonites are found within concretions from which they must be removed, at least in part.