We recently received a large shipment of stunning Late Cretaceous ammonite fossils from the Fox Hills and Pierre Shale Formations of South Dakota. Ammonites are an extinct type of cephalopods, often with coiled shells that died out at the end of the Cretaceous period, some 65 million years ago.
Approximately 70 million years ago, much of what is now central North America was covered by an inland sea known as the Western Interior Seaway. This seaway divided the continent into two land masses stretching from the Gulf of Mexico through to the Arctic Ocean. Many large marine reptiles such as Mosasaurs inhabited this sea as well as a very diverse fauna of ammonites.
The ammonites now collected from inside of concretions, a hard ball of rock that tends to form around organic matter. These concretions are split open to reveal the ammonites and other fossil preserved inside. The fossils must then be prepared by hand using tools such as air scribes to remove the rock around the fossil and expose additional detail.
The shell of many of the ammonites from within these formation are preserved with an iridescent or opalescent shell which shines with brilliant hue’s of pinks and greens in the right light. The iridescence is due to the microstructure of the aragonite, a primary mineral with in the shell. Light rebounds off of thin platelets that make up the aragonite creating the color. The thicker the layers, the more reds and greens are produced; the thinner the layers, the more blues and violets predominate.