Very fine examples of fossilized ammonites, Sphenodiscus sp. The brilliant red opalescent color is due to mineral crystallization, which occurred during the fossilization process. This makes the fossils very valuable on the commercial market. The ammonites (subclass Ammonoidea) were marine mollusks, which had a clearly defined head with tentacles for gathering food. They first appeared in the Lower Devonian period (400 million years before present), becoming extinct at the end of the Cretaceous period (65 million years BP). (1991)