IRIDESCENT AMMONITE

IRIDESCENT AMMONITE

Sphenodiscus species

Late Cretaceous, Fox Hills Formation, South Dakota, USA

Measuring approximately 12 inches in diameter. 

Ammonites are among the most instantly recognizable of fossils and, having existed on Earth for almost 350 million years, serve as excellent index fossils for the dating of the geological formations in which they are found. Under certain conditions related to fossilization, the nacreous shell becomes highly iridescent; the resulting opalescent gem-like material is called "ammolite." This somewhat laterally compressed specimen displays metallic colors of red and orange and is partially encrusted in its original matrix. The genus Sphenodiscus was a member of the acanthoceratacean ammonite group which were some of the last ammonoids to have evolved before the mass K-T extinction