Spectacularly Iridescent Fox Hills Ammonite

Hoploscaphites nicolletti

Phylum Mollusca, Class Cephalopoda, Subclass Ammonoidea, Order Ammonitida

Geological Time: Upper Cretaceous

Size: 75 mm across

Fossil Site: Fox Hills Formation, North Central South Dakota

Hoploscaphites nicolletti ammoniteDescription: This is an example of the ammonite Hoploscaphites nicolletti of the Fox Hills Formation deposits of South Dakota. During the Cretaceous, much of the Western United States was covered by what is termed the Western Interior Seaway, a broad shallow (100-300 m maximum depth) sea that was home to various aquatic reptiles, fish, and ammonites. It was open to the North to Canada’s boreal seas and to the South to what is now the present-day Gulf of Mexico. During the Late Cretaceous, fluctuations in the sea level led to various bouts of exposure of some regions; as a consequence, there is some intertonguing of terrestrial deposits, the most famous of which is Hell Creek, home of T. rex. Ammonites of this degree of preservation are difficult to come across. The high degree of preservation of its natural mother-of-pearl shell makes it appear to be preserved as opal. This is a consequence of light passing through the various layers of aragonite and conchiolin deposited by the ammonite in life. Conchiolin is secreted by various other mollusks, including oysters, and is a complex protein. Notice how the colors change with changing perspective.

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