Sphenodiscus lenticularis Ammonite Fossil Fox Hills Formation Cretaceous USA COA South Dakota Maastrichtian Marine Specimen

Description

Genuine Sphenodiscus lenticularis Ammonite Fossil

This listing is for a genuine Sphenodiscus lenticularis ammonite fossil from the famous Fox Hills Formation of South Dakota, USA. Dating from the Maastrichtian Stage of the Upper Cretaceous, this specimen represents one of the final chapters in ammonite evolution, shortly before ammonites disappeared at the end of the Cretaceous Period.

Sphenodiscus lenticularis is a highly collectible Late Cretaceous ammonite species, valued for its elegant, compressed shell form and its association with the ancient marine environments of North America. This fossil is a carefully chosen specimen, and the photo shows the actual piece you will receive. Full sizing can be seen in the photo.

Geology, Age and Location

This ammonite comes from the Fox Hills Formation, a well-known Upper Cretaceous geological unit exposed across parts of the northern Great Plains of the United States, including South Dakota. The formation was deposited near the end of the Cretaceous Period, during the retreat of the Western Interior Seaway, a vast inland sea that once divided North America.

The age of this fossil is Maastrichtian, the final stage of the Late Cretaceous, approximately 72 to 66 million years old. Fossils from this interval are especially interesting because they come from the time immediately before the Cretaceous–Paleogene extinction event, which brought an end to ammonites, marine reptiles, many plankton groups, and the non-avian dinosaurs.

The Fox Hills Formation records shallow marine, shoreline, deltaic, and near-coastal environments formed as sea levels changed and the ancient seaway withdrew. Ammonites such as Sphenodiscus lenticularis lived in these marine waters alongside other Cretaceous sea life, including bivalves, gastropods, fish, sharks, marine reptiles, and other cephalopods.

Fossil Type and Species Details

Sphenodiscus lenticularis was a marine cephalopod, related to modern squid, cuttlefish, octopus, and nautilus. Like other ammonites, it possessed a coiled external shell divided internally into chambers. The living animal occupied the final body chamber, while the earlier chambers helped regulate buoyancy as it moved through the water.

This species is known for its relatively smooth, lens-shaped, strongly compressed shell. The name “lenticularis” reflects this flattened, lens-like form. Sphenodiscus ammonites typically have involute whorls, meaning the outer whorl overlaps much of the earlier shell, leaving only a small umbilicus visible. The shell form is streamlined, with a narrow profile and a sharply defined outer margin, making it visually distinct from many more heavily ribbed or ornamented ammonites.

Scientific classification places Sphenodiscus lenticularis within the Phylum Mollusca, Class Cephalopoda, Subclass Ammonoidea, Order Ammonitida, Superfamily Acanthoceratoidea, and Family Sphenodiscidae. The genus Sphenodiscus was named by Meek in 1871, while the species Sphenodiscus lenticularis is attributed to Owen, 1852.

A Collectible Late Cretaceous Ammonite

Ammonites are among the most iconic fossils, widely collected for their beauty, geological importance, and evolutionary history. Sphenodiscus lenticularis is particularly desirable because it belongs to one of the last ammonite lineages before their extinction. This makes it an excellent fossil for collectors interested in the end-Cretaceous world, the Western Interior Seaway, or the marine life that existed at the same time as the last dinosaurs.

The specimen’s natural preservation, geological age, and South Dakota origin make it a highly appealing addition to any fossil collection, educational display, natural history cabinet, or geology-themed gift selection. Its association with the Fox Hills Formation gives it strong scientific and collector interest, especially for those who focus on North American Cretaceous fossils.

Authenticity and Presentation

This fossil is a genuine specimen and includes a Certificate of Authenticity lifetime guarantee generic card. The fossil shown in the photograph is the actual specimen you will receive, making it ideal for buyers who want to see the exact item before purchase.

A superb genuine Upper Cretaceous ammonite fossil from South Dakota, representing the final age of ammonites and the ancient marine ecosystems of the Fox Hills Formation.