Description
Genuine Sphenodiscus lenticularis Ammonite Fossil – Maastrichtian, South Dakota
This genuine fossil ammonite is identified as Sphenodiscus lenticularis, a highly collectable Upper Cretaceous ammonite from the Fox Hills Formation of South Dakota, USA. Dating to the Maastrichtian Stage, this specimen comes from the final chapter of the Cretaceous Period, approximately 72–66 million years ago, shortly before the extinction event that ended the age of the ammonites and the non-avian dinosaurs.
This fossil is a carefully chosen specimen, with the photograph showing the actual fossil you will receive. Full sizing can be seen in the photo. It is supplied as a genuine specimen and includes a Certificate of Authenticity lifetime guarantee generic card, making it an excellent piece for fossil collectors, natural history displays, educational collections, and anyone interested in rare Late Cretaceous marine life from North America.
Fossil Type and Species
Ammonites were extinct marine cephalopods related to modern squid, cuttlefish, octopus, and nautilus. They lived inside coiled, chambered shells, with the animal occupying the final body chamber while the earlier chambers helped regulate buoyancy. Ammonites were among the most successful marine animals of the Mesozoic Era and are highly important fossils for dating and correlating ancient marine rocks.
Sphenodiscus lenticularis belongs to the order Ammonitida, family Sphenodiscidae, and superfamily Acanthoceratoidea. Sphenodiscus ammonites are particularly distinctive because of their smooth, compressed, lens-shaped shells. Compared with many heavily ribbed Cretaceous ammonites, Sphenodiscus often has a more refined and streamlined appearance, making it instantly recognisable and very appealing to collectors.
The species name lenticularis refers to the lenticular, lens-like form of the shell. This flattened, elegant profile is one of the key visual features of the genus and gives the fossil a clean, sculptural quality. Sphenodiscus is a classic Late Cretaceous ammonite group and is especially associated with the marine deposits of the Western Interior Seaway and related coastal environments of North America.
Shell Morphology and Notable Features
Sphenodiscus ammonites are known for their compressed, involute shell form, where the outer whorl strongly overlaps and covers much of the earlier whorls. This gives the fossil a neat, rounded outline and a smooth, disc-like shape. The whorl section is typically narrow and streamlined, with a sharp or narrow venter in many examples.
Unlike many Cretaceous ammonites that display bold ribs, nodes, and tubercles, Sphenodiscus lenticularis is valued for its smooth to subtly ornamented shell surface and elegant symmetry. This makes it a striking contrast to more rugged acanthoceratid ammonites and gives it strong display appeal. The shell form would have been well suited to movement through the water column, reflecting a successful ammonite design near the end of the Cretaceous Period.
As with all ammonites, the original shell was divided into internal chambers connected by a siphuncle. These chambers helped the animal control buoyancy, while the living cephalopod occupied the outer body chamber. After death, the shell became buried in sediment and fossilised over millions of years.
Fox Hills Formation Geology
This specimen comes from the Fox Hills Formation, an important Upper Cretaceous geological unit exposed across parts of the northern Great Plains, including South Dakota. The formation represents sediments deposited as the great Western Interior Seaway retreated from North America near the end of the Cretaceous.
The Fox Hills Formation typically records shallow marine, shoreface, deltaic, and coastal environments. Sands, silts, and marine sediments accumulated as coastlines shifted and sea levels changed. These deposits preserved a range of marine fossils, including ammonites, bivalves, gastropods, fish remains, and other organisms that lived in or near the Late Cretaceous sea.
The presence of Sphenodiscus lenticularis in the Fox Hills Formation links this fossil to a fascinating time when inland seas were shrinking, coastal habitats were changing, and the marine ecosystems of North America were approaching the end of the Cretaceous world.
Maastrichtian Age and Scientific Interest
The Maastrichtian is the final stage of the Cretaceous Period. It is famous because it immediately precedes the end-Cretaceous extinction event, which eliminated ammonites, many marine reptiles, and the non-avian dinosaurs. Fossils from this interval are especially desirable because they represent some of the last ammonite faunas before their extinction.
Sphenodiscus lenticularis is an important Late Cretaceous ammonite because it helps represent the final evolutionary radiation of ammonites in North America. Named specimens with formation, stage, and locality information are particularly valued because they provide geological context as well as visual interest. This makes the fossil suitable for both display and educational use.
Collectable Late Cretaceous Ammonite
This Sphenodiscus lenticularis ammonite fossil is a desirable specimen for collectors of ammonites, American fossils, Cretaceous fossils, and natural history specimens. Its South Dakota origin, Fox Hills Formation provenance, Maastrichtian age, and distinctive lens-shaped shell all add to its appeal.
The fossil is ideal for a labelled fossil collection, display cabinet, educational geology set, or natural history collection focused on marine life from the age of dinosaurs. Its connection to the last stage of the Cretaceous gives it particular significance, as it represents an ammonite lineage close to the end of a major chapter in Earth’s history.
Authenticity and Presentation
This fossil is a genuine specimen and includes a Certificate of Authenticity lifetime guarantee generic card. The specimen has been carefully selected, and the photograph shows the actual fossil you will receive. Full sizing can be seen in the photo.
As a genuine Sphenodiscus lenticularis ammonite from the Fox Hills Formation of South Dakota, this fossil offers an attractive and scientifically interesting example of Maastrichtian marine life from the Upper Cretaceous of North America.
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