RARE: CLIOSCAPHITES PLATYGASTUS Heteromorph Ammonite Fossil – Santonian, Cretaceous – South Dakota, USA – Certified Specimen

Description RARE CLIOSCAPHITES PLATYGASTUS – HETEROMORPH AMMONITE FOSSIL FROM SOUTH DAKOTA (CRETACEOUS: SANTONIAN) This listing features an exceptionally rare and scientifically significant specimen of Clioscaphites platygastus, a heteromorph ammonite from the Santonian Stage of the Late Cretaceous, found in South Dakota, USA. This well-preserved fossil represents a distinctive scaphitid cephalopod known for its partially uncoiled shell and important role in biostratigraphy. You will receive the exact fossil pictured, from the distinguished Alice Purnell Collection, with provenance assured. The photo includes a 1cm scale cube for accurate sizing. Full measurements are visible in the listing images. All of our fossils are 100% genuine and come with a Certificate of Authenticity. Fossil Identification: Species: Clioscaphites platygastus Genus: Clioscaphites Family: Scaphitidae Superfamily: Scaphitoidea Order: Ammonitida Class: Cephalopoda Phylum: Mollusca Common Name: Heteromorph Ammonite Geological and Stratigraphic Context: Geological Period: Cretaceous Stage: Santonian (approx. 86.3 – 83.6 million years ago) Formation: Likely part of the Pierre Shale Formation, known for its marine fossil content Location: South Dakota, United States Depositional Environment: Shallow epicontinental sea (Western Interior Seaway); fine-grained marine shales and mudstones Biozone Significance: Clioscaphites platygastus is a key zonal index fossil used to date Santonian strata within the Western Interior Basin Notable Stratigraphic Role: Defines part of the ammonite zonation within the Niobrara-Pierre transition Morphological Features: Clioscaphites platygastus is characterised by its heteromorphic shell—a planispiral early whorl that evolves into a slightly uncoiled or recurved body chamber. It often exhibits strong ribbing, ventral tubercles, and a compressed, laterally flattened whorl profile. These features provide key insights into Cretaceous marine adaptation and paleoecology. Its unique shape, midway between coiled and uncoiled, makes it a visually distinctive fossil and a highly desirable collector’s piece. Scientific Importance: This species is a hallmark of Santonian ammonite faunas in North America and is critical in biostratigraphic correlation of Late Cretaceous marine sediments. It reflects evolutionary trends in ammonoid morphology and offers paleobiogeographic links between Western Interior Basin faunas and those of Europe. Provenance: From the highly respected Alice Purnell Collection, known for high-integrity sourcing, accurate labelling, and scientifically curated specimens. Item Details: Genuine Clioscaphites platygastus ammonite fossil Geological Age: Santonian Stage, Late Cretaceous Origin: South Dakota, USA Included: Certificate of Authenticity Display ready – professionally selected and preserved Exact specimen in listing photos Scale reference: 1cm cube shown for sizing A rare opportunity to own a fine example of Clioscaphites platygastus, a standout ammonite fossil with both aesthetic appeal and scientific value. Ideal for collectors, educators, and anyone fascinated by the marine history of Cretaceous North America.