Description
Eutrephoceras dekayi Nautiloid Fossil – Genuine Upper Cretaceous Specimen
This is a genuine Eutrephoceras dekayi nautiloid fossil from the Pierre Shale of Dawson County, Montana, USA. Dating to the Upper Cretaceous, Campanian stage, this specimen represents a highly collectable marine cephalopod from one of North America’s most famous fossil-bearing formations. Eutrephoceras is an extinct nautiloid, related to the modern chambered nautilus, and is valued by fossil collectors for its classic coiled shell form, long geological history, and connection to the ancient Western Interior Seaway.
This fossil is a carefully chosen specimen, with the photograph showing the actual fossil you will receive. It also includes a Certificate of Authenticity lifetime guarantee generic card, making it an excellent choice for fossil collectors, natural history displays, geology students, Cretaceous fossil enthusiasts, and anyone interested in genuine marine fossils from the American West.
Geology, Age and Location
This fossil comes from the Pierre Shale, a widespread Upper Cretaceous marine formation exposed across parts of the western and central United States. In Dawson County, Montana, the Pierre Shale preserves fossils from the ancient Western Interior Seaway, a vast inland sea that once stretched from the Gulf of Mexico to the Arctic Ocean and divided North America into eastern and western landmasses.
The specimen is Campanian in age, placing it roughly between 83 and 72 million years old. During this time, Montana was not the inland landscape seen today, but part of a broad marine environment filled with cephalopods, fish, marine reptiles, bivalves, gastropods, crustaceans, and other sea life. The fine-grained shale of the Pierre Shale formed from muds deposited in this offshore marine setting, creating excellent conditions for preserving many types of Cretaceous fossils.
Fossil Type and Species
Eutrephoceras dekayi is a fossil nautiloid, a type of shelled cephalopod. Nautiloids are related to ammonites, squid, cuttlefish, octopus, and the living chambered nautilus. Unlike ammonites, which became extinct at the end of the Cretaceous Period, the nautiloid lineage survived and is still represented today by modern Nautilus and Allonautilus.
Eutrephoceras belongs to the order Nautilida and family Nautilidae. The species Eutrephoceras dekayi was originally described as Nautilus dekayi by Morton in 1834 and later placed within Eutrephoceras. It is one of the best-known Late Cretaceous nautiloids from North America and is associated with marine deposits such as the Pierre Shale and related Western Interior Seaway formations.
As a nautiloid, this fossil had a chambered shell divided internally by septa. The living animal occupied the final outer chamber, while the older internal chambers helped control buoyancy through gas and fluid regulation. This allowed the animal to move through the water column and live as an active marine predator or scavenger.
Morphology and Collector Appeal
Eutrephoceras is recognised for its smooth, rounded, tightly coiled shell, giving it a classic nautiloid appearance. Unlike many ammonites, which often show strong ribs, spines, or complex ornamentation, Eutrephoceras typically has a more streamlined shell with a rounded whorl profile and simple, elegant form. This makes it especially appealing to collectors who appreciate natural symmetry and the clean geometry of fossil cephalopods.
The shell of Eutrephoceras dekayi is generally involute, meaning the outer whorl wraps around and partly covers the earlier whorls. The sutures are simpler than those of ammonites, reflecting the more conservative shell structure of nautiloids. This contrast makes Eutrephoceras an excellent educational fossil for comparing ammonites and nautiloids, two major groups of extinct and living cephalopods.
Specimens from the Pierre Shale may preserve as natural internal moulds, shell-bearing examples, or mineralised fossils depending on local conditions. Their association with the Western Interior Seaway gives them strong scientific and display interest, especially for collectors of Upper Cretaceous marine fossils from Montana.
Depositional Environment
During the Campanian stage of the Upper Cretaceous, the Pierre Shale was deposited in a broad marine seaway with relatively quiet offshore conditions. Fine mud settled through the water and accumulated on the seabed, eventually compacting into shale. This environment supported a rich marine ecosystem in which nautiloids such as Eutrephoceras lived alongside ammonites, inoceramid bivalves, fish, sharks, mosasaurs, plesiosaurs, turtles, and many other marine organisms.
The Western Interior Seaway was one of the great marine environments of the Cretaceous world. Fossils from this setting help reveal how life flourished in warm inland seas at a time when dinosaurs still lived on the surrounding landmasses. A nautiloid fossil from the Pierre Shale is therefore more than a display piece; it is a direct link to a vanished ocean that once covered large parts of North America.
Authenticity and Display
All of our fossils are 100% genuine specimens and come with a Certificate of Authenticity. This Eutrephoceras dekayi nautiloid fossil is supplied with a lifetime guarantee generic COA card for confidence and peace of mind.
Full sizing: please see photo.
The fossil shown in the photograph is the actual specimen you will receive. This genuine Eutrephoceras dekayi from the Pierre Shale of Dawson County, Montana is a superb addition to a fossil collection, natural history cabinet, geology teaching set, Cretaceous marine display, or collection of genuine American fossil cephalopods.
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