Iridescent-Pearly Eutrephoceras with Marine Shell Fossil- Pierre Shale Montana | River Walkers Minerals

Locale: Pierre Shale Formation, Glendive, Montana, USA 🇺🇸 
Age: Late Cretaceous (~70–80 million years old)
Dimensions: 371g | 3.7 x 2.7 x 2 inches

This attractive fossil association from the Pierre Shale of eastern Montana features a beautifully preserved Eutrephoceras dekayi accompanied by an associated marine shell fossil preserved in natural matrix. Both fossils display subtle iridescence with flashes of blue, green, pink, and gold that become especially noticeable under direct light. The contrasting textures and sculptural arrangement create a unique display that captures a small piece of the ancient Western Interior Seaway.

Eutrephoceras belonged to a group of cephalopods closely related to modern nautiluses. Unlike ammonites, which disappeared during the mass extinction at the end of the Cretaceous, nautiloids survived and are still represented today by the living chambered nautilus. Fossils of Eutrephoceras provide an important link between ancient seas and one of the oldest surviving animal lineages on Earth.

Dating to approximately 70–80 million years ago, this specimen formed when eastern Montana lay beneath a shallow inland sea teeming with marine life. The preserved pearly shell and natural association with another marine fossil offer a fascinating glimpse into that long-lost ecosystem.

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