2.5" Fossil Nautilus (Eutrephoceras) - South Dakota

About Nautilus Fossils

autilus is a classic shelled cephalopod with a fossil history spanning more than 500 million years. Fossil nautilus shells are instantly recognizable by their tightly coiled, chambered structure, created as the animal grew and adjusted buoyancy using a central siphuncle. These fossils often preserve beautiful suture patterns, internal chambers, and sometimes even traces of original nacre. Found in marine deposits worldwide and frequently mineralized with calcite or chalcedony, fossil Nautilus specimens provide a striking glimpse into ancient seas and showcase one of the most enduring designs in natural history.

These 70 million year old ammonites lived when South Dakota was a shallow inland sea. They were found preserved in concretions when split open. They then had to be hand-prepared to remove the hard rock surrounding them from their shells, a very time consuming task.