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Eutrephoceras Nautilus Fossil Late Cretaceous Montana #17536


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This is a nautilus fossil from the Pierre Shale in Montana.  It dates back to the Late Cretaceous time period, approximately 100-66 million years ago.  The nautilus is a type of soft-bodied marine mollusc with a spiral, compressed, multi-chambered shell.  The first members of the family Nautilidae appeared in the Late Triassic time period about 230 million years ago, and nautiluses have remained in existence to the present day. Having survived relatively unchanged for hundreds of millions of years, these creatures are often considered "living fossils".  Nautiluses are sometimes confused with ammonites, which are similar in appearance in many ways.  However, ammonites went extinct approximately 66 million years ago during the K-T event that killed the dinosaurs.  Also, a nautilus can be distinguished from an ammonite because of the smooth appearance of the sutures dividing the chambers of its shell.  In contrast, ammonites have more complex and squiggly sutures lines.  This undulation in an ammonite's sutures also creates a ribbed look to the outer surface of its shell, while the shell of the nautilus is smooth.

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