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Ammonite - Placenticeras

This ammonite is a member of the genus Placenticerus defined by its grooved venter, ornate sutures and moderately involuted umbilicus.  The specimen is from the Pierre Shale in western South Dakota.  Note the “scar” on the flank of the shell.  This could have been caused by a Mosasaur which fed on ammonites during the late Cretaceous period.

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