Fossilized ammonite. Fossilized shell of an ammonite (Hoploscaphites nicolleti). Ammonites are extinct cephalopods which lived between 380 and 65 million years ago (mya). They are distantly related to the modern-day nautilus and lived in coastal waters and inland seas. Ammonites are zone fossils, as certain species existed at specific times within the geological timescale, allowing accurate dating of marine rocks. This specimen, from the Fox Hills Formation of South Dakota, USA, lived during the Late Cretaceous period (97.5-65 mya).
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