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Ancyloceras is an extinct genus of heteromorph ammonites found in the marine strata of Europe and Morocco dating to the Lower Cretaceous, from 117-113 million years ago.

Ancyloceras ammonites are known as heteromorph shaped, with a partly uncoiled shell and the aperture directed toward the coiled part. Most ammonites are homomorph, as they maintain the same shape throughout the growth, while the ammonites in this genus have uncoiled shells (heteromorph or different-shaped ammonites), that would have precluded fast swimming.

This authentic specimen of Ancyloceras is 17 inches long, 11 inches high and 8 inches wide and weighs 19 pounds.