It was found during work to expand Rotterdam's port and went on display Thursday.
Reumer, a paleontologist, said the dung was dated by its sediment layer. He hopes a second example is found so it can be broken apart, studied and dated more exactly.
"Cave" hyenas were once common in Europe, and the fossil closely resembles modern hyena dung. But its color is misleading since before fossilization it would have been chalky gray.