This story appears in the December 2019 issue of National Geographic magazine.
The fossils resembled blades of circular saws. Frozen in time, some of the ancient impressions revealed up to 150 razor-sharp wedges in tightly wrapped spirals, or whorls. For more than a century, scientists questioned: What were they? Answer: The killer jaws of the Helicoprion, an evolutionary outlier from 275 million years ago.