You are in the coastal swamps of North America, 310 million years ago.

The water was a murky, warm green, shallow enough that the sunlight pierced through, illuminating the thick mud of the Carboniferous seabed 300 million years ago. You were exploring the lush flora of the swamp, when suddenly, the water ahead surged.

A massive form, nearly twenty feet long, cut through the calm. It wasn’t a sleek, modern shark. Its snout was blunt, and its mouth was dominated by an impossible structure: a single, curved row of serrated teeth, curling outward like a grotesque pair of pinking shears, designed to cut through prey with a thrashing motion…