You are in the swamps of North America, 300 million years ago.

The air in the Carboniferous swamp was thick, hot, and smelled of decaying ferns. You sat in your small, wooden skiff, rowing silently through the murky, tea-colored freshwater channel. The silence was broken only by the buzzing of massive dragonflies.

Suddenly, the water near the bank exploded.

You caught a flash of brownish-green—the long, deadly dorsal spine of a 3-foot Orthacanthus. It had been lurking in the shallows. The shark wasn't hunting fish, however. Its jaws were locked onto a smaller, thrashing shark of its own species—a juvenile.

It was your first direct look at the brutal nature of "filial cannibalism" described by scientists. Swallowing the smaller fish whole, the Orthacanthus then circles around your spot on the skiff…