You are in the dark oceans of North America, 270 million years ago.

The water was unusually murky, filled with the silt of a shallow Permian sea under the stormy clouds. Diving deeper, into the dimly lit twilight zone, you were scanning the edge of a reef when a shadow—far too large and narrow for a shark—cut across the dim sunlight. It wasn't one; it was three. A pod of Helicoprion.

These were not the predators you knew. They were graceful but eerily thin, roughly 20 feet long each, moving with a subtle, serpentine motion. As they circled, the largest of them turned its head toward you, with an indifferent predatory look in its eyes, opening a long, deep, cartilaginous mouth.

It wasn't a set of serrated teeth you saw, but a nightmare in a spiral. The tooth whorl, a "buzzsaw" coil of older teeth curling inward with new, razor-sharp teeth at the front, gleamed with an iridescent sheen.

The Helicoprion pivoted, its narrow body swishing, and in a burst of speed, swims past you instead of attacking you. It had just snagged a passing, long-dead squid-like ammonoid, its lower jaw rotated, the teeth slicing through the shell as easily as a machine, leaving a perfect, torn segment.

More of these ammonoids were seen floating around you, and you soon realize that you are in the middle of a feeding ground for the Helicoprion pod. The other two Helicoprion begin to move in for the kill as you watched…