You are in the oceans of North America, 360 million years ago.

The water above the shale seafloor was surprisingly warm, thick with prehistoric algae and murky with suspended sediment. You hovered near a cluster of small ammonoids, observing their delicate, corkscrew shells. The silence of this ancient world was absolute.

Suddenly, the water pressure shifted, pushing against your dive suit. The school of tiny fish ahead vanished in a flash of movement.

Then, you saw it.

It did not move like a fish; it moved like a tank. A massive, shadowy shape emerged from the gloom, its head and upper torso covered in a thick, matte-gray dermal armor that looked more like stone than skin. Its eyes, encased in bony, protective rings, were locked onto something below.

The beast was over fifteen feet long, its body sleek and powerful behind the massive helmet. It didn't have teeth. Instead, as its jaw unhinged with terrifying speed—less than a 50th of a second—you saw two jagged, razor-sharp bony blades acting as, shearing against each other.

It was hunting a small prehistoric shark, a foolish, younger cartilaginous fish. It opened its mouth, creating an instant vacuum, and the sheer suction force pulled the struggling shark directly toward its maw. Snap. The shear-blades closed, slicing through armor and flesh with an estimated force of over 6,000 newtons. The shark was gone, swallowed in a single, brutal, and efficient movement.

For a moment, the armored predatory fish hovered, turning its head towards you…