You are in the forested swamps of South America, 270 million years ago.

The air in this Permian swamp is thick, a humid haze that clings to the skin, smelling of rotting ferns and shallow mud. You are standing on a bank of dark sediment, looking out over a stagnant, shallow lake surrounded by a dense forest.

Fifty feet out, a line of bubbles disrupts the murky water. Then, it breaks the surface. At first, it’s just the eyes. Two dark, unblinking orbs sitting high on a flat, bony skull, looking exactly like a modern gharial, but bloated to astronomical proportions. The snout is absurdly long, a slender "saw-croc" rod lined with hundreds of razor-sharp, cone-shaped teeth.

It’s a Prionosuchus. It rises, showing a 16-foot, slender body that is less a land-walker and more a perfected fish-trap. Its scaly, mottled skin, covered in mucus to keep it hydrated, glints in the humid sun. It moves with deceptive grace, tail swishing slowly, commanding the water with absolute, silent dominance.

It turns its massive, five-foot skull toward the bank, watching you...