You are in the forests of North America, 220 million years ago.

The air in the Triassic floodplain was thick, tasting of petrified wood and sulfur from nearby rivers. You were walking by the riverbank when the undergrowth in front of you shifted.

Emerging from the low-lying ferns was a Desmatosuchus—a "living tank" stretching nearly sixteen feet from its pig-like snout to the tip of its armored tail. Its back was a mosaic of interlocking bony plates—osteoderms—and running down its flanks were smaller, sharp bony scutes. What made your heart pound, however, were the massive, 45-centimeter shoulder spikes projecting sideways, honed to a wicked point, obviously meant as a last resort if the armor wasn’t enough against foolish predators.

It ignored you, preoccupied with the soft mud at the river's edge as it plods over on its sprawling, quadrupedal gait.