You are in the swamplands of South America, 8 million years ago.

The humid air of the Miocene Amazon was thick, almost chewable, as you paddled your small canoe along the edge of a vast river system that felt more like an inland sea. The water was still, reflecting the dense prehistoric jungle, until a ripple—too large to be a mere fish—disturbed the surface.

You froze.

Twenty feet ahead, what you thought was a moss-covered log suddenly rose, revealing an armored back that seemed to span the width of the river. It was a Purussaurus, the monster caiman of the Miocene, measuring nearly 40 feet from its snout to its crushing tail.

Its head, a massive, heavily armored skull, slowly turned toward you, its eyes placed high on its head offering a cold, calculated gaze…