You are in the tropical swamps of South America, 60 million years ago.

The air in the Paleocene swamp was thick, humid, and smelled intensely of sulfur and rotting vegetation. You were pushing through the thick, muddy bank of a slow-moving river when the ground seemed to shift to your left.

It wasn’t the ground.

It was a shell—a massive, mottled brown dome of bone larger than a modern armchair. You froze as a head slowly emerged, thick-necked and covered in armored skin. The Carbonemys turned its beaked face toward you, its eyes dark and intensely…