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

The water of the shallow Devonian sea was warm and murky, filled with suspended sediment. You were swimming near a patch of crinoids when a shadow moved, not with the quick, lateral movements of a modern fish, but with a strange, gliding motion. It was a Stethacanthus, barely three feet long.

At first glance, it looked entirely conventional, a dark grey, sleek shark patrolling the seafloor. But as it turned in the sunlit, shallow water, the bizarre, high-rising dorsal fin caught the light. It wasn’t a fin, really—it was an "ironing board" structure, a flat, bristled anvil resting on its shoulders, topped with rows of tiny, backward-pointing spikes.

As you drifted closer, the Stethacanthus seemed to take notice, perhaps viewing you as a threat. It arched its back, making the spiky "brush" stand out more prominently against the blue. The front of its head, also covered in small, protective denticles, matched the bristly top of the anvil. It hung in the water, vibrating slightly, the strange structure a terrifying, alien silhouette—a 300-million-year-old threat display that made it look much larger and more formidable than its size warranted.