Between 75 and 72 million years ago in what is now Chilean Patagonia, the carcass of a stubby-tailed dinosaur somehow ended up buried in a river delta, where the fine sediments exquisitely preserved the bones as they fossilized.

By dinosaur standards, this creature wasn’t huge. As it walked about on all fours, it stood less than two feet tall and spanned less than seven feet long, snout to tail. But it was a tough little animal in a world of giants. The dinosaur had armor-studded skin to defend itself—and a unique weapon bringing up the rear.

The end of this creature’s tail was unlike anything scientists have seen before: a mass of fused bone resembling a jagged cricket bat. “It’s entirely unprecedented,”