Casablanca, MoroccoAt the end of a dim hallway in Casablanca’s Université Hassan II, I’ve walked into a dusty room containing a remarkable set of fossils—bones that raise foundational questions about Spinosaurus aegyptiacus, one of the weirdest dinosaurs ever discovered.

Longer than an adult Tyrannosaurus rex, the 50-foot-long, seven-ton predator had a large sail on its back and an elongated snout that resembled the maw of a crocodile, bristling with conical teeth. For decades, reconstructions of its bulky body have ended in a long, narrowing tail like the ones on its many theropod cousins.

The red-brown remains laid before me are altering that picture. These bones assemble into a mostly complete tail, the first yet found for Spinosaurus. It’s so large, five