A 500-million-year-old clawed predator rewrites the origin of spiders and horseshoe crabs

It had been a long day of teaching for Rudy Lerosey-Aubril. As a reward, he returned to cleaning an intriguing Cambrian arthropod fossil he had recently received for review. At first, the specimen showed all the expected characteristics of its time—yet, something was off. In place of an antenna, there appeared to be a claw. "Claws are never in that location in a Cambrian arthropod," said Lerosey-Aubril, "It took me a few minutes to realize the obvious, I had just exposed the oldest chelicera ever found."