The Cambrian explosion was one of the most dramatic chapters in the history of evolution. Around 540 million years ago, a sudden, astonishing burst of diversification effectively marked the beginning of most animal life on our planet. The fossil record rapidly became much richer, revealing strange new animal body plans with hard shells and spines , appendages , eyes , and specialized mouthparts . But there's been a long-standing mystery about where one key group of organisms fits in. Those