Few things get armchair paleontologists as excited as the phrase “soft tissue preservation.” But a new study is casting doubt on some of the most stunning of these finds: Researchers argue that claims of brains, nerves and blood vessels preserved in animals for 520 million years are just a bunch of microbial goo called biofilm.

For the last decade, researchers working in Southern China have described a number of fossils from Stage 3 of the Cambrian Period, more than 500 million years ag