'My jaw just dropped': 500 million-year-old larva fossil found with brain preserved

Researchers have discovered how the brain of the largest animal group evolved after finding the remains of a microscopic, worm-like creature that lived half a billion years ago. 

The creature died while still in its early development, or larval stage, and belongs to a new species named Youti yuanshi, which combines the standard Chinese words "yòutǐ," meaning "larva," and "yuánshǐ," meaning "primitive," according to a new study published Wednesday (July 31) in the journal Nature. 

Y. yuanshi inhabited the Cambrian seas and helped give rise to living arthropods like insects, spiders and crabs. Despite being about the size of a grain of sand, the fossil is exceptionally well-preserved, revealing never-before-seen details that help explain how arthropods developed complex brains. 

"When I used to daydream about the one fossil I'd most like to discover, I'd always be thinking of an arthropod larva, because developmental data are just so central to understanding their evolution," study lead author Martin Smith, an associate professor in paleontology at Durham University in the U.K., said in a statement. "But larvae are so tiny and fragile, the chances of finding one fossilised are practically zero — or so I thought!" 

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A team of researchers recovered the fossil from the Yu'anshan rock formation in the Yunnan Province of China. They scanned the fossil with X-rays to create virtual 3D images of its internal structures. The images revealed a brain and primitive circulatory system, including traces of nerves serving the larva's simple legs and eyes, according to the study.

"I already knew that this simple worm-like fossil was something special, but when I saw the amazing structures preserved under its skin, my jaw just dropped — how could these intricate features have avoided decay and still be here to see half a billion years later?" Smith said.