Side view of Youti Yuanshi worm fossil shows internal organ systems – by Emma J. Long via SWNS

A 520-million-year-old worm fossil has solved the mystery of how modern insects, spiders and crabs evolved.

The “incredibly rare and detailed” fossil, named Youti yuanshi, offered a peek inside one of the earliest ancestors of many species today, as it dates back to the Cambrian period when major animal groups were first evolving.

What makes the fossil so special is the exceptional preservation of the larva and its internal organs, despite its tiny size.

The research team led by Durham University in the UK says it is one of the first arthropod ancestors belonging to a diverse group called euarthropoda, which includes modern insects, spiders, centipedes, and crustaceans.

Scientists published their findings in the journal Nature, saying the level of complex anatomy proves the early arthropod-relatives were much more advanced than previously thought.

“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,” said Durham’s lead researcher Dr. Martin Smith.

“But larvae are so tiny and fragile, the chances of finding one fossilized are practically zero—or so I thought.

“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?”

Virtual dissection of Youti yuanshi shows internal structure of body cavity with head to the left – Emma J. Long via SWNS

Using advanced scanning techniques of synchrotron X-ray tomography at Diamond Light Source, a national UK science facility, the research team generated 3D images of miniature brain regions, digestive glands and a primitive circulatory system. They even found traces of the nerves supplying the larva’s simple legs and eyes.

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“It’s always interesting to see what’s inside a sample using 3D imaging, but in this incredible tiny larva, natural fossilization has achieved almost perfect preservation,” said study co-author Dr. Katherine Dobson, of the University of Strathclyde.

Studying the ancient larva has provided “key” clues about the evolutionary steps required for simple worm-like creatures to transform into the sophisticated arthropod body plan with specialized limbs, eyes and brains.

For example, the fossil reveals an ancestral proto-cerebrum brain region that would later form the nub of the segmented and specialized arthropod head with its various appendages such as antennae, mouthparts and eyes.

Composite image showing internal organ systems at front, middle and rear of Youti yuanshi – By Emma J. Long via SWNS

Scientists explained that the complex head allowed arthropods to take on a range of lifestyles and allowed them to become the dominant organisms in the Cambrian oceans.

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The remarkable specimen was originally discovered in China, and is housed at Yunnan University.

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