A new study led by researchers at Durham University, has uncovered an incredibly rare and detailed fossil, named Youti yuanshi, that gives a peek inside one of the earliest ancestors of modern insects, spiders, crabs and centipedes.

No bigger than a poppy seed the fossil dates back over 520 million years when the major animal groups we know today were first evolving. It has its internal organs preserved in exceptional quality.

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

This fossil allows researchers to look under the skin of one of the first arthropod ancestors. The level of complexity anatomy shows these early arthropod-relatives were much more advanced than the researchers thought.

Lead researcher, Dr Martin Smith of Durham University said:

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.

But larvae are so tiny and fragile, the chances of finding one fossilised 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?

Christopher Rau, Principle Beamline Scientist on Diamond’s I13-2 beamline commented:

“The research team produced a truly amazing piece of work. I am very pleased to see that we at Diamond could contribute the essential element for their study using XCT techniques. The micro-tomography station at I13-2 delivered these very detailed insight into the hundreds of millions old larva, which never would have otherwise been revealed. The precious and unique samples were examined, preserving all internal features.

Details like these also help trace how modern arthropods gained their incredible anatomical complexity and diversity and came to be the most abundant group of animals today.

The researchers point out that fossil fills an important gap in our understanding of how the arthropod body plan originated and became so successful during the Cambrian Explosion of life.

This remarkable specimen is housed at Yunnan University in China, where it was originally discovered.