Evrim Yazgin
Cosmos science journalist
A fossil of a larva which lived more than half a billion years ago might shed light on the earliest ancestors of modern insects, spiders, crabs and centipedes.
Named Youti yuanshi, the fossil is described in a new paper published in Nature.
Y. yuanshi lived during the Cambrian period (539–485 million years ago). The 1-millimetre-long worm was found in Yunnan Province in southern China. It was unearthed in a layer of rock which dates to about 520 million years ago.
The Cambrian period saw an “explosion” of life and the emergence all the basic body plans for life on Earth we see today.
This includes animals with external skeletons of the group euarthropods which includes Y. yuanshi, spiders, crabs, insects, scorpions and their relatives.
What makes the Y. yuanshi fossil so special is that its internal organs, including its brain and digestive glands, are preserved in exquisite detail. Detailed analysis even revealed a primitive circulatory system and traces of nerves supplying the larva’s simple legs and eyes.
The researchers were able to probe the tiny fossil using advanced scanning techniques. They used synchrotron X-ray tomography at the Diamond Light Source in the UK.
“Larvae are so tiny and fragile, the chances of finding one fossilised are practically zero – or so I thought!” says lead researcher Martin Smith from the UK’s Durham University.
“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 says the fossil could help understand the earliest evolutionary history of euarthropods.
“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.”
It might help understand how earlier worm-like creatures developed into arthropods with legs, eyes, mouth parts and antennae.
For example, the fossil shows evidence of an ancestral protocerebrum – the brain region which would later form the segmented, complex head of later arthropods.
