520-Million-Year-Old Fossil Reveals Arthropod Evolution

An international team of scientists has completed a new analysis of the anatomy of a worm larva fossil that lived 520 million years ago, during the incredibly remote Cambrian Period (540 to 485 million years ago).

This exceptionally rare specimen comes from a long-extinct species of eurarthropod (primitive arthropod) known as Youti yuanshi, and it is one of the earliest ancestors of modern arthropods (insects, spiders, centipedes, and crabs) ever discovered. The larva’s unique anatomical features, which have been miraculously preserved despite the immense passage of time, offer scientists an unprecedented glimpse into the remote evolutionary history of the Earth’s most common category of animal life.

Tiny Specimen Packed With Data

This minute specimen is no bigger than a flower seed. Yet the Y. yuanshi fossil was successfully recovered from inside a rock formation in a virtually intact state. The latest analysis has revealed the intimate details of its interior and exterior structures, which has allowed scientists to identify it as a clear forerunner of the modern arthropod family.

The larva essentially represents an ancient template of a species grouping that presently comprise about 80% of all the animal species living on the planet. Since the forces driving evolution acted on the foundational shape and structure of Y. yuanshi to eventually produce insects, spiders, centipedes, and crabs, this ultra-rare fossil gives expert researchers an ideal starting point for launching a deep dive into the evolutionary story of an extraordinarily successful animal category.

Scanning electron micrograph of the left side of Youti yuanshi, showing external anatomy. (Yang Jie / Zhang Xiguang/Nature)

A Microscopic Time Traveler from 520 Million Years Ago

Researchers from the Earth Sciences Department at Durham University in the United Kingdom, supported by colleagues from Yunnan University in China, recently published the results of their analysis of the worm larva in the journal Nature.

“The Cambrian radiation of euarthropods can be attributed to an adaptable body plan,” they wrote in their paper.

This brief statement summarizes their discovery that the shape and structure of the ancient species known as Y. yuanshi featured a built-in flexibility that would have allowed it to evolve in a suitable direction in just about any environment. On the land, in the sea, or even inside the Earth, different species of arthropod would have been able to evolve, and they would have been able to thrive.

Still preserved in its full three-dimensional shape, the worm larva fossil was recovered from a bank of shale rock in China’s fossil-rich Yu’anshan Formation. It was carefully extracted with acetic acid, and later shipped to a special laboratory in the UK where it could be analyzed using X-ray computed tomography equipment. With the images obtained researchers were able to generate 3D re-creations of the larva’s brain (it was surprisingly complex and advanced), its digestive and circulatory systems, and clusters of nerves connecting the brain to its primitive “legs” and “eyes.”

This fossil has proven to be both extremely ancient and extremely complex, and it is both rare and remarkable for each of these reasons.

“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 R. Smith, an associate professor of paleoanthropology from Durham University, said in a press release. “But larvae are so tiny and fragile, the chances of finding one fossilized are practically zero – or so I thought!”

As exciting as this discovery was, what the analysis of it found exceeded Smith’s wildest fantasies.

“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?”

Yet the worm larva was preserved for that long in a nearly pristine state, to the good fortune of scientists like Smith who understand the importance of what it represents.

Anatomical overview of  Youti yuanshi. (Emma J. Long/Nature)

Solving the Mystery of the Cambrian Evolutionary Explosion

Through a process of comparative analysis, the scientists involved in this new study will be able to trace some of the evolutionary steps that must have taken place for Y. yuanshi to ultimately transform into several million diverse species in the far-off future.

Some of this work has already been performed. For example, the fossil was found to possess a region in its brain that would later morph into a segmented arthropod head, which would include the familiar appendages seen on insects and other arthropods today (i.e., the antennae, the distinctive eyes, and various parts of the mouth). This movement toward greater complexity in the shape of the head helped the earliest arthropod species to adapt to different environments, leading to so much early diversity that the arthropods became the dominant lifeforms in the vast Cambrian oceans.

This unique fossil is currently being kept in storage at Yunnan University. Researchers are hopeful that other Cambrian larval specimens will eventually be discovered, since they offer so much useful information about early animal evolution. It was during the so-called ‘Cambrian Explosion’ that animal life from different categories began evolving in abundance, and there are still many secrets to learn about how and why this happened.

Top image: Top; The head of Youti yuanshi, showing (from left) external view from below; internal organs, with ventral organs omitted; internal organs; (right) oblique view showing internal anatomy. Bottom; Composite image showing internal organ systems at front, middle and rear of Youti yuanshi.  Source: Emma J. Long/Nature

By Nathan Falde