Rare 520-million-year-old larva could reveal more about early animal evolution

The Cambrian Period saw the rapid radiation of complex life and gave rise to many of the major animal groups we see today.

Fossils from this time can tell us more about how these groups evolved, but the soft tissue rarely preserves well in the fossil record. This means information about the internal organs of these animals is limited.

But the discovery of a fossil larva that is over half a billion years old that is so exceptionally well preserved in three-dimension that traces of the brain and other parts of the nervous system have been revealed.

The larva is from a species named Youti yuanshi. This was an early arthropod, the highly successful group of animals whose living members include insects, centipedes and crabs.

The tiny worm-like fossil was found in the Yu’anshan Formation in China and is almost four millimetres long, which is smaller than a grain of rice. X-ray computed tomography (XCT) were required to study the internal structures of the animal in fine detail. These analyses, published in the journal Nature, can help scientists understand more about the development of early animals.

Although it is rare for internal anatomy to be preserved, previous Cambrian fossils of a similar age from the Burgess Shale in Canada have been found to have preserved nervous tissue. But millions of years of geological activity have compressed these fossils, which makes it challenging to distinguish between different organ systems and interpret how they would have been arranged.

Emma Long, a researcher at the Museum and co-lead author of the study, says, “Youti's organ systems are preserved in 3D, offering a remarkable comparison to the carbonaceous compression fossils found in the Burgess Shale. Youti provides a framework for interpreting these flattened fossils, enabling us to better understand the complex organ systems of these early animals.”

“Although softer material like the internal organs have not been preserved, we can still see the voids left inside the tissue where structures like the nerve cord and the brain would have resided. It’s amazing to see such detailed preservation in a fossil so small!”

What was Youti yuanshi like?

During the Cambrian most animals – including Youti yuamshi – lived around the ocean floor.

The larva of Y. yuanshi comprises multiple segments that each contained a pair of tiny legs. It also had a highly developed head, a complex brain and a digestive tract.

Youti yuanshi is closely related to the gilled lobopodians, which includes the enigmatic Hallucigenia, and the radiodonts, a highly successful Cambrian group that includes Anomalocaris. It is likely that adult Y. yuanshi may have resembled something similar to these two different arthropods, which had flaps running along their bodies, likely used for swimming, and well-developed frontal appendages for capturing prey.

“We have to be cautious when speculating about what this animal may have looked like as an adult because it was only a larva and hadn’t yet reached maturity,” Emma says. “So we cannot assume that features absent in the larva were also absent in the adult.”

“However, we can look at what is present in the larva, such as the organ systems, which have allowed us to pinpoint where Youti yuanshi fits in the evolutionary tree. We can then use this to give us an idea as to what the adult might have looked like based on its closest relatives.”

“This has helped us to get a feel for what some of the earliest ancestors of modern arthropods looked like and where they came from.”

Why are arthropods so successful?

Arthropods are the most diverse animal group alive today. They can be found all over the world and are thought to account for over two-thirds of all animal species.

A number of factors have contributed to the success of arthropods. This includes jointed legs that allow them greater mobility on land and a hard exoskeleton that protects them from predators and prevents them from drying out. But it is their very adaptable body plan that is mainly responsible for their incredible success.

Their bodies are divided into segments, which are often repeated with the same set of structures. For example, the trunk of a centipede is made up of many repeating segments that each contain a single pair of legs. Their heads also have appendages that can adapt to suit the way they feed. This has allowed them to modify individual segments to evolve a vast diversity of body plans.

“Arthropods are an incredibly diverse group of animals,” Emma says. “Their evolutionary success is often attributed to their segmented body plan. The basic blueprint of a single segment, which includes a pair of appendages, can be modified into something highly specialised. For instance, limbs become sensory antennae and claws become jaws.”

“This modular adaptability was likely a key driver in the Cambrian Explosion and continues to drive the diversification of arthropods today. This is what has made them such a successful and enduring group through the ages.”