An artists reconstruction of Wufengella in black and white. A flattened slug-like animal facing the viewer, with a number of small, swirling, scale-like shells running along its back, and tufts of bristles coming out the sides of the body. Underneath these bristles, you can just about see a feshy lobe running along the entire side of the body.

Living some 518 million years ago, Wufengella would have looked unlike anything alive today ©Bob Nicholls

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An ancient hairy, armoured worm lived in the oceans over 500 million years ago

A tiny half-inch long fossil from China has filled in a major gap in the evolutionary tree for life on Earth.

Looking like a hairy, armoured worm, Wufengella would have crept around the oceans some 518 million years ago. 

For a few tens of millions of years, many of the animals that lived on Earth were largely worm and jellyfish-like creatures.

But around 550 million years ago something changed. During a period of roughly 20 million years, known as the Cambrian explosion, life in the oceans started to rapidly diversify, with a whole range of innovative body plans suddenly appearing in the fossil record.

Figuring out how these different body plans are related has been a challenge, but the fossil from China of a hairy armoured worm has now bridged one of those gaps. Called Wufengella, the fossil worm reveals what the ancient ancestor of three major groups of animals would have looked like.

Dr Greg Edgecombe is a palaeontologist at the Museum whose work has focused on this critical point in the evolution of animals.

'This discovery highlights how important fossils can be for reconstructing evolution. We get an incomplete picture by only looking at living animals; even though we can draw on evidence from molecular biology to infer how different phyla are related to each other, there are relatively few anatomical characters that are shared between phyla.'

'With fossils like Wufengella, we can trace each lineage back to its roots and find combinations of anatomical features that weren’t anticipated by living species.'

The paper describing Wufengella has been published in Current Biology.

The fossil of Wufengella, which is almost cigar-like in shape, with criss-crossing lines visable along its entire length.

At only half an inch long the fossil might not look like much, but it fills in a significant gap in evolutionary history ©Guo et al. 2022

An explosion of life

All species in the entire animal kingdom fall into roughly 30 distinct body plans. Known as 'phyla', these different body arrangements involve a set of features that define the individual animal groups, such as the dorsal nerve chord in vertebrates or segmented body parts of insects and other jointed-legged animals.

It is believed that many of these defined body plans first appeared around the same time some 550 million years ago during the Cambrian explosion. At this point in Earth's history, there was a rapid flurry of evolution as early life in the oceans diversified from more simple animals into the ancestors of everything we see today, from molluscs to fish.

What caused this sudden shift is still not really known, although it was likely a combination of factors such as changes to the climate and chemistry of the oceans.

Figuring out what the animals looked like before the rapid dash of evolution is tricky. The extreme age and delicate nature of these ancient critters all compromise their chance of being preserved in the fossil record.

But now researchers have been able to fill in at least one of these gaps, uniting three of the defined phyla with a common ancestor that more closely resembles phyla outside this trio than anything previously known.

Unearthed in China, at first glance the half-inch fossil of Wufengella might not look like much. Yet it sheds light on one of the most critical periods in the evolution of animal life. Dating to 518 million years old, the little cigar-shaped worm has a curious mix of features.

Dr Luke Parry from the University of Oxford and a co-author of this study, says, 'When it first became clear to me what this fossil was that I was looking at under the microscope, I couldn't believe my eyes.'

'This is a fossil that we have often speculated about and hoped we would one day lay eyes on.'

A drawing of the fossil, highlighting some of the key features, including the patterns created by the shells that ran down its back and the bristles along its side.

By examining the fossil in extraordinary detail, the researchers were able to build up a much clearer picture of what Wufengella looked like, including the bristles that ran down the side of its body ©Guo et al. 2022 

Hairy, armoured worm

Understanding Wufengella's place in evolutionary history requires looking at three distinct groups of animals living in the oceans today: brachiopods, bryozoans and horseshoe worms.

Brachiopods might look superficially like bivalve molluscs, such as oysters, but they evolved the hinged shells that protects their soft bodies entirely independently. Found in oceans around the world, they attach themselves to the seafloor, rocks and reefs, and feed using a pair of tentacles that emerge from the shells and then fold back up into a U-shaped organ known as the 'lophophore'.

Bryozoans, on the other hand, superficially look like coral but again are only distantly related to these animals. Made up of a colony of small animals which come together to create a number of different forms, such as fans or sheets, the individual animals also feed in the water column using a lophophore.

Finally, horseshoe worms are not dissimilar to bryozoans except the animals live on their own in little tubes. It is from these tubes that the tiny animals emerge, thrusting their tentacular lophophore into the water to filter out tiny particles of food.

United by this U-shaped feeding organ, scientists knew that these three groups of animals must have been related, but were unsure what their last common ancestor would have looked like or how it linked to other invertebrate phyla.

That is where Wufengella steps in. The fossils show that this small creature was a freely-creeping worm-like animal with a number of unusual characteristics unlike anything seen today. Its body was covered in a pattern of swirling scale-like shells, flattened lobes projecting from the side of its body, and a fringe of bristles filling in between those lobes.

The shells, lobes and clusters of bristles all have a one-to-one repetition along the body, showing that Wufengella was segmented.

'Wufengella belongs to a group of Cambrian fossils that are crucial for understanding how lophophorates evolved,' explains Luke. 'They're called tommotiids, and thanks to these fossils we have been able to understand how brachiopods evolved to have two shells from ancestors with many shell-like plates arranged into a cone or tube.'

What is more, since Wufengella was segmented it means that the team of researchers can unite the brachiopods, bryozoans and horseshoe worms with another major phylum of animals, the segmented worms, known as annelids. Annelids and lophophorates share another feature that Wufengella also shows, clusters of bristles known as chaetae.

One tiny fossil has just had an outsized impact on how we understand the history of life on Earth.