A well-preserved fossilized worm from 518 million years ago looks like the ancestor of three major groups of living animals

An international team of scientists, including from the universities of Bristol and Oxford, and the Natural History Museum, have found that a well-preserved fossilized worm dating from 518 million years ago resembles the ancestor of three large groups of living animals.

Measuring half an inch long, the fossil worm – named Wufengella and unearthed in China – was a stocky creature covered in a dense, evenly overlapping network of plates on its back, belonging to an extinct group of shellfish organisms called tommotiids.

Surrounding the asymmetrical armor was a fleshy body with a series of flattened lobes protruding from the sides. Bundles of hair emerged from the body between the lobes and the armor. The numerous lobes, bundles of hairs, and array of shells on its back are evidence that the worm was originally serialized or segmented, like an earthworm.

The results are published today in the journal Current biology. Study co-author Dr Jakob Vinther from the University of Bristol’s School of Earth Sciences said: “It looks like the unlikely offspring between a silkworm and a chiton snail. Interestingly, he does not belong to any of these groups. »

The animal kingdom consists of over 30 major body planes classified into phyla. Each phylum harbors a set of characteristics that distinguish them from each other. Only a few features are shared by more than one group, reflecting the very rapid rate of evolution in which these major groups of animals originated, called the Cambrian Explosion, around 550 million years ago.

Brachiopods are a phylum that superficially resemble bivalves (like clams) in having a pair of shells and living attached to the seabed, rocks, or reefs. However, looking inside, brachiopods turn out to be very different in many ways. In fact, brachiopods filter water using a pair of tentacles folded into a horseshoe-shaped organ.

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Such an organ is called a lophophore and the brachiopods share the lophophore with two other major groups called the phoronids (“horseshoe worms”) and the bryozoans (“moss animals”). Molecular studies – which reconstruct evolutionary trees using amino acid sequences – agree with anatomical evidence that brachiopods, bryozoans and phoronids are each other’s closest living relatives, a group called Lophophorata according to their filtering organ.

Co-author Dr Luke Parry of the University of Oxford added: ” Wufengella belongs to a group of Cambrian fossils that is crucial to understanding how lophophorates evolved. They are called tommotiids, and through these fossils we have been able to understand how brachiopods evolved to have two ancestor shells with many shell-like plates arranged in a cone or tube.

“We have known about this group of tommotiids called camenellans for a long time. Paleontologists thought these shells were attached to an agile – crawling – organism rather than being fixed in one place and feeding on a lophophore. »

The team, made up of paleontologists from the University of Bristol, Yunnan University, the Chengjiang Museum of Natural History, the University of Oxford, the Natural History Museum in London and the National Museum of Natural History of Paris, demonstrates that Wufengella is a complete tommotiid camenellan, meaning it reveals what the long-sought wormy ancestor of the lophophorates looked like.

Dr Parry added: “When I first realized what this fossil was that I was looking at under the microscope, I couldn’t believe my eyes. It’s a fossil we’ve often speculated on and hoped we might one day lay our eyes on. »

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While the fossil meets the paleontological prediction that the ancestral line of lophophorates was an agile, armored worm, the appearance of its soft anatomy sheds light on some hypotheses about how lophophorates may be related to segmented worms.

Dr Vinther said: “Biologists had long noted how brachiopods had multiple paired body cavities, unique kidney structures and bundles of hair on their backs as larvae. These similarities led them to notice how much brachiopods resemble annelid worms. »

“We can now see that these similarities are a reflection of a shared ancestry. The common ancestor of lophophorates and annelids had an anatomy most resembling annelids.

“At some point, the tommotiid ancestor of lophophorates became sessile and evolved by suspension feeding (capturing suspended particles in water). Then a long, worm-like body with many repeating body units became less useful and shrunk. »

Co-author Greg Edgecombe of the Natural History Museum said: “This discovery highlights the importance of fossils for reconstructing evolution.

“We get an incomplete picture by looking only at living animals, with the relatively few anatomical characters that are shared across different phyla. With fossils like Wufengellawe can trace each line back to its roots, realizing how they once looked completely different and had very different ways of life, sometimes unique and sometimes shared with more distant relatives. »