An armored worm reveals the ancestors of three groups of animals

Sep. 27 () –

A well-preserved fossil worm dating back 518 million years resembles the ancestor of three major groups of living animals, according to a study published in the journal ‘Current Biology’.

The fossil worm, called Wufengella and unearthed in China by scientists from the universities of Bristol and Oxford, and the Natural History Museum, in the United Kingdom, was half an inch long and a squat creature covered by a dense set of plates that regularly superimposed on his back, and that it belonged to an extinct group of gelatinous organisms called tommotia.

Surrounding the asymmetrical armor was a fleshy body with a series of flattened lobes protruding from the sides. Bundles of bristles protruded from the body between the lobes and the armor. The numerous lobes, the bundles of bristles, and the set of shells on the back are evidence that the worm was originally serial or segmented, like an earthworm.

Study co-author Dr Jakob Vinther, from the University of Bristol’s School of Earth Sciences, comments it’s a statement which “looks like the unlikely offspring between a bristle worm and a chiton mollusk. Curiously, it doesn’t belong to either group.”

The animal kingdom consists of more than 30 major body plans classified as phyla. Each phylum harbors a set of characteristics that differentiate them from one another. Only a few traits are shared by more than one group, which shows the rapid rate of evolution in which these large groups of animals originated, the so-called Cambrian Explosion, about 550 million years ago.

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

This organ is called the lophophore, and brachiopods share the lophophore with two other large groups called phoronids (“horseshoe worms”) and bryozoans (“moss animals”). molecular studies -which reconstruct evolutionary trees from amino acid sequences- They match the anatomical evidence that brachiopods, bryozoans, and phoronids are each other’s closest living relatives, a group named Lophophorata after their filtering organ.

Co-author Dr Luke Parry, from the University of Oxford, adds: “Wufengella belongs to a group of Cambrian fossils that is crucial to understanding how lophophores evolved. They’re called thommotids, and thanks to these fossils we’ve been able to understand how brachiopods evolved to have two shells from ancestors with many shell-like plates arranged in a cone or tube”.

“We have known about this group of tommotians called camenellans for a long time,” he continues. “Paleontologists have thought that these shells were attached to an agile organism – which crawled – instead of 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 Natural History Museum, Oxford University, the Natural History Museum in London and the Muséum national d’Histoire Naturelle in Paris (France) , shows that Wufengella is a complete camenelic thommotid, which means that it reveals what the long-sought worm ancestor of the loforates was like.

Although the fossil fulfills the paleontological prediction that the ancestral lineage of the loforates was an agile and armored worm, the appearance of its soft anatomy reveals some hypotheses about the kinship of the loforates with the segmented worms.

Dr Vinther explains that “biologists have long observed how brachiopods have multiple paired body cavities, unique kidney structures and bundles of bristles on their backs when they are larvae. These similarities led them to observe how closely brachiopods resemble annelid worms.“.

“Now we can see that those similarities are a reflection of shared ancestry. The common ancestor of the lophophores and the annelids had an anatomy very similar to that of the annelids,” he continues. it became sessile and evolved towards suspension feeding (capturing suspended particles in the water). So a long, wormy body with numerous repeating body units became less useful and shrank.”

Co-author Greg Edgecombe of the Natural History Museum said, “This discovery highlights how important fossils can be in reconstructing evolution. We get an incomplete picture if we only look at living animals, with the relatively few anatomical characters that are shared across different phyla With fossils like Wufengella, we can trace each lineage back to its roots, realizing how they once looked totally different and had very different, sometimes unique, ways of life and other times shared with more distant relatives“, he highlights.