Armored worm reveals the ancestry of three major animal groups

A well-preserved fossilized worm from 518 million years ago has been found by an international team of scientists, including researchers from the Universities of Bristol and Oxford and the Natural History Museum, to resemble the ancestor of three significant groups of living creatures.

The fossil worm, known as Wufengella and discovered in China, was a stubby creature with a half-inch long body covered in a dense, regularly overlapping array of plates on its back. It belonged to an extinct class of shelled animals known as tommotiids.

A fleshy body with several side-projecting flattened lobes surrounded the asymmetrical armour. Between the lobes and the armor, bundles of bristles protruded from the body. The worm was previously serialized or segmented, like an earthworm, as evidenced by the numerous lobes, bundles of bristles, and array of shells on the back.

The results have been published in the journal Current Biology today. Dr. Jakob Vinther, a study co-author from the School of Earth Sciences at the University of Bristol, said: "It appears to be the improbable progeny of a bristle worm and a chiton mollusk. It's interesting that it doesn't belong to either of those.

There are more than 30 phyla, or primary body plans, in the animal kingdom. Each phylum possesses a unique set of traits that distinguishes it from the others. Only a few characteristics are shared by multiple groups, which is evidence of the extremely rapid rate of evolution that gave rise to these main animal groupings during the Cambrian Explosion, which occurred around 550 million years ago.

Brachiopods are a class of animals that, on the surface, resemble bivalves (such as clams) because they have two shells and are rooted to the seafloor, rocks, or reefs. Brachiopods, however, show themselves to be substantially different in many ways when examined from the inside. In reality, brachiopods use a pair of tentacles folded into a horseshoe-shaped organ to filter water.

This type of organ is known as a lophophore, and the phoronids (also known as "horseshoe worms") and bryozoans are two additional important species that share it with brachiopods ("moss animals"). Brachiopods, bryozoans, and phoronids, collectively known as the Lophophorata for its filter-feeding organ, are each other's closest living cousins, according to molecular studies that reconstruct evolutionary trees using amino acid sequences.

Dr. Luke Parry, a co-author from the University of Oxford, added: "Wufengella is part of a critical group of Cambrian fossils for comprehending the evolution of lophophorates. They are known as tommotiids, and it is because of these fossils that we are able to comprehend how brachiopods changed from having many shell-like plates grouped into a cone or tube to having two shells.

"We have long known about this tommotiid group known as the camenellans. Instead than being anchored in one location and feeding with a lophophore, paleontologists believed that those shells were attached to an agile organism that was crawling around.

The group of palaeontologists from the Universities of Bristol, Yunnan, Oxford, the Natural History Museum in London, and the Muséum national d'Histoire Naturelle in Paris show that Wufengella is a complete camenellan tommotiid, revealing the appearance of the long-sought-after wormy ancestor to lophophorates.

Dr. Parry continued, "I couldn't believe my eyes when I first realized what this fossil was that I was looking at under the microscope. We had often wondered about this fossil and hoped to someday see it in person.

The fossil supports the palaeontological hypothesis that the ancestor of lophophorates was a quick, armored worm, but it also highlights various theories about how lophophorates might be connected to segmented worms due to the look of its soft anatomy.

Brachiopods have several, paired body chambers, distinctive kidney structures, and bundles of hairs on their back as larvae, according to Dr. Vinther. They were aware of how much brachiopods resemble annelid worms because of these similarities.

We now understand that the commonalities are a result of ancestry. The annelids' anatomy was most similar to that of the lophophorates' common ancestor.

"At some time, the tommotiid ancestor of the lophophorates evolved suspension feeding and became sessile" (catching particles suspended in the water). A lengthy, wormlike body with numerous, repetitive body units was reduced and became less effective at that point.

Greg Edgecombe, a co-author from the Natural History Museum, said: "This discovery emphasizes the use of fossils in reconstructing evolution.

"By focusing just on live creatures and the relatively few physical traits that are shared by various phyla, we only receive a partial picture. When we trace each lineage back to its origins using fossils like Wufengella, we can see how they formerly had very distinct appearances and extremely varied lifestyles, sometimes unique and other times shared with more distant relatives.

University of Bristol