The well-preserved fossil of a 518-million-year-old worm helps trace the origins of three animal phyla. The newly discovered fossil worm Wufengella bengtsoni had a segmented, armored body with laterally projecting bristles. Anatomical studies show that it is a primordial relative of the brachiopods, bryozoans and horseshoe worms - three animal phyla whose evolutionary relationships have so far been a mystery.
Biologists divide the animal kingdom into phyla, each with a number of unique characteristics that distinguish them from other animal phyla. Certain traits are found across multiple tribes. For example, there are three animal tribes that feed with the help of a so-called lophophore, a ring-shaped, tentacled organ with which they swirl water and the food particles contained in it into themselves. These include the brachiopods (Brachiopoda), which look like shells; the bryozoa, so called because the tiny marine creatures clump together to resemble a carpet of moss; and the horseshoeworms (Phoronida), which anchor themselves in burrows in the seabed. Although the evolutionary relationship of these animal strains has not yet been fully clarified, they are grouped together as Lophophorata because of the lophophore found in all representatives.
Early Cambrian fossil
A team led by Jin Guo from Yunnan University in China has now taken a decisive step towards the common origin of these three animal phyla. In the Chengjiang faunal community, an Early Cambrian fossil site in southwest China, they discovered a fossil worm that turned out to be an ancestral relative of the three animal phyla. "When I first realized what this fossil I was looking at under the microscope was, I couldn't believe my eyes," says co-author Luke Parry of the University of Oxford. "We have often speculated about this fossil and hoped that one day we would see it."
The worm, which the researchers named Wufengella bengtsoni and is believed to be around 518 million years old, has an asymmetric carapace made up of overlapping plates. Below was a fleshy body with a series of flattened lobes protruding from the sides. Bunches of bristles protruded from the body between the lobes and the carapace. Overall, the worm is about 16 millimeters long. "He looks like the unlikely offspring of a cross between a bristleworm and a chiton mollusk. Interestingly, however, he does not belong to either of these two groups,” says co-author Jakob Vinther from the University of Bristol.
Relationship with ringworms
Instead, it appears to be a representative of the Camenellan tommotiids, a group of invertebrates that lived in the early Cambrian and is considered the ancestral group of brachiopods and horseshoeworms. Parry explains: “Wufengella belongs to a group of Cambrian fossils that is crucial for understanding the evolution of Lophophorata. Thanks to these fossils, we now understand how ancestors with many shell-like plates evolved into brachiopods, which have only two conch-like shells.”
Unlike today's representatives, who anchor themselves sedentarily to the seabed or to surfaces, Wufengella was motile. The clearly segmented physique suggests an evolutionary relationship with the annelids, which also include today's earthworm. Biologists had previously noticed similarities between the larvae of brachiopods and annelids. "Now we can see that these similarities stem from a common ancestry," says Vinther. "The common ancestor of lophophorates and annelids had an anatomy most similar to annelids."
From mobile to sedentary
According to the researchers, the worm-like ancestor of the lophophorates developed a sedentary lifestyle over the course of evolution and developed a diet in which it whirls food particles towards itself with the water. "A long, worm-shaped body with many body units then became less useful and was therefore reduced," explains Vinther.
"This discovery shows how important fossils can be in reconstructing evolution," says co-author Greg Edgecombe of the Natural History Museum in London. “We get an incomplete picture just looking at the living animals, with the relatively few anatomical features shared between the different phyla. With fossils like Wufengella, we can trace each lineage back to its roots and see that they once looked very different and had very different ways of life, sometimes unique and sometimes shared with more distant relatives.”
Source: Jin Guo (Yunnan University, China) et al., Current Biology, doi: 10.1016/j.cub.2022.09.011