An armored-worm from the Cambrian could be an important missing link

A 518-million-year-old worm resembles the ancestor of three significant subgroups of modern animals.
Christopher McFadden
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A well-preserved fossilized worm that dates back 518 million years has been found by an international team of experts, including researchers from the Universities of Bristol and Oxford and the Natural History Museum. While interesting in and of itself, the new fossil appears to be the ancestor of three major groups of living animals.

The fossil worm called Wufengella, a half-inch (roughly 1.27 cm) long armored worm-like creature, was discovered in China and belonged to an extinct class of shelled animals known as tommotiids. The fossil seemed to indicate that the animal was pretty sturdy in life, given its dense, regularly overlapping array of plates on its back.

A fleshy body with several side-projecting, flattened lobes surrounded the asymmetrical armor. 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 an array of shells on the back.

The results have been published in Current Biology today.

An armored-worm from the Cambrian could be an important missing link
The Cambrian explosion was an interesting time for life.

“It looks like the unlikely offspring between a bristle worm and a chiton mollusk. Interestingly, it belongs to neither of those groups,” Dr. Jakob Vinther, a study co-author from the School of Earth Sciences at the University of Bristol, said.

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.

The new fossil might be a missing link between several extinct and modern phyla

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.

The now-extinct phylum known as brachiopods were a class of animals that, on the surface, resemble bivalves (such as clams) because they have two shells and are rooted on the seafloor, rocks, or reefs. Brachiopods, however, are substantially different in many ways when examined from the inside. They also used a pair of tentacles folded into a horseshoe-shaped organ to filter water.

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An armored-worm from the Cambrian could be an important missing link
The discovered fossil.

The term for such an organ is a lophophore, and brachiopods share it with the phoronids (also known as horseshoe worms) and bryozoans (also known as moss creatures). Brachiopods, bryozoans, and phoronids are each other's closest living relatives, according to morphological evidence and molecular studies that reconstruct evolutionary trees using amino acid sequences.

This group is known as the Lophophorata for its filter-feeding organ.

Co-author Dr. Luke Parry from the University of Oxford added: “Wufengella belongs to a group of Cambrian fossils crucial for understanding how lophophorates evolved. 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."

“We have known for a long time about this tommotiid group called camenellans. Paleontologists have thought that those shells were attached to an agile organism—crawling around—rather than being fixed in one place and feeding with a lophophore,” he added.

The paleontologists 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 added: “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.”

The new fossil solves one important evolutionary mystery

The fossil supports the palaeontological hypothesis that the ancestors of lophophorates were quick, armored-worm-like creatures. Still, it also highlights various theories about how lophophorates might be connected to segmented worms due to the look of their soft anatomy.

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

“We now can see that those similarities are reflections of shared ancestry. The common ancestor of lophophorates and annelids had anatomy closely resembling the annelids," Vinther explained.

An armored-worm from the Cambrian could be an important missing link
The fossil's proposed position in the tree of life.

“At some point, the tommotiid ancestor to the lophophorates became sessile and evolved suspension feeding (catching particles suspended in the water). Then a long, wormy body with numerous, repeated body units became less useful and was reduced,” he said.

Co-author Greg Edgecombe from the Natural History Museum said, “this discovery highlights how important fossils can be for reconstructing evolution.“

"We get an incomplete picture by only looking at living animals, with the relatively few anatomical characters shared between different phyla. With fossils like Wufengella, we can trace each lineage back to its roots, realizing how they once looked altogether different and had very different modes of life, sometimes unique and sometimes shared with more distant relatives,” Edgecombe explained.

Study abstract:

Among extant animals, Lophotrochozoa accounts for the majority of phyla.1 This bilaterian clade radiated rapidly during the Cambrian explosion, obfuscating its phylogenetic relationships and rendering many aspects of its early evolution uncertain. Many early lophotrochozoans are known only from isolated skeletal microfossils, “small shelly fossils,” often derived from larger animals with complex multi-element skeletons.2 The discovery of articulated fossils has revealed surprising insights into the animals from which these skeletal pieces were derived, such as paired shells in the mollusc Halkieria.3 Tommotiids are a key group of phosphatic early skeletal fossils that first appear in the late early Cambrian.4,5 Although their affinities were previously obscure, discoveries of partial scleritomes and investigations of growth and microstructure6 provide links with Brachiopoda7,8 and Phoronida,9 two of the lophophorate phyla. By contrast, the body plan of camenellan tommotiids remains a palaeontological mystery, with hypothetical reconstructions representing motile, benthic, dorsally armored worms.4,10 Here, we describe an articulated camenellan (Wufengella bengtsoni gen. et sp. nov.) from the Cambrian Chengjiang Biota, China, revealing the morphology of the scleritome and the first soft tissues from an adult tommotiid. Wufengella carries two dorsal rows of sclerites in a highly asymmetric arrangement, flanked by smaller, cap-shaped sclerites. The scleritome was fringed by iterated fascicles of chaetae and two layers of flattened lobes. Phylogenetic analysis confirms that camenellans occupy a deep branch in lophophorate phylogeny, prior to the acquisition of a sessile lifestyle. Wufengella reveals direct evidence for a metameric body plan reminiscent of annelids early in the evolutionary history of lophophorates.

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