Wufengella bengtsoni, an extinct species of tommotiid worm that lived during the Cambrian period, resembles the ancestor of three major groups of living animals.
Wufengella bengtsoni lived in what is now China during the Cambrian period, approximately 518 million years ago.
The ancient animal belonged to an extinct group of shelly organisms called tommotiids.
Measuring only 1.3 cm (0.5 inches) long, it was a stubby creature covered in a dense, regularly overlapping array of plates on its back.
Surrounding the asymmetrical armor was a fleshy body with a series of flattened lobes projecting from the sides. Bundles of bristles emerged from the body in between the lobes and the armor.
The many lobes, bundles of bristles and array of shells on the back are evidence that the worm was originally serialised or segmented, like an earthworm.
“It looks like the unlikely offspring between a bristle worm and a chiton mollusk. Interestingly, it belongs to neither of those groups,” said Dr. Jakob Vinther, a researcher with the School of Earth Sciences at the University of Bristol.
The animal kingdom consists of more than 30 major body plans categorized as phyla. Each phylum harbors a set of features that set them apart from one another.
Only a few features are shared across more than one group, which is a testament to the very fast rate of evolution during which these major groups of animals originated, called the Cambrian explosion, about 550 million years ago.
Brachiopods are a phylum that superficially resemble bivalves — such as clams — in having a pair of shells and living attached to the seafloor, rocks or reefs.
However, when looking inside, brachiopods reveal themselves to be very different in many respects. In fact, brachiopods filter water using a pair of tentacles folded up into a horseshoe-shape organ.
Molecular studies agree with anatomical evidence that brachiopods, bryozoans and phoronids are each other’s closest living relatives, a group called Lophophorata after their filter-feeding organ.
“Wufengella bengtsoni belongs to a group of Cambrian fossils that’s crucial for understanding how lophophorates evolved,” said Dr. Luke Parry, a researcher with the University of Oxford.
“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.”
Wufengella bengtsoni is a complete camenellan tommotiid, revealing what the long sought-after wormy ancestor to lophophorates looked like.
“While the fossil fulfils the paleontological prediction that the lophophorates’ ancestral lineage was an agile, armored worm, the appearance of its soft anatomy brings into focus some hypotheses about how lophophorates may be related to segmented worms,” Dr. Parry 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,” Dr. Vinther added.
“We now can see that those similarities are reflections of shared ancestry. The common ancestor of lophophorates and annelids had an anatomy most closely resembling the annelids.”
“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.”
The findings appear today in the journal Current Biology.
Jin Guo et al. A Cambrian tommotiid preserving soft tissues reveals the metameric ancestry of lophophorates. Current Biology, in press;