Scientists from the Universities of Bristol and Oxford and the Natural History Museum have found that a fossilized worm from 518 million years ago looks like the ancestor of three major groups of animals that are still alive today.
The fossil worm, named Wufengella and discovered in China, was a stout creature covered in a dense, regularly overlapping array of plates on its back. It belonged to an extinct group of shelled organisms known as tommotiids.
Encircling the asymmetrical armor was a fleshy body with a series of protruding, flattened lobes. In between the lobes and the armor, bundles of bristles emerged from the body. The many lobes, groups of bristles, and array of shells on the back of the worm show that, like an earthworm, it was once segmented or serialized.
The findings were published today in the journal Current Biology. Dr. Jakob Vinther from the University of Bristol’s School of Earth Sciences stated, “It appears to be the improbable offspring of a bristle worm and a chiton mollusk. Interestingly, it does not belong to either of these categories. ”
The animal kingdom comprises more than thirty phyla, or major body plans. Each phylum possesses characteristics that distinguish it from the others. Only a few traits are shared by more than one group. This is evidence of how quickly these major animal groups evolved about 550 million years ago, during what is called the Cambrian Explosion.
Brachiopods are a phylum that superficially resembles bivalves (such as clams) in having two shells and living on the seafloor, rocks, or reefs. However, upon closer inspection, brachiopods are remarkably distinct in numerous ways. In fact, brachiopods use a pair of tentacles folded into a horseshoe-shaped organ to filter water.
This organ is known as a lophophore, and brachiopods share it with two other major groups: the phoronids (also known as “horseshoe worms”) and bryozoans (“moss animals”). Molecular studies, which use amino acid sequences to build evolutionary trees, agree with anatomical evidence that brachiopods, bryozoans, and phoronids are the closest living relatives of each other. Together, they make up a group called Lophophorata, after the organ they use to filter food.
Dr. Luke Parry from the University of Oxford added, “Wufengella belongs to a group of Cambrian fossils that are essential to comprehending the evolution of lophophorates. These fossils, called tommotiids, have helped us figure out how brachiopods went from having many shell-like plates arranged in a cone or tube to having two shells.
“We have long known about the tomotoxic group known as the camenellans. Palaeontologists have theorized that these shells were attached to a mobile, crawling organism as opposed to a stationary one that fed from a lophophore.
The team, comprised of palaeontologists from the University of Bristol, Yunnan University, the Chengjiang Museum of Natural History, the University of Oxford, the Natural History Museum in London, and the Muséum national d’Histoire naturelle de Paris, demonstrates 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 realized what this fossil was that I was observing under the microscope. This is a fossil about which we have often speculated and hoped to see one day.
The fossil proves that the ancestor of lophophorates was a fast, armored worm. It also sheds light on hypotheses about the relationship between lophophorates and segmented worms by showing how soft its body was.
Dr. Vinther stated, “Biologists have known for a long time that brachiopod larvae have multiple, paired body cavities, distinct kidney structures, and bundles of bristles on their backs. They observed that brachiopods resemble annelid worms because of these similarities.
“We can now see that these similarities are the result of common ancestry. The common ancestor of lophophorates and annelids had a body shape that was most like that of annelids.
“At some point, the tommotiid ancestor of the lophophorates evolved suspension feeding and became sessile” (catching particles suspended in the water). Then, a lengthy, worm-like body with numerous, repeated body units became ineffective and was diminished.
Greg Edgecombe, a co-author from the Natural History Museum, stated, “This discovery demonstrates the significance of fossils for reconstructing evolution.”
By examining only living animals with the relatively few anatomical characteristics shared between different phyla, we obtain an incomplete picture. With fossils such as Wufengella, we are able to trace each lineage back to its origins, realizing that they once had radically different appearances and lifestyles, sometimes unique and sometimes shared with more distant relatives.
