Ancestry of Three Major Animal Groups Revealed by 518-Million-Year-Old Armored Worm
Ancestry of Three Major Animal Groups Revealed by 518-Million-Year-Old Armored WormA well-preserved fossilized worm dating from 518-million-years-ago resembles the ancestor of three major groups of living animals.
Named Wufengella and unearthed in China, the fossil worm measures about half an inch long. It was a stubby creature covered in a dense, regularly overlapping array of plates on its back. It belongs to an extinct group of shelly organisms called tommotiids.
Surrounding the asymmetrical armor of the worm was a fleshy body with a series of flattened lobes projecting from the sides. In between the lobes and the armor, bundles of bristles emerged from the body. The many lobes, bundles of bristles, and the array of shells on the back are evidence that the worm was originally serialized or segmented, like an earthworm.
The animal kingdom consists of more than 30 major body plans categorized as phyla. Each phylum contains a unique 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. This was during a period called the Cambrian Explosion, approximately 550 million years ago.
Brachiopods are a phylum that superficially resembles bivalves (such as clams) in that they have a pair of shells and live attached to the seafloor, rocks, or reefs. However, when looking inside, brachiopods reveal themselves to be significantly different in many respects. In fact, brachiopods filter water using a pair of tentacles folded up into a horseshoe-shaped organ.
Such an organ is called a lophophore and brachiopods share the it with two other major groups called the phoronids (“horseshoe worms”) and bryozoans (“moss animals”). Molecular studies – which reconstruct evolutionary trees using amino acid sequences – agree with anatomical evidence that brachiopods, bryozoans, and phoronids are each other’s closest living relatives. This is a group called Lophophorata after their filter-feeding organ.
“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.”
The team, which consists of paleontologists from the University of Bristol, Yunnan University, the Chengjiang Museum of Natural History, University of Oxford, the Natural History Museum in London, and the Muséum national d’Histoire Naturelle in Paris, demonstrate that Wufengella is a complete camenellan tommotiid. This means that it reveals what the long sought-after wormy ancestor to lophophorates looked like.
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.”
While the fossil fulfills the palaeontological 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. 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 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.”
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 that are 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.”