An article published in the journal “Current Biology” describes a research on Kootenayscolex barbarensis, a bristle worm (class Polychaeta) that lived about 508 million years ago in the site of today’s Marble Canyon, in the Burgess Shale, Canada. Karma Nanglu and her supervisor Jean-Bernard Caron of the University of Toronto and the Royal Museum of Ontario studied this very ancient annelid. It’s a sea animal although it’s related to the current earthworms. The most important thing is that its discovery provides new information on the origin of the annelids’ head.
The annelid phylum (Annelida) includes many modern animals such as ragworms, leeches and earthworms. The bristle worms class is one that includes generally sea species. The great variety of species of annelids made it difficult to understand what their primitive ancestors might have looked like, a common problem in the field of paleontology but even more serious in the case of annelids. They’re in fact soft-bodied animals with the consequence that their fossils are rare.
In the case of bristle worms, the fossils are mainly of jaws of some species and mineralized tubes. In many cases fossils have been found with some similarities with bristle worms belonging to what’s called Ediacaran biota but their classification is very uncertain. There are a lot of arguments around these fossils because it’s difficult to establish their relationships with reasonable certainty.
Fossils of other species of ancient annelids foundi in the past were in a state of conservation far from perfect. Their study suggested that their heads were indistinguishable from the rest of their bodies but it was impossible to establish it with reasonable certainty. The situation changed with the new discoveries made in the Burgess Shale, one of the world’s best known sites for very well-preserved fossils.
During the last century, several fossil deposits dating back to the Cambrian period (between 541 and 485 million years ago) have been discovered in the Burgess Shale. In 2012, one was discovered in which over 500 fossils of annelids were found, including the species Kootenayscolex barbarensis.
This bristle worm shows the presence of the appendages called chaetae on its head. This convinced Karma Nanglu and Jean-Bernard Caron that the annelids’ head evolved from their body’s posterior segments, which also had similarly positioned chaetae.
These fossils of Kootenayscolex barbarensis are so well preserved that sediments in their guts could also be examined. The result suggests that these ancient bristle worms recycled organic materials from sediments like their modern cousins. These materials returned to the food chain thanks to other animals that fed on these annelids.
Kootenayscolex barbarensis was a tiny animal, so much so that the largest specimen is about 2.5 centimeters (1″) long, and the annelids are not the organisms that stimulate the greatest interest in the public. However, they play their part in the ecosystems in which they live and this research suggests they already did it half a billion years ago.