Christopher Cameron of the University of Montreal's Department of Biological Sciences and his colleagues reported the discovery of a fossil relative of the acorn worm family in Canada’s Burgess Shale in the journal Nature on March 13, 2013, that pushes back the known fossil record in the area by 200 million years.
Spartobranchus tenuis was confirmed to be a member of the acorn worms group that is closely related to today's sea stars and sea urchins through the hemichordates group.
The fossil specimen is remarkably similar to modern day acorn worms. Spartobranchus tenuis had fibrous tubes that the researchers claim to be the distinguishing mark of hemichordate evolution that separated the two main branches: the enteropneusts and the pterobranchs, including graptolites. The tubes may have served as dwelling structures for the animals.
Spartobranchus tenuis was a filter feeder with a pharynx perforated with gill slits. The worms averaged 10 centimeters in length, had a flexible body consisting of a short proboscis, collar and narrow elongate trunk terminating in a bulbous structure, which may have served as an anchor. The paleontologists found thousands of Spartobranchus tenuis fossils in the Walcott Quarry in Yoho National Park.
The research establishes the first molecular evidence that enteropneusts and pterobranchs are closely related and evolutionary diversification began in the Cambrian period.