Earlier this year, in March, Nature reported that soft-bodied worms from the Burgess Shale fossil beds in Canada, given the name Spartobranchus tenuis, have been identified as ancient examples of acorn worms. They were hailed as a “missing link” in the vertebrate family tree: “a crucial evolutionary link between two distinct living groups of animals: enteropneusts and pterobranchs.” The evidence supporting this was said to be the tubes constructed by Spartobranchus tenuis. Living enteropneusts (acorn worms) do not have tubes, whereas living pterobranchs (minute colonial organisms) do. Professor Simon Conway Morris affirmed the significance of the newly discovered fossil tubes with these words: “By finding enteropneusts in tubes we begin to bridge this evolutionary gap.” At the time, these issues were discussed in a blog here, and questions were raised about the evolutionary narrative. More now needs to be said, as a recent paper in Nature Communications has documented modern tube-forming acorn worms found in Antarctic benthic communities.
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