Tubular worms from the Burgess Shale

Abstract

The discovery, in 500-million-year-old rocks, of fossil acorn worms that lived in tubes illuminates the debate about whether the ancestor of vertebrates was a mobile worm-like animal or a sessile colony dweller. See Letter p.503 Enteropneusts, or acorn worms, are mud-dwelling creatures that live from the foreshore down to the deep ocean. They are related to pterobranchs, small, colonial, tube-dwelling animals that superficially look completely different. Both are related to echinoderms (starfish and allies) and chordates — the group of animals that includes ourselves. Here Jean-Bernard Caron and colleagues describe fossil enteropneusts from the famous Cambrian Burgess Shale of British Columbia. They look much like some modern enteropneusts, but lived in tubes, more like modern pterobranchs. This adds to the diversity of the fossil fauna and gives pointers to what the common ancestor of enteropneusts and pterobranchs looked like. It might also fuel the debate about whether the ancestor of chordates was a free-living worm, or a sessile, colonial creature.