REDESCRIPTION OF OESIA DISJUNCTA AS A CAMBRIAN ENTEROPNEUST (HEMICHORDATA) AND MARGARETIA DORUS AS ITS ASSOCIATED TUBE

The phylogenetic status of Oesia disjuncta from the Burgess Shale has been ambiguous. First described as an annelid by Walcott in 1911, affinities to tunicates and, more recently, chaetognaths have also been proposed. Here we redescribe Oesia as a primitive enteropneust based on unpublished material from the Walcott Quarry and thousands of new Burgess Shale specimens from Marble Canyon (Kootenay National Park). Furthermore, we interpret dozens of fossil associations that always include a single O. disjuncta specimen preserved within Margaretia dorus as a genuine biological association.

O. disjuncta is almost indistinguishable from living enteropneusts, with an anterior acorn-shaped proboscis, a collar, and a trunk. The heart-kidney-stomochord complex is preserved as a dark and reflective concentration of carbon in the posterior half of the proboscis coelom. A nuchal skeleton extends from the proboscis through the neck and bifurcates in the antero-dorsal collar. The trunk is stout and not as serpentine as in extant enteropneusts or in its Burgess Shale contemporary Spartobranchus tenuis. The pharynx is supported by serially repeated, collagenous U-shaped gill bars for most of the length of the trunk and is followed by a short intestine. The extended pharynx, short intestine and the sessile tubiculous habitat (M. dorus) suggests that O. disjuncta was primarily a filter feeder rather than primarily a deposit feeder like extant forms.

M. dorus was previously regarded as a cylindrical shaped algae sometimes branching with spirally organized papillae. We redescribe it as a perforated tube constructed by O. disjuncta. The tube may have provided a safe refuge from predators, while its pores would have facilitated water exchange for filter feeding. Oesiid worms probably had a wide geographic and stratigraphic distribution during the Cambrian period since M. dorus, presumably more decay resistant than O. disjuncta itself, has also been reported from the Sinsk Formation in Russia (Cambrian Stage 4) and the Wheeler Shale in the USA (Drumian Stage).