Fig. 3

Margaretia dorus tubes and associations with Oesia disjuncta from the Burgess Shale. Specimens in a and d come from the Raymond Quarry; all other specimens come from Marble Canyon. (a–h) Taphonomic gradient of the worm inside its tube from generally poorly preserved (a) to better preserved (h); the tubes tend to preserve more poorly at Marble Canyon relative to tubes from the Raymond Quarry showing similar amounts of decay of the worm. a Holotype of M. dorus with worm preserved as a dark/reflective band along the central axis of the tube (USNM 83922). b, c Small fragments of tubes containing worms showing only few recognizable features (b: ROM 63955, c: ROM 63956). d Part of a tube excavated to reveal a poorly preserved worm inside (ROM 63715). e Tripartite body plan recognizable but worm heavily decayed (ROM 63953). f Clear posterior structure but indistinct proboscis and trunk (ROM 63957). g Poorly preserved trunk and faded tube (ROM 63952). h Close-up of framed area in g on counterpart, showing gill bars readily visible. i, j Specimen showing clear tripartite body plan and evidence of gill bars (ROM 63715). k The extant acorn worm Saccoglossus pusillus after 48 hours of decay at 25 °C showing dissociated parts, although the tripartite body plan is still recognizable. l, m O. disjuncta outside of its tube, showing extreme signs of decay comparable with k. Direct light (l) is contrasted with polarized light (m) to reveal different aspects of fossil morphology (ROM 63954). The ectoderm is fraying off, the proboscis is indistinct and the trunk has lost turgidity. Most worms preserved inside their tubes show a similar level of preservation. Direct light images: a, b, d, l; polarized light images: c, e–i, m. Bi: node of bifurcation, Fe: fibrous elements, Wo: worm, other acronyms see Figs. 1 and 2. Scale bars: a–c, e–g, k–m = 10 mm, d, i = 5 mm