Oesia possesses the canonical enteropneust body plan of proboscis, collar and elongate
trunk (Figs. 1, 2a, b) but is unusual in that posterior to the pharynx there is a bilobed structure, rather
than a vermiform intestine. Body length averages 53 mm (?=?187, size range 2.4–120 mm), but the width seldom exceeds 10 mm. The proboscis
is relatively elongate (ratio of length to width is 1.35?±?0.58) and variable in shape
(Figs. 2a, d, e, g, h; Additional files 1A, D–F, 2A, F–I, 3C, 4). A conspicuous ovoid area at the medial base of the proboscis appears darker or
more reflective than the surrounding area (Fig. 2a–c, f; Additional files 2F–I, 3C). This is interpreted as the heart-kidney-stomochord complex 11]. More irregular structures across the proboscis probably represent decayed musculature
(Fig. 2c; Additional file 2F–I). The collar is rectangular, but with rounded edges (Fig. 2a–c, f, g; Additional files 1A, D–F, 2F–I, 3C, 5D, E, G). In proportion it is shorter than the proboscis (average ratio is 0.39?±?1.12)
but has an equivalent width (average proboscis to collar width is 1.08?±?0.23 mm).
At the posterior margin of the collar (Fig. 2b, c, f; Additional file 2F–I), a dark or reflective band probably represents the circum-collar ridge, while
a thin, longitudinal structure between the proboscis base and collar (Fig. 2d, e; Additional file 5A, B, D–I) is interpreted as the nuchal skeleton. The pharyngeal region houses a series
(about 3 bars/mm) of approximately U-shaped gill bars (Fig. 2g–j; Additional file 5A, C) but is remarkable in that it occupies approximately 80 % of the trunk length
(Fig. 2a, g; Additional files 2A, C, 5D, E, G). The posterior end of the trunk is bulbous (Fig. 2b, g; Additional files 1D–E, 2H, 4, 5D, E, G) and sometimes terminates in a bilobed structure (Fig. 2a, f; Additional files 1B, C, 2A–D, F, H) that is usually wider than long (average width-to-length ratio is 1.48?±?0.63).
Fig. 1. Schematic anatomy of Oesia disjuncta. Co: collar, Cr: circum-collar ridge, Dg: digestive groove, Pr: proboscis, Hks: heart-kidney-stomochord
complex, Gb: gill bars, Gp: gill pores, Mo: mouth, Po: pores, Ps: posterior structure,
Tr: trunk, Tu: tube. Dashed lines indicate transverse cross sections
Fig. 2. General morphology of Oesia disjuncta from the Burgess Shale. (Specimens in d, e and j come from the Walcott Quarry; all other specimens come from Marble Canyon).a Note bilobed posterior structure and extended pharyngeal area (ROM 63737, part and
counterpart are superimposed at the dashed line). b, c Tripartite body plan and internal organs in the proboscis (ROM 63711). d, e Large proboscis and possible nuchal skeleton (USNM 509815), see also Additional file
5A–C. f Well-developed bilobed posterior structure (ROM 63713). g–i Details of the pharyngeal area (h, partial counterpart of g, highlighted by vertical dashed line; i is close-up of framed area in g, ROM 63710). j Left and right pairs of gill bars preserved in lateral view (USNM 277844). Direct
light images: a, b, h; polarized light images: c–g, j; SEM image: i. Co: collar, Cr: circum-collar ridge, Dg: digestive groove, Dm: dorsal midline, Gb:
gill bars, Hks: heart-kidney-stomochord complex, Ll: lateral side left, Lr: lateral
side right, Ns: nuchal skeleton, Pr: proboscis, Ps: posterior structure, Tr: trunk.
Scale bars: a?=?10 mm, b–e?=?1 mm, f–h?=?5 mm, i?=?500 ?m, j?=?2 mm
The preservation of Oesia (?=?45 in Marble Canyon, ?=?6 in Raymond Quarry) inside Margaretia (now a junior synonym of Oesia disjuncta) suggests an original association (Fig. 3a; Additional file 6). Only single worms are found within tubes, suggesting a solitary mode of life, although
due to breakage during transport, it is conceivable that tubes may have been inhabited
by more than one worm (Fig. 3b–d). Typically the tube is at least twice the width of the worm, suggesting the worm
could move freely within its dwelling (Fig. 3e–j; Additional files 3B, 7, 8). Three-dimensional preservation of both sides of the tube (Fig. 4d, e) shows that the internal cavity of the tube was spacious, and that the tube was at
least semi-rigid. The total length and extremities of the tubes are poorly known.
This is because of either prior breakage or concealment (Fig. 4a), but at least one end (presumably the top of the tube) appears rounded and closed
(Additional file 9B, C).
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, mO. 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
Fig. 4. a, b Spirally arranged pores perforate the tube (ROM 63716; see also Additional file 9A). c Two examples of multiple bifurcation points in a single specimen. Extreme size variation
underscores the fragmentary nature of most tubes (left: KUMIP 204373, right: KUMIP 241392). d, e Tube showing three-dimensional preservation. d Large section of the tube has been broken off revealing the other side of the tube.
e The broken segment has been placed back in its original configuration to illustrate
the three-dimensionality of the tube (KUMIP 147911). f, g Close-up of the pores and fibrous texture of the tube. Individual fibres are micrometre
small (ROM 63705). Bi: node of bifurcation, Fe: fibrous elements, Lo: lower surface,
Po: pores, Up: upper surface, Wo: worm, other acronyms see Fig. 2. Scale bars: a, b, f, g?=?5 mm, c–e?=?10 mm
Tubes with irregular undulations and lacking the spiral pattern were previously interpreted
as prostrate subterranean rhizomes (Fig. 4.2-3 18]). While the reassignment from alga to organically produced tube invalidates this
identification, it remains plausible that subterranean, lateral extensions of the
tube could serve as an anchor. In any individual the width of the tube is usually
consistent along the length, but otherwise it varies considerably (4–20 mm). Occasionally
a tube shows one (Fig. 3i; Additional files 7A, C, 8A–C, 9D–F) or, more rarely, two bifurcations (Fig. 4c). Each bifurcates at approximately the same angle and has the same width as the primary
tube. The tube wall is perforated by spirally arranged pores (about 10 openings per
revolution; Fig. 4a, b). In a single tube pore size varies. Some may be almost closed, but others have diameters
equivalent to about a third of the tube width (Fig. 4a, b, d, e; Additional file 9A). Pore shape varies from circular to oblong ellipse and rhombic. That these might
simply be taphonomic variations is less likely given that the specimens are preserved
parallel to the bedding plane (Fig. 4a, b, g–e; Additional file 9A). The margins of the pores tend to be raised, imparting a semi-corrugated texture
to the external surface of the tube (Fig. 4a, b; Additional file 9A). The tube is composed of narrow fibres (about 7 ?m) that are braided and/or overlain
in bundles (Fig. 4f, g).
Margaretia dorus is unlike any known species of Paleozoic algae. In particular, the combination of
a fibrous composition and elaborate pore architecture are inconsistent with an algal
grade of organization, as are its biotic associations and size in relation to well-established
Cambrian macroalgae 19]. This in turn argues against Oesia being an example of inquilinism. While the dozens of co-occurrences of O. disjuncta and its tube strongly suggest an original association, the preservation of large
numbers of isolated Oesia specimens on single bedding surfaces (Additional files 3, 4) at Marble Canyon also needs an explanation. One possibility is that the association
was facultative and Oesia could alternate between a tubicolous and non-tubicolous existence. Alternatively
the worm may have been forced to vacate the tube as an en masse evacuation prior to
final burial. This may be related to both the high-energy burial events 17] and the resultant dysoxic conditions that such events create 20], although this hypothesis is weakened by the lack of obvious exit structures (i.e.
there is no evidence the worms could enter or leave the tubes at either end).
In this context, fragmentation of the tubes and dispersal during transport is perhaps
a more plausible explanation as to how the worms became isolated. This appears to
be reasonable given the observation that although tubes with a length of up to 544 mm
are known (Fig. 4c), tubes of comparable width can be not only significantly shorter (e.g. Figs. 3b, 4c), but sometimes are even smaller than the worms themselves. A related observation
is that along the tube margins showing evidence for breakage, the bundles of fibres
may exhibit a pattern of ’unbraiding.’ This suggests that originally the tubes were
vulnerable to damage (Fig. 3b).
The second factor is that in at least some cases the tube evidently serves to conceal
the worm. For a worm to be readily visible, the tube either needs to be prepared mechanically,
split more or less along the axis or be sufficiently degraded so as to allow a view
of the interior. Accordingly, tubes showing such evidence of degradation also contain
worms in an evident state of decay (Fig. 3b–h). In such cases worms are poorly preserved and are effectively reduced to a narrow
band of reflective carbon (Fig. 3k–m). Worms in such late stages of decay also show a tendency to bend at sharp angles
into semi-discrete sections (Figs. 2g, 3e, f, l, m). This appearance may represent adjacent sets of gill bars maintaining their articulation
through attachment to the collagenous basal lamina, but at points where this basal
lamina has degraded, the more acutely angled bending occurs 11].