Gut Content Analysis

As with the majority of non-biomineralizing fossils from the Burgess Shale, Ottoia prolifica is preserved as compressed aluminosilicate and carbonaceous films [46], [47] (Fig. 2). Ottoia resembles Recent priapulids [48], [49] (Fig. 3) in having a retractile introvert armed with hooks and an invaginable pharynx lined with small teeth, two features of key-importance in locomotion and feeding [50]. The gut of Ottoia appears as a colored or reflective strip of constant width (1.4 to 2.3 mm in specimens 60–100 mm long [21]) running axially from the pharynx to the anus. It is either straight, sinuous or looped. EDX elemental mapping reveals anatomical partitioning of the gut with elevated C, Fe and P that probably reflects its organic-rich original composition and early diagenetic mineralizations in pyrite, apatite or calcite (Fig. 4). More than 50% of the studied specimens possess empty guts (Fig. 1) and about 20% display three-dimensionally preserved gut contents (GC) that preferentially concentrate in the posterior half of their digestive tract. GC typically occur as compacted ribbon-like features or fragmented blobs containing skeletal elements (e.g. hyolithid conchs, brachiopod valves), smaller debris of uncertain origin, and sediment. Thin section, SEM and EDX analyses do not show any significant compositional difference between GC and the aluminosilicate host rock, except from being enriched in organic matter (Fig. 5). Furthermore, acritarchs and sponge spicules found in comparable quantities in GC and the host rock [21] confirm that Ottoia ingested sediment.

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larger image TIFF original image Download: Figure 2. General morphology of Ottoia prolifica from the middle Cambrian Burgess Shale. A, ROM 61780a, high concentration of complete specimens. B–D, ROM 61759, ROM 61752 and ROM 61757, complete specimens. E, F, ROM 61751 and ROM 61765, details of anterior part. G, ROM 61760, details of introvert bearing curved scalids. H, I, ROM 61769 and ROM 61764, details of posterior part showing bursa and posterior hooks. Abbreviations: a, anus; an, trunk annulation; bu, bursa; gu, gut; in, introvert; m, mouth; ph, posterior hook; pt, pharyngeal teeth; px, pharynx; px(e), everted pharynx; px(i), inverted pharynx; sc, scalid; tr, trunk. Scale bar: 1 cm for A–D and 5 mm for E–I. https://doi.org/10.1371/journal.pone.0052200.g002

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larger image TIFF original image Download: Figure 3. General morphology of Recent priapulid worms exemplified by Priapulus caudatus collected from near the Kristineberg Marine Station, Gullmarfjord, Sweden, depth ca. 30 m. A, B, general view of a live specimen in sea water and simplified section through body showing major anatomical features. C, section through pharynx (sclerotized pharyngeal teeth in orange; introvert removed. D, F, frontal view of a slightly everted pharynx showing pentagonal pattern of pharyngeal teeth around mouth opening and details of pharyngeal teeth. E, G, scalid rows along bulbous introvert and details of scalids (tip perforated). H–J, feces of Priapulus caudatus filled with compacted undigested material and enclosed by a transparent membrane, bundles of undigested polychaete chaetae and undetermined gut contents (mainly sediment and detritus of various origin). D–G, I, J, are scanning electron micrographs of dessicated specimens. Abbreviations: a, anus; an, trunk annulation; bu, bursa; ca, caudal appendage; cc, coelomic cavity; fc, feces contents; fm, feces membrane; go, gonads; gu, gut; in, introvert; m, mouth; pm, pharyngeal muscles; pt, pharyngeal tooth; px, pharynx; rm, retractor muscle; sc, scalid; sr, scalid row; tr, trunk. Scale bar: 1 cm for A, B; 5 mm for C; 500 µm for D, E, H; 200 µm for F; 100 µm for G; 10 µm for I; 5 µm for J. https://doi.org/10.1371/journal.pone.0052200.g003

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larger image TIFF original image Download: Figure 4. Elemental mapping of the gut of Ottoia prolifica from the middle Cambrian Burgess Shale. The mapping reveals anatomical partitioning of the gut, with elevated C, Fe and P that probably reflects its organic-rich original composition and early diagenetic mineralizations in pyrite, apatite or calcite. A–E, ROM 61758b. A, B, general view under normal and polarized light (white arrow to indicate mapped area). C, D, back scattered image of gut showing patches of carbonaceous film; this film is interpreted as remains of the gut wall, rather than gut contents. E, elemental mapping. Abbreviations: a, anus; cf, carbonaceous film; gc, gut content; gu, gut; m, mouth. Scale bar: 1 cm for A, B; 5 mm for E; 1 mm for C; 20 µm for D. https://doi.org/10.1371/journal.pone.0052200.g004