Siboglinidae Caullery, 1914

Georgieva, Magdalena N., Little, Crispin T. S., Watson, Jonathan S., Sephton, Mark A., Ball, Alexander D. & Glover, Adrian G., 2019, Identification of fossil worm tubes from Phanerozoic hydrothermal vents and cold seeps, Journal of Systematic Palaeontology 17 (4), pp. 287-329 : 297-301

?Family Siboglinidae Caullery, 1914 (?vestimentiferan)

‘Omagari tubes’

( Fig. 8 )

2003 possible vestimentiferan worm tubes Hikida, Suzuki, Togo, & Ijiri: 336, figs 3.1, 4.1, 4.4, 4.5, 5, 8.

Material. Many small, similar-diameter tubes preserved together. OMG03-4a, pinkish calcite with many worm tubes with brown walls ( Fig. 8A, B ). OMG03-4b, brown-walled tubes observed in thin section. OMG03-1, many tubes in muddy, crumbly matrix ( Fig. 8C, D ). OMG03-2, OMG03-3a, OMG03-3b, many tubes with light-coloured walls preserved in hard cement. Donated by Y. Hikida.

Occurrence. Omagari, Nakagawa-cho region, north-western Hokkaido, northern Japan (44 º 39.58 ' N, 142 º 2.22 ' E). Approximately 10 m wide seep carbonate deposit, Omagari Formation, Yezo Supergroup, Campanian, Cretaceous ( Hikida et al. 2003; Majima et al. 2005; Kiel et al. 2008a).

Description. Non-agglutinated tubes, many of which are partially or fully replaced by silica or siderite, resulting in a dark brown-black or reddish colour, respectively. The tubes do not appear to be branching ( Fig. 8A–D ), and have walls that, where visible, appear smooth ( Fig. 8B, D ). Tube diameters range from 0.8 to 2.7 mm, and tubes do not appear to taper distinctly along their lengths. Tube walls are uncompressed, suggesting that they may have originally been rigid, and are multi-layered ( Fig. 8E, F ). Tube walls are generally not very thick, apart from in a subset of tubes in which the walls are thick and exhibit many layers ( Fig. 8F ). Preserved tears of the tube wall reveal an originally fibrous nature ( Fig. 8G ).

Remarks. These tubes were interpreted as those of vestimentiferans by earlier work ( Hikida et al. 2003), but are difficult to identify owing to their lack of ornamentation. They fall among siboglinids when more homoplasy is permitted in the cladistic analysis ( Fig. 23B ). The tubes were clearly organic, due to preserved wall tears that reveal a fibrous nature. Transverse ornamentation typical of some frenulates or chaetopterids is absent, and the size and clumped nature of the tubes, combined with walls that are sometimes thick and neatly multi-layered, suggests that the Omagari tubes are most likely the roots/posterior portions of vestimentiferan tubes. The morphology of Omagari tube clumps very closely resembles a clump of the roots of the seep vestimentiferan Lamellibrachia luymesi ( Fig. 8H, I ). We therefore tentatively suggest that the Omagari tubes may be vestimentiferan tube roots.

Figures

Figure 8. A–H, ‘Omagari tubes’, Campanian, Hokkaido, Japan; A, OMG03-4a, hand specimen with many, similar-diameter tubes with brown walls encased in carbonate; B, OMG03-3b, detail of tube with brown wall, outer wall appears smooth; C, D, OMG03-1, cluster of tubes (C) with mineralized walls but not encased in carbonate, and detail of individual tubes (D); E, OMG03-2, tubes with brown walls in section; F, OMG03-4b, large-diameter tube in transverse section with wall comprised of many layers; G, OMG03-1, preserved tear in the wall of a tube suggesting an originally fibrous nature; H, clump of Omagari tubes (reproduced from Hikida et al. 2003). I, clump of the roots of the seep vestimentiferan Lamellibrachia luymesi (donated by C. Fisher). Scale bars: A = 10 mm; B = 1 mm; C = 5 mm; D, E = 2 mm; F = 500 µm; G = 50 µm; H, I = 20 mm.

Figure 23. Strict consensus cladograms constructed using a total of 64 modern and fossil annelid taxa and 48 mostly morphological tube characters. Analyses were performed using implied character weighting, with the concavity constant set as default (k = 3; A), and also set to downweight homoplastic characters less (k = 4; B). Numbers on nodes represent groups present/contradicted support values. Modern taxa are coloured according to taxonomic groups; fossil taxa are in grey. A, consensus of 271 most parsimonious trees (best score = 15.387, consistency index = 0.195, retention index = 0.264); B, consensus of 60 most parsimonious trees (best score = 13.568, consistency index = 0.232, retention index = 0.569). Symbols/colours indicate taxonomic affinities.