Oncousoecia dilatans ( Johnston, 1847 )

Taylor, Paul D. & Zatoń, Michał, 2008, Taxonomy of the bryozoan genera Oncousoecia, Microeciella and Eurystrotos (Cyclostomata: Oncousoeciidae), Journal of Natural History 42 (39 - 40), pp. 2557-2574 : 2563-2567

Oncousoecia dilatans ( Johnston, 1847)

( Figures 3A–D ; 4A–G ; 5A–C )

Alecto dilatans Johnston 1847, p. 281 , Plate 49, Figures 5 –8.

? Alecto dilatans Thompson : Busk 1875, p. 24, Plate 32, Figure 2 .

Stomatopora dilatans (Johnston) : Hincks 1880, p. 429, Plate 57, Figures 3, 3a .

Oncousoecia dilatans (Johnston) : Hastings 1963, p. 117 (part), non Plate 3, Figures 2 , 3 [5 Oncousoecia lobulata Canu ].

Alecto compacta Norman 1867, p. 204 .

Eurystrotos compacta (Norman) : Hayward and Ryland 1985a, p. 1074 (part), non Figure 1A,B [5 Microeciella suborbicularis (Hincks) ]; Hayward and Ryland 1985b, p. 94 (part), non Figure 32A–D [5 Microeciella suborbicularis (Hincks) ].

non Eurystrotos compacta (Norman) : Hayward and McKinney 2002, p. 116, Figure 55F–H [5 Microeciella suborbicularis (Hincks) ].

Material

Lectotype (here chosen). NHM 47.9.18.59, Sana Island , Mull of Kintyre, UK, encrusting the interior of a small bivalve shell, Johnston Collection, figured by Johnston (1847, Plate 49, Figures 7,8) ( Figures 1A , 3A–D herein).

Other material. NHM 11.10.1.63, Outer Haaf , Shetland, Norman Collection, the type material of Alecto compacta Norman, 1867 , comprising a large, broken scallop shell with several colonies encrusting the interior surface of which the colony depicted here as Figure 1B is chosen as the lectotype of Norman’s species. NHM 11.10.1.64, Shetland, 80–170 fathoms (44–93 m), Norman Collection, several colonies encrusting the interior of a small scallop shell .

Description

Colony encrusting, small, up to about 15 mm in diameter, comprising bifurcating, lobate, flat-topped, oligoserial branches ( Figures 1A, B ; 3A, B ; 4A, B, F ) with autozooids diverging slightly from branch axes and kenozooids developed along sloping lateral edges; branch width variable, typically about 0.6 mm; lateral branches originating at edges of some primary branches. Ancestrula ( Figure 5B ) with large protoecium, about 0.17 mm in diameter, a single row of tiny pseudopores around the perimeter ( Figure 5C ); distal tube short, 0.09–0.15 mm long, straight or curved, aperture 0.05–0.06 mm in diameter; one distal autozooid budded from ancestrula ( Figure 5A ). Growing edge low ( Figure 3B ), basal lamina extending a short distance beyond budding zone with surface microstructure of broad fibres imbricated distally.

Autozooids elongate ( Figure 3C ), 0.36–0.57 mm long by 0.10–0.13 mm wide, frontal walls flat proximally, gently convex distally; pseudopores regularly spaced, small, teardrop-shaped, pointed distally, 4–5 mm wide, lacking spines ( Figure 4G ). Apertures subcircular, 0.06–0.11 mm in diameter, distributed more or less quincuncially, not clustered or connate; preserved peristomes short to moderate in length; terminal diaphragms with scattered pseudopores occluding some apertures and buds at inactive growing edges.

Gonozooids small with short, longitudinally ovoidal to rounded subtriangular brood chambers ( Figure 4C–E ), 0.38–0.45 mm long by 0.30–0.46 mm wide, bulbous, margins indented by neighbouring autozooids but roof not pierced by peristomes; frontal wall densely pseudoporous, the pseudopores identical in shape to those of autozooids; floor pustulose ooeciopore terminal or subterminal, slightly transversely elliptical, smaller than an autozooidal aperture, about 0.04–0.07 mm in diameter; ooeciostome simple, short.

Remarks

As pointed out by Hastings (1963, p. 118), Johnston’s description of A. dilatans was based on a specimen encrusting the outer side of a bivalve, probably that shown in his Plate 49, Figures 5 and 6 , which was loaned to him by W. King of Newcastle-upon-Tyne and came from the coast of Northumberland. The whereabouts of this specimen, if it still exists, is unknown. The specimen here chosen as the lectotype ( Figure 1A ), which is from the west of Scotland, although described as a variety of A. dilatans in Johnston’s figure caption, was listed along with the Northumbrian material in his text without further comment and can therefore be considered as a syntype available to serve as a lectotype. Hastings (1963), who also regarded this specimen as a syntype, noted that it had no gonozooecia. In fact, this is not quite true as there is one underdeveloped (?aborted) gonozooid ( Figure 3D ).

Even in the absence of a complete gonozooid in the lectotype of A. dilatans , it is clear that this species is a senior synonym of A. compacta Norman, 1867 , the type species of Eurystrotos Hayward and Ryland, 1985 (see below). Colony-form in the two species is identical (compare Figures 3A and 4A ), as are the size and morphology of the autozooids and pseudopores.

Figures

Figure 1. Photographs of type specimens of some Recent oncousoeciid cyclostome species. (A) Lectotype of Oncousoecia dilatans (Johnston, 1847) showing colony ramifying across the interior of a small bivalve shell (NHM 47.9.18.59). (B) Lectotype of Alecto compacta Norman, 1867, a subjective junior synonym of O. dilatans (NHM 11.10.1.63). (C) Lectotype of Oncousoecia lobulata Canu, 1918, the colony figured by Hincks (1880, Plate 50, Figure 5, 5a) as Tubulipora lobulata Hassall, 1841 which apparently formed the basis for Canu’s type species of Oncousoecia (NHM 1911.10.1.86). (D) Lectotype of Microeciella suborbicularis (Hincks, 1880), a subcircular colony with associated spirorbids (NHM 99.5.1.1427a). Scale bars: 5 mm (A, D); 1 mm (B, C).

Figure 2. Oncousoecia lobulata Canu, 1918, lectotype (NHM 1911.10.1.86). (A) Lobate, oligoserial branches; (B) autozooids; (C) gonozooid; (D) gonozooidal pseudopores showing partial occlusion by spines. Scale bars: 1 mm (A); 200mm (B, C); 50 mm (D).

Figure 3. Oncousoecia dilatans (Johnston, 1847), lectotype (NHM 47.9.18.59). (A) Oligoserial branches close to colony origin; (B) slightly oblique view of branches showing growing tips; (C) autozooids; (D) incompletely-formed gonozooid represented by a depression (left of centre) and opening into the fertile zooid. Scale bars: 1 mm (A); 500 mm (B); 200 mm (C, D).

Figure 4. Oncousoecia dilatans (Johnston, 1847). (A, C, D) Lectotype of Alecto compacta Norman, 1867 (NHM 11.10.1.63); (A) oligoserial branches close to colony origin; (C) gonozooid; (D) another gonozooid illustrating within-colony variability in shape. (B, G) Colony sharing the same substrate as the lectotype (NHM 11.10.1.63); (B) mass of branches overgrowing colony origin; (G) frontal wall of autozooids showing pseudopores and a larger boring. (E, F) Bleached topotype colony (NHM 11.10.1.64); (E) gonozooid; (F) two branches with intact and broken gonozooids. Scale bars: 1 mm (A); 500 mm (B, F); 200mm (C, D, E); 50 mm (G).

Figure 5. Comparison of early astogeny in (A–C) Alecto compacta (Norman, 1867) [5Oncousoecia dilatans (Johnston, 1847)] (NHM 11.10.1.64) and (D–F) Microeciella suborbicularis (Hincks, 1880), two species formerly considered synonymous. (A) Ancestrula and first budded zooids; (B) ancestrula with short distal tube; (C) large protoecium with tiny pseudopores around perimeter (best seen top right); (D) fan-shaped young colony; (E) ancestrula with long distal tube; (F) protoecium with a few, non-perimetrical pseudopores and two larger, enigmatic punctures or borings (left). Scale bars: 500 mm (D); 200mm (A); 100 mm (B, E); 50 mm (C, F).

Figure 6. Microeciella beliensis Taylor and Sequeiros, 1982, holotype from the Lower Jurassic of Spain (NHM D53321a). (A) Subcircular colony; (B) gonozooid; (C) another gonozooid; (D) early astogeny with partly overgrown ancestrula at centre; (E) autozooidal frontal wall showing pseudopores. Scale bars: 1 mm (A); 200 mm (B, C, D); 50 mm (E).