Placostegus Philippi, 1844

Słowiński, Jakub, Vinn, Olev, Jäger, Manfred & Zatoń, Michał, 2022, Middle and Late Jurassic tube-dwelling polychaetes from the Polish Basin: diversity, palaeoecology and comparisons with other assemblages, Acta Palaeontologica Polonica 67 (4), pp. 827-864 : 849

Genus Placostegus Philippi, 1844

Type species: Serpula tridentata ( Fabricius, 1779) ; Recent, Greenlan- dic part of the Arctic Ocean.

Placostegus planorbiformis (Münster in Goldfuss, 1831)

Fig. 11A .

1831 Serpula planorbiformis sp. nov.; Münster in Goldfuss 1831: 231, pl. 68: 12a, b.

1956 Serpula (Tetraserpula) planorbiformis Münster 1831 ; Parsch 1956: 225, pl. 19: 10, pl. 20: 1, 2.

Material.—Seven relatively well-preserved specimens encrusting sponges from the lower Oxfordian (Upper Jurassic) of Zalas, Polish Jura; GIUS 8-3746.

Description.—Tubes medium-sized, planispirally (sinistrally) and tightly coiled; diameter of the tube up to 2.5 mm, while the diameter of the entire spiral is ca. 10 mm. Spiral leaves only a narrow but deep central umbilicus open; however, coiling is evolute, leaving parts of the inner whorls well visible and countable. The tubes consist of four whorls increasing moderately rapidly in diameter. Tubes attached to the substrate along most of their length, except for the anterior part which may be attached predominantly to the previous whorl. The tube base widened. Cross-section subtriangular due to the presence of an explicit keel running uniformly along the entire tube length and forms a spine above the aperture. The anteriormost part projecting upwards; possibly it grew vertically (see Radwańska 2004); however, this is not evident in the material studied. The tube wall thick.

Remarks.—The studied specimens are very similar to those described by Radwańska (2004) as Placostegus conchophilus ( Radwańska, 2004) . However, P. conchophilus possesses a rather convex outer wall, whereas in P. planorbiformis the cross-section is roof-shaped, topped by a low, consistent keel.

Stratigraphic and geographic range. —The material studied herein comes from lower Oxfordian (Upper Jurassic) of Zalas, Polish Jura. P. planorbiformis is also common in the Oxfordian (Upper Jurassic) of South Germany (e.g., Parsch 1956; MJ own observation).

Genus Pseudovermilia Bush, 1907

Type species: Spirobranchus occidentalis ( McIntosh, 1885) ; Recent , The Bermuda Archipelago .

Figures

Fig. 11. Serpulid polychaetes from the Jurassic of Poland. A. Placostegus planorbiformis (Münster in Goldfuss, 1831) encrusting a sponge fragment from the Oxfordian of Zalas (GIUS 8-3746/8). B. Pseudovermilia sp. encrusting a rock fragment from the Callovian of Zalas (GIUS 8-3589/15). Scale bars 1 mm.