Pleurotomaria sp.

Monari, Stefano, Valentini, Mara & Conti, Maria Alessandra, 2011, Earliest Jurassic patellogastropod, vetigastropod, and neritimorph gastropods from Luxembourg with considerations on the Triassic-Jurassic faunal turnover, Acta Palaeontologica Polonica 56 (2), pp. 349-384 : 366

Pleurotomaria sp.

Fig. 11D .

Material.— One specimen: MNHNL GL344 , Burmerange , Hettangian–Lower Sinemurian, (exact stratigraphical level uncertain) .

Dimensions.—See Table 2.

Description.—Trochiform, slightly coeloconical, gradate shell. Whorls with subtrapezoidal cross−section. Ramp moderately oblique and feebly convex, limited by a subrounded outer angulation. Outer whorl face subvertical, convex on the last whorl. Selenizone wide, flush, running slightly below the middle of the outer whorl face. Periphery marked by a rounded angulation. Base slightly convex. Apertural region not preserved. Ornament made of spiral and collabral threads. Five to six, widely spaced and sharp spiral threads ornament the ramp. Two to three relatively strong spiral threads run on the surface of the whorls between the selenizone and the periphery. Collabral threads densely spaced and of uneven size. Angulation of the whorl surface bearing small and numerous nodes which attenuate during the adult growth. Peripheral angulation ornamented with small nodes, slightly elongated and prosocline on the base surface and disappearing during the growth of the last whorl. Selenizone bordered by moderately thin, sharp marginal spiral threads. It bears a submedian lira which becomes relatively prominent on the last whorls. Base ornamented with strong, sharp and relatively widely spaced spiral threads. Growth lines prosocline and widely prosocyrt on the ramp, orthocline and prosocyrt on the whorls surface between the selenizone and the periphery, opisthocyrt on the base.

Remarks.—This specimen is poorly preserved. The shell wall is partly eroded and the apertural region is strongly incomplete. Its stratigraphical position is uncertain. As for the previous species, the sediment embedding the shell is a grey marl suggesting that the specimen comes from the Elvingen marl (lowermost Hettangian) or from the Strassen marl and limestone (Lower Sinemurian).

The specimen is reminiscent of that figured by Orbigny (1854: 396, pl. 346: 1–4) as Pleurotomaria anglica ( Sowerby, 1815) . It differs in the position of the selenizone, which runs slightly below the middle of the outer whorl face, whereas in Orbigny’s (1854) specimen the selenizone is placed above it. Moreover, the nodes of the ornament are slightly less prominent.

Figures

Fig. 11. Pleurotomariid gastropods from Hettangian–Early Sinemurian, Early Jurassic of Grand−Duchy of Luxembourg. A. Pleurotomaria hennocquii Terquem, 1855, MNHNL BR881, dorsal (A1), lateral (A2), apical (A3) views, and detail of the ornament (A4), Brouch (Mersch, Grand−Duchy of Luxembourg), Late Hettangian (Schlotheimia angulata Zone, Schlotheimia complanata Subzone). B, C. Pleurotomaria cognata Chapuis and Dewalque, 1853. B – MNHNL HE197−1, apertural (B1), basal (B2), and dorsal (B3) views. C – MNHNL HE197−2, dorsal (C1) and basal (C3) views, Burmerange, exact stratigraphical level uncertain. D. Pleurotomaria sp., MNHNL GL344, dorsal (D1), apertural (D2), and basal (D3) views, Burmerange, exact stratigraphical level uncertain.