Pleurotomaria wanderbachi Terquem, 1855

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 : 362-363

Pleurotomaria wanderbachi Terquem, 1855

Fig. 9 .

1855 Pleurotomaria wanderbachi sp. nov.; Terquem 1855: 270, pl. 16: 13, 13a.

1855 Pleurotomaria mosellana sp. nov.; Terquem 1855: 274, pl. 16: 14, 14a.

1855 Pleurotomaria densa sp. nov.; Terquem 1855: 274, pl. 17: 1a, b.

1988 Pleurotomaria mosellana Terquem ; Meier and Meiers 1988: 30–31, pl. 6: 12a, b.

Material.— Five specimens: MNHNL BR 306−2, MNHNL BR 308, MNHNL BR 359, MNHNL BR 432−1, MNHNL BR 677, Brouch, Upper Hettangian ( Schlotheimia angulata Zone , Schlotheimia complanata Subzone ). One specimen: MNHNL GL121, Mamer (Kaatzefiels), Luxembourg Sandstone Formation, stratigraphical level unknown.

Dimensions.—See Table 2.

Description.—Shell trochiform, gradate. Whorls rather high, angulated and with trapezoidal cross−section. Juvenile shell feebly cyrtoconical, adult shell almost conical. Ramp oblique, moderately wide and significantly narrower than the outer whorl face. During the adult growth, the ramp becomes slightly convex and the angulation of the whorls tends to fade out. Sutures impressed. Selenizone running on or slightly above the middle of the outer whorl face. It is flat on the juvenile spire and becomes convex and rather wide on the adult shell. In some specimens the selenizone tends to become flat again on the last growth stage. Periphery subangulated, not exposed on the juvenile spire, becoming exposed on the last whorls. Base flattened with feebly convex surface. Umbilicus absent to very narrow. Aperture subtrapezoidal, slightly larger than high. Peristome discontinuous on the parietal lip. Inner lip robust, reinforced by a callus, slightly prosocline and passing to the basal lip through an evenly rounded curve. Umbilical cavity partly closed by the outward reflection of the callus of inner lip. Parietal lip orthogonal to the columellar lip and covered by a very thin shelly coating. Collabral ornament consists of densely and regularly spaced threads. These are clearly visible especially on the ramp and on the part of the outer whorl face below the selenizone of the juvenile whorls. Angulation of the whorls and peripheral angulation bearing marked nodes. Nodes on the peribasal angulation slightly more numerous and smaller than those on the angulation of the whorls. The angulation of the whorls bears about twenty−five nodes per whorl on the adult shell. On the last whorls, the nodes of the peripheral angulation tend to fade out. Spiral ornament consists of about four to six, relatively strong, widely spaced spiral threads on the ramp and on the part of the outer whorl face above the selenizone. Two to three widely spaced spiral threads run below the selenizone. Selenizone delimited by thin marginal spiral threads. On the juvenile spire, a sharp lira appears along the mid−line of the selenizone which becomes more and more prominent and wide during the adult growth so that the selenizone becomes bulge−shaped. Strong, dense, evenly spaced spiral threads ornament the base. Growth striae feebly prosocline and prosocyrt above the selenizone, slightly opisthocline to orthocline and prosocyrt below the selenizone, widely opisthocyrt on the base.

Remarks.—The material consists of a juvenile and several adult specimens showing a relatively low variability in the shell proportions and sculpture. Pleurotomaria wanderbachi Terquem, 1855 is here synonymised with other two species erected by Terquem (1855) in the same paper, namely Pleurotomaria mosellana Terquem, 1855 and Pleurotomaria densa Terquem, 1855 . The holotype of P. mosellana differs from P. wanderbachi only in having stronger and less numerous nodes, a difference considered here to be included in the intraspecific variability. The holotype of P. densa differs from P. wanderbachi in a more rounded last whorl which tends to lose the nodes. These changes frequently appear during the latest growth of the pleurotomariid species. As a matter of fact, the sequence represented by P. mosellana , P. wanderbachi and P. densa corresponds to the transition from the juvenile to the adult shell of the same species, as also supported by the size of the specimens figured by Terquem (1855).

Pleurotomaria anglica Sowerby, 1815 ( Sowerby 1815: 95, pl. 142) closely resembles P. wanderbachi in the general shape of the shell, in the main elements of the ornament and in the position of the selenizone. However, P. anglica has the outer whorl face and the nodes of the basal angulation ornamented by a pattern of thin and dense spiral lines whereas in P. warderbachi these parts of the shell are sculptured with few and marked spiral threads. Moreover, in P. anglica the selenizone is less prominent.

Stratigraphic and geographic range.—Hettangian, Hettange−Grande (Lorraine, France); Late Hettangian ( Schlotheimia angulata Zone , Schlotheimia complanata Subzone ), Brouch ( Luxembourg).

Figures

Fig. 9. Pleurotomariid gastropod Pleurotomaria wanderbachi Terquem, 1855 from Brouch (Mersch, Grand−Duchy of Luxembourg), Late Hettangian (Schlotheimia angulata Zone, Schlotheimia complanata Subzone), Early Jurassic. A. MNHNL BR359, dorsal (A1), basal (A2), apertural (A3) views, and detail of the apical spire (A4). B. MNHNL BR306−2, lateral (B1) and sublateral (B2) views. C. MNHNL BR432−1, apertural (C1), basal (C2), and dorsal (C3) views. D. MNHNL BR308, apertural (D1), dorsal (D2), and basal (D3) views.