Trochotoma, Eudes-Deslongchamps, 1843

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 : 374

Trochotoma clypeus Terquem, 1855

Fig. 16A .

1855 Trochotoma clypeus sp. nov.; Terquem 1855: 268, pl. 16: 9. 1934 Ditremaria (Ditremaria) clypeus (Terquem); Haber 1934: 332–333.

Material.— Two specimens: MNHNL BR 124, MNHNL BR 305−1, Brouch, Upper Hettangian ( Schlotheimia angulata Zone , Schlotheimia complanata Subzone ).

Dimensions.—See Table 2.

Description.—Trochiform, low conical−gradate shell consisting of about five whorls. Early whorls strongly convex. Adult whorls bearing a rounded angulation delimiting a flat to slightly convex, relatively broad and feebly oblique ramp. Outer whorl face flat to slightly concave. Periphery exposed on spire and marked by a strong spiral bulge. Sutures impressed, running along the mid−line of the peripheral bulge. Selenizone slightly elevated, placed at the angulation of the whorls. Ornament made of dense, somewhat strong and subregularly sized spiral threads separated by thin interspaces. Three to four threads run on the ramp and four to five on the outer whorl face. Selenizone corresponding to a duplicated spiral thread which is slightly wider than the spiral threads of the ornament. Peripheral bulge bearing thin spiral threads. Ramp ornamented also by fairly prosocline, unevenly sized and spaced collabral riblets. Growth striae strongly prosocline and widely prosocyrt on the spire.

Remarks.—Although the specimens have the base embedded into the sediment, they correspond well to the description and illustration of Trochotoma clypeus Terquem, 1855. The only difference concerns the size which is smaller in Terquem’s (1855) type specimen which, therefore, most probably represents a juvenile shell. According to Terquem (1855), T. clypeus shows a slightly concave base with funnel−like umbilicus. This is a typical feature of the species belonging to the genus Trochotoma Eudes−Deslongchamps, 1843 .

Trochotoma clypeus differs from Trochotoma vetusta Terquem, 1855 in having a smaller size, a more obtuse apical angle, and more rounded juvenile whorls. Moreover, in T. clypeus the ramp is sculptured with a stronger collabral ornament.

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

Figures

Fig. 16. Trochotomid and turbinid gastropods from Brouch (Mersch, Grand−Duchy of Luxembourg), Late Hettangian (Schlotheimia angulata Zone, Schlotheimia complanata Subzone), Early Jurassic. A. Trochotoma clypeus Terquem, 1855, MNHNL BR305−1, lateral (A1), apical (A2), and dorsal (A3) views. The arrow indicates the position of the selenizone.B, C. Meiersia disarmata sp. nov. B. Holotype, MNHNL BR358, apertural (B1), dorsal (B2), basal (B3), and apical views. C. Paratype, MNHNL BR873, apical (C1), basal (C2), and dorsal (C3) views.