Meiersia disarmata, Monari & Valentini & Conti, 2011

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 : 375-376

Meiersia disarmata sp. nov.

Fig. 16B, C .

1988 Asperilla calcar (Orbigny) ; Meier and Meiers 1988: 35, pl. 8:

18a–c.

Etymology: From Latin disarmata , without arms; referred to the absence of spines on the peripheral flange.

Type material: Holotype: specimen MNHNL BR 358 ( Fig. 16B ) . Paratypes: six specimens: MNHNL BR 758, MNHNL BR 869, MNHNL BR 870, MNHNL BR 872, MNHNL BR 873, MNHNL BR 874 .

Type locality: Brouch (Mersch, Grand−Duchy of Luxembourg) .

Type horizon: Luxembourg Sandstone Formation, Upper Hettangian ( Schlotheimia angulata Zone , Schlotheimia complanata Subzone ).

Dimensions.—See Table 2.

Diagnosis.—Base provided with a smooth median spiral angulation which almost disappears during the last growth stages. Peripheral flange densely crenulated, without spines.

Description.—Sinistral shell with flat spire, consisting of about five whorls. Exposed surface of whorls feebly and evenly convex, delimited by slightly incised sutures. Last whorls slightly bending downward during the final growth. Periphery marked by a sharp, prominent, slightly upward curved flange which is covered by the previous whorls on the spire and becomes slightly exposed on the last whorl. Base strongly convex, widely phaneromphalous. Median band of the surface of the base marked by a spiral angulation which tends to disappear on the last whorl. Umbilical edge bearing a smooth, rounded angulation. Basal angulation running nearly at the mid−distance between the periumbilical angulation and the peripheral flange. Aperture rounded−trapezoidal. Peristome angulated at the peripheral flange. Basal angulation ending on the peristome at the junction of the outer lip with the basal lip. Peristome feebly angulated by the periumbilical angulation at the junction of the inner lip with the basal lip. Apart from the peripheral flange the shell is essentially smooth. Outer rim of peripheral flange densely crenulated and lacking spines. Very shallow, wrinkle−shaped collabral ribs can originate from the crenulations of the peripheral flange. They tend to fade out towards the suture, and on the peripheral band of the base. Growth lines very strong, prosocline and distinctly prosocyrt on the whorl surface, opisthocyrt on the base, acutely backward angular on the peripheral flange.

Remarks.—The variability of this species concerns the angulations of the base and the elements of the ornament. Both the basal and periumbilical angulations vary in strength. The collabral ribs are of variable strength and distribution.

Meiersia disarmata sp. nov. is very close to Meiersia doi:10.4202/app.2010.0098

calcar (Orbigny, 1853) , from the Upper Pliensbachian deposits of Calvados ( France). However, Orbigny’s (1853) species differs from M. disarmata in the peripheral flange, which has long, widely and evenly spaced spines (seven to eight on the last whorl). Moreover, in the holotype of M. calcar , the basal angulation is absent or obscure.

Stratigraphic and geographic range.—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.