Ptychomphalus rotellaeformis ( Dunker, 1847 )

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 : 368-369

Ptychomphalus rotellaeformis ( Dunker, 1847)

Fig. 12C .

1847 Pleurotomaria rotellaeformis sp. nov.; Dunker 1847: 111–113, pl. 13: 12a–e.

1855 Pleurotomaria rotellaeformis Dunker ; Terquem 1855: 272–273, pl. 16: 11a, b.

1907 Cryptaenia rotellaeformis Dunker [sic]; Sieberer 1907: 24–25, pl. 2: 4.

1988 Ptychomphalus rotellaeformis (Dunker) ; Meier and Meiers 1988: 32–33, pl. 7: 15a, b.

Material.— Two specimens: MNHNL BR 364, MNHNL BR 703, Brouch, Upper Hettangian ( Schlotheimia angulata Zone , Schlotheimia complanata Subzone ).

Dimensions.—See Table 2.

Description.—Shell low turbiniform, tending to lenticular, consisting of four moderately convex teleoconch whorls. Suture slightly impressed. Periphery formed by a somewhat rounded angulation, not exposed on the spire. Selenizone flush, moderately narrow, placed on the upper part of the peripheral band and with its lower edge running on the peripheral line. Labial slit short, extending about one eighth of the length of the last whorl. Base convex, with swollen surface and depressed axial region. Aperture sublanceolate, with prosocline, discontinuous peristome. Outer lip thin. Inner lip robust, covered by a callosity. The callus extends to the axial depression of the base where it forms a concave pad. Shell smooth. Selenizone delimited by very thin marginal spiral lines. Growth striae thin and scarcely visible, prosocline and prosocyrt on the spire, prosocyrt below the selenizone.

Remarks.— Ptychomphalus rotellaeformis ( Dunker, 1847) is a species known from Upper Hettangian deposits of western Europe. It is characterised by a slightly angulated periphery and a smooth callus that completely fills and flattens the axial depression of the base.

The species was frequently quoted by previous authors. However, some of them refer to specimens likely belonging to other species. For example, the material illustrated by Gründel (2003: 3, pl. 1: 1–3) clearly differs from P. rotellaeformis in the presence of a prominent sutural band which forms a shallow spiral bulge. Moreover, the axial depression of the base bears a strong basal callus and is encircled by an outer spiral furrow. These characters are present also in the specimen ascribed by Dareste de la Chavanne (1912: 384, pl. 17: 2) to Pleurotomaria (Cryptaenia) rotellaeformis . They are reminiscent of Pleurotomaria dewalquei Terquem and Piette, 1865 ( Terquem and Piette 1865: 59, pl. 4: 24, 25), although the presence of an outer furrow around the basal cavity is unclear in P. dewalquei .

The specimen ascribed by Sieberer (1907: 24, pl. 2: 4) to Cryptaenia rotellaeformis differs from the type material of Dunker’s (1847) species in having a slightly more globose shape of the shell. This difference is here considered to represent species variability.

Other quotations of P. rotellaeformis (e.g., Quenstedt 1858; Dumortier 1864, 1867; Onetti 1915; Szente 1992) are not supported by sufficient data and/or concern poorly preserved specimens.

Ptychomphalus heliciformis ( Eudes−Deslongchamps, 1849) is a Late Pliensbachian species very similar to P. rotellaeformis . Despite its rather different stratigraphical distribution, P. heliciformis was synonymised by Orbigny (1854) and Sieberer (1907) with Dunker’s (1847) species. However, Fischer and Weber (1997: 155, pl. 25: 15, 16) considered it as a distinct species. These authors noted that the type material is lost and established a neotype and a paraneotype. These differ from P. rotellaeformis in a more rounded periphery and more convex whorl surface giving to the shell a slightly more turbiniform and less lenticular shape. Moreover, according to Fischer and Weber (1997), P. heliciformis has a more acute spire angle with respect to that of P. rotellaeformis . However, an extensive analysis of the known data concerning the two species seems to demonstrate a morphological continuity of these characters. Fischer and Weber (1997) mentioned also differences in the shape of the callus on the base, but they neither described them nor illustrated the basal view of the type material of P. heliciformis . According to the illustration made by Eudes−Deslongchamps (1949: 149, pl. 17: 2b) and Szabó (2009: 25, fig. 19B), in P. heliciformis the basal cavity is deeper and narrower than that of P. rotellaeformis . Moreover the callus on the axial depression of the base is thinner. It is not clear whether these differences are sufficient to keep the two species distinct. As underlined by Szabó (2009), only a full revision of all the material ascribed by previous authors to these species could give answers.

Stratigraphic and geographic range.—Early Hettangian ( Psiloceras planorbis Zone ), Halberstadt (Saxony−Anhalt, Germany); Hettangian, Hettange−Grande (Lorraine, France); Late Hettangian ( Schlotheimia angulata Zone , Schlotheimia complanata Subzone ), Brouch ( Luxembourg); Late Hettangian ( Schlotheimia angulata Zone ), Swabia (southern Germany).

Figures

Fig. 12. Pleurotomariid and ptychomphalid gastropods from Brouch (Mersch, Grand−Duchy of Luxembourg), Late Hettangian (Schlotheimia angulata Zone, Schlotheimia complanata Subzone), Early Jurassic. A, B. Anodomaria schroederi sp.nov. A – Holotype, MNHNL BR342, dorsal (A1) and apical (A2) views. B – Paratype, MNHNL BR871, dorsal (B1), lateral (B2), and apical (B3) views. C. Ptychomphalus rotellaeformis (Dunker, 1847), MNHNL BR364, apertural (C1), dorsal (C2), basal (C3), apical (C4), and lateral (C5) views.