Pleurotomaria hettangiensis 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 : 363-365

Pleurotomaria hettangiensis Terquem, 1855

Fig. 10A–D .

1855 Pleurotomaria hettangiensis sp. nov.; Terquem 1855: 273, pl. 17: 2a, b.

?1907 Pleurotomaria princeps (Koch and Dunker) ; Sieberer 1907: 12–13, pl. 1: 2a, b.

1988 Pleurotomaria hettangiensis Terquem ; Meier and Meiers 1988: 29–30, pl. 5: 11a, b.

Material.— Five specimens: MNHNL BR 294, MNHNL BR 304, MNHNL BR 375−1, MNHNL BR 443, MNHNL BR 676, Brouch, Upper Hettangian ( Schlotheimia angulata Zone , Schlotheimia complanata Subzone ). One specimen MNHNL GL281, Hespérange, Upper Hettangian.

Dimensions.—See Table 2.

Description.—Shell trochiform with a moderately gradate outline. Juvenile spire conical, composed of relatively high whorls. Ramp rather narrow and appearing from about the third whorl. Adult spire cyrtoconical, with an oblique and relatively narrow ramp delimited by an obtuse angulation. Ramp flat to slightly concave. Surface of the outer whorl face slightly inclined and flat. Sutures moderately impressed. Periphery angulated and slightly swollen. Base narrowly umbilicated and with a feebly convex surface. Selenizone rather wide, running at or slightly below the middle of the outer whorl face. It is convex and bulge−shaped in the adult shell. Aperture subtrapezoidal, wider than high, with discontinuous peristome on the parietal lip. Inner lip robust, inclined, and orthogonal to the basal lip. Inner lip callus relatively strong, outward reflected and partly closing the umbilical cavity. Parietal region covered by a very thin shelly coating. Ornament of the first observable whorls seemingly faint. Adult ornament consisting of spiral threads, collabral riblets and nodes at the angulation of the whorls and at the peripheral angulation. Spiral threads sharp and well spaced. Three spiral threads ornament the ramp. Two to three spiral threads run on the outer whorl face either above and below the selenizone. Collabral riblets feeble, prosocline on the ramp. They start from nodes on the angulation of the whorls and disappear before reaching the upper suture. The nodes at the angulation of the whorls are slightly prosocline and do not extend to the outer whorl face; those on the peripheral angulation are smaller, more elongated and opisthocline. Last whorls with about thirty nodes on the upper angulation. Selenizone almost smooth on the adult shell, delimited by sharp, marginal spiral lines. Base ornamented with dense spiral threads. Irregularly spaced and sized collabral ribs, made by strong and sinuous growth striae, arise from the peribasal nodes and tend to disappear during the last growth. Growth striae feebly prosocline and prosocyrt above the selenizone, slightly opisthocline to orthocline and prosocyrt below the selenizone, widely opisthocyrt on the base.

Remarks.—In Terquem’s collection housed in the UCBL, the specimen indicated as the holotype of Pleurotomaria hettangiensis Terquem, 1855 is somewhat different from the original illustration given by the author ( Terquem 1855: 273, pl. 17: 2) and from the specimens described herein. Indeed, it exhibits more numerous and smaller nodes and sharper collabral ribs. Moreover, during the growth of the last whorl the outer rim of the ramp becomes rounded and tends to lose the nodes. The analysis of the material from Luxembourg re−

doi:10.4202/app.2010.0098

veals that these differences are representative of the intraspecific variability and of the adult ontogenetic changes of P. hettangiensis , respectively.

Pleurotomaria hettangiensis Terquem, 1855 differs from most of the other Jurassic relatives in having a much less gradate shell owing to a narrower ramp. Consequently, the general shape of the shell tends to be conical. For this reason, it shows some similarities with Pleurotomaria princeps Koch and Dunker, 1837 ( Koch and Dunker 1837: 26, pl. 1: 18). However, in P. princeps , the whorls lack a ramp and, consequently the shell is definitely conical. Moreover, the selenizone is lower on the whorl surface than in P. hettangiensis . In agreement with Fischer and Weber (1997: 156, 160, pl. 34: 10, 11), P. princeps should be assigned to the genus Pyrgotrochus Fischer, 1885 .

Joly (1936: 75, pl. 1: 2) ascribed to P. hettangiensis some conical, not gradate shells that most probably should be assigned to P. princeps . In contrast, the specimen figured by Sieberer (1907) as P. princeps has a gradate shell and an ornament pattern similar to those of P. hettangiensis . It differs from the type material of Terquem’s (1855) species and from the specimens described here in having a sharper ornament and a wider umbilicus. These differences could fall within the variability of P. hettangiensis .

Sacchi Vialli (1964: 5, pl. 1: 7–9) reviewed the gastropod fauna from the Sinemurian of Lombardy (northern Italy) published by Parona (1893) and synonymised Pleurotomaria oblita Parona, 1893 ( Parona 1893: 166, pl. 6: 4) and Pleurotomaria granulatocincta Parona, 1893 ( Parona 1893: 166, pl. 6: 6), with P. hettangiensis . However, these species are clearly distinct from P. hettangiensis owing to their general shape of the shell and ornament details. Moreover, Parona (1893) described P. oblita and P. granulatocincta as anomphalous, whereas P. hettangiensis has a narrow umbilicus.

Pleurotomaria wanderbachi Terquem, 1855 differs from P. hettangiensis in having a more acute spiral angle, higher whorls, a wider ramp and a much less inclined outer whorl face. Moreover, the juvenile shell of P. wanderbachi is definitely gradate whereas in P. hettangiensis it tends to be conical. Finally, in P. wanderbachi the nodes on the outer edge of the ramp are slightly less numerous and more widely spaced.

Pleurotomaria anglica ( Sowerby, 1815) differs from P. hettangiensis in the same characters listed above concerning the comparison between Sowerby’s (1815) species and P. wanderbachi . Moreover, P. hettangiensis has a tendentially less high spire and a wider spiral angle.

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

Figures

Fig. 10. Pleurotomariid gastropods from Brouch (Mersch, Grand−Duchy of Luxembourg), Late Hettangian (Schlotheimia angulata Zone, Schlotheimia complanata Subzone), Early Jurassic. A–D. Pleurotomaria hettangiensis Terquem, 1855. A. MNHNL BR294, dorsal (A1), basal (A2), and apertural (A3) views. B. MNHNL BR676, lateral (B1) and dorsal (B2) views. C. MNHNL BR375−1, dorsal (C1) and apertural (C2) views. D. MNHNL BR443, detail of the ornament (D1), dorsal (D2), and basal (D3) views. E. Pleurotomaria cf. hettangiensis Terquem, 1855, MNHNL BR432−2, dorsal (E1), basal (E2), and apertural (E3) views.