Platyacra aff. sinistrorsa ( 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 : 378

Platyacra aff. sinistrorsa ( Terquem, 1855)

Fig. 17D, E .

1988 Platyacra sinistrorsus [sic] (Deshayes); Meier and Meiers 1988: 29, pl. 4: 10a, b.

Material.— Two specimens: MNHNL BR 373, MNHNL BR 876, Brouch, Upper Hettangian ( Schlotheimia angulata Zone , Schlotheimia complanata Subzone ).

Dimensions.—See Table 2.

Description.—Sinistral, dimorphic shell. Early shell planispiral anisostrophic composed of about four whorls. Its apical side is concave and consists of flat whorls which are delimited by a sharp peripheral angulation which is not prominent on the spire surface. Suture running near the peripheral angulation. Adult spire pagodiform, consisting of about three whorls. The peripheral angulation becomes well−exposed on the spire. It delimits a wide and steep ramp, and a relatively narrow outer whorl face which is inclined opposite to the ramp. Last whorl with peripheral angulation slightly below mid−whorl. Suture oblique and impressed, running at or near to an obtuse angulation of the base. Base distinctly convex, with a wide umbilicus encircled by a periumbilical angulation. Apertural region not preserved. Main spiral ornament made of a strong peripheral keel and of a spiral cord marking the angulation of the base. Outer whorl face and base of the adult spire ornamented with subequally sized and widely spaced spiral threads. There are two spiral threads on the outer whorl face and four on the base below the spiral cord marking the angulation of the base. Peripheral keel crenulated by shallow, widely spaced, spirally elongated nodes. About twelve nodes on the last whorl forming shallow undulations on the ramp and on the base. The basal cord bears also shallow nodes axially aligned with those of the peripheral keel. Growth striae prosocline and prosocyrt on the ramp, seemingly opisthocyrt on the base.

Remarks.—The material is poorly preserved. The largest specimen ( MNHNL BR 373) is partly embedded in the sediment. The other specimen ( MNHNL BR 876) is an incomplete adult shell seemingly preserving obscure traces of a fine spiral ornament on the ramp. The specimens are closely reminiscent of the material on which Terquem (1855: 264, pl. 15: 14) erected Trochus sinistrorsus Terquem, 1855 . However, Terquem’s (1855) species, which was refigured by Cossmann (1916: 207, pl. 9: 8, 9) and assigned by him to the genus Platyacra Ammon in Zittel, 1882, exhibits some differences with respect to the material described here. In particular, the type material of P. sinistrorsa is smaller in size and lacks spiral cords on the outer whorl face and base. Moreover, it has less pronounced nodes undulating the peripheral keel. The ramp is ornamented with distinct collabral ribs which form nodes at the upper suture.

Platyacra impressa ( Schafhäutl, 1863) (Zittel 1882: 190, fig. 237; Ammon 1893: 169, fig. 7), type species of the genus Platyacra , is also smaller in size and with a slightly narrower adult spiral angle. Moreover, the base lacks a spiral cord and the ramp is crossed only by growth striae.

The specimens described here most likely belong to a species different from both P. sinistrorsa ( Terquem, 1855) and P. impressa . However, both present specimens are so poorly preserved that species identification is not recommended.

Figures

Fig. 17. Eucycloidean gastropos from Luxembourg Sandstone Formation, Early Jurassic. A. Eucyclus sp., MNHNL GL205, apertural (A1), basal (A2) and dorsal (A3) views, Hettangian–Early Sinemurian, exact locality and stratigraphical level uncertain. B–E. Brouch (Mersch, Grand−Duchy of Luxembourg), Late Hettangian (Schlotheimia angulata Zone, Schlotheimia complanata Subzone). B, C. Spirocirrus weisi sp. nov., B. Holotype, MNHNL BR299, lateral view (B1) and detail of the ornament (B2). C. Paratype, MNHNL BR374, apertural (C1) and subdorsal (C2) views. D, E. Platyacra aff.sinistrorsa (Terquem, 1855). D. MNHNL BR373, dorsal view (D1) and detail of the ornament of the base (D2). E. MNHNL BR876, dorsal (E1), subapertural (E2), and basal (E3) views, and detail of the apical spire (E4).