Spirocirrus weisi, 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 : 376-378

Spirocirrus weisi sp. nov.

Fig. 17B, C .

1907 Trochus sp. ; Joly 1907: 53, pl. 1: 1a–d.

1988 Spirocirrus calisto (Orbigny) ; Meier and Meiers 1988: 28, pl. 4: 9a, b.

Etymology: The species is dedicated to Robert Weis (National Museum of Natural History of Luxembourg).

Type material: Holotype: specimens MNHNL BR 299 ( Fig. 17B ) . Paratypes: one specimen: MNHNL BR 374.

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.—Sinistral, high turbiniform shell. Umbilicus very narrow. Aperture rounded with continuous peristome on the parietal lip. Outer face of whorls delimited by two weak angulations of which the lower one is more pronounced. Angulations fading on the last whorl. Collabral ornament made of varicose, swollen and widely spaced ribs which become much less prominent on the lower angulation of the outer whorl face. Spiral ornament composed of three primary spiral cords which alternate with secondary spiral cords on the juvenile shell and form elongated nodes on the collabral ribs. Secondary spiral cords becoming as thick as the primary ones on the last whorl. Base ornamented with few and well spaced spiral cords.

Description.—Sinistral, high turbiniform shell. Whorls strongly convex separated by strongly impressed and oblique sutures running well below the periphery. Periphery of the juvenile whorls delimited by two weak and rounded angulations, of which the lower one is slightly more pronounced. The periphery becomes rounded on the last whorl. Last whorl relatively swollen. Base evenly convex, narrowly umbilicated. Aperture circular. Peristome slightly prosocline, continuous along the parietal lip. Inner lip weakly reflected outwards. Outer lip not observable. Ornament composed of collabral ribs and spiral cords. About ten, regularly spaced prosocline collabral ribs on the adult shell, which are varicose, swollen and rounded. Three primary spiral cords ornament the juvenile shell and form spirally elongated nodes overlapping the collabral ribs. The uppermost cord runs on the ramp, whereas the other two ones delimit the outer face of the whorl. Thinner spiral cords appear during growth which rapidly become almost as thick as the primary cords. Last whorl ornamented with six strong spiral cords. The collabral ribs become less prominent in correspondence to the spiral cord marking the lower angulation. Base ornamented with spiral cords separated by inter−

doi:10.4202/app.2010.0098

vals which are much wider than those on the whorls. Growth lines prosocline and slightly prosocyrt on the whorls, feebly opisthocyrt on the base. They form dense and thin growth threads.

Remarks.— Spirocirrus weisi sp. nov. represents the oldest member of the genus Spirocirrus , a genus previously known from Late Toarcian to Bathonian of the northern Paris Basin and England (Orbigny 1853; Hudleston 1892; Cossmann 1900, 1916; Fischer and Weber 1997). The species has strong similarities with Spirocirrus calisto (Orbigny, 1853) , from the Late Bathonian of the Paris Basin (Orbigny 1853; Cossmann 1885, 1900, 1916; Fischer and Weber 1997). On the basis of that similarity, Meier and Meiers (1988) assigned the specimens from Luxembourg to S. calisto . However, S. calisto differs from S. weisi in having more thinner and denser spiral threads on the spire and on the base. Moreover, in S. calisto the lower spiral thread of the peripheral band is thicker and the associated angulation is more pronounced. The peripheral angulations are more persistent on the last whorl in S. calisto , whereas in S. weisi they fade out so that the last whorl is almost rounded.

The specimen described by Joly (1907) as Trochus sp. differs from the holotype of S. weisi only in being smaller. It is interpreted here as a juvenile specimen of that species.

Scaevola busambrensis Gemmellaro, 1879 (Gemmellaro 1879: 341, pl. 27: 1, 2), from the Sinemurian carbonate platform sediments of north−western Sicily ( Italy) probably belongs to the genus Spirocirrus Cossmann, 1916 . It differs from S. weisi in having three strong spiral carinae on the adult shell and in having thin and dense spiral threads on the base.

Stratigraphic and geographic range.—Late Hettangian ( Schlotheimia angulata Zone , Schlotheimia complanata Subzone ), Brouch ( Luxembourg). Late Hettangian ( Schlotheimia angulata Zone ), Metzert ( Belgium).

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).