Pleurotomaria hennocquii 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 : 365-366

Pleurotomaria hennocquii Terquem, 1855

Fig. 11A .

1855 Pleurotomaria hennocquii sp. nov.; Terquem 1855: 275, pl. 16: 12, 12a.

Material.— One specimen: MNHNL BR 881, Brouch, Upper Hettangian ( Schlotheimia angulata Zone , Schlotheimia complanata Subzone ).

Dimensions.—See Table 2. Description.—Trochiform, slightly cyrtoconical shell composed of about six whorls. Juvenile whorls moderately convex, with a weak, rounded angulation delimiting a slightly concave, relatively narrow ramp. Selenizone clearly raised on the whorl surface, running below the angulation and on the upper part of a convex outer whorl face. Convexity of the whorls increasing during the adult growth. Last whorls markedly and almost evenly convex, with feebly concave subsutural band. Suture impressed. Peripheral rim of the base subangulated. Selenizone of adult whorls relatively wide, running below the mid−line of the whorl surface and delimited by thin spiral lines. Spiral ornament composed of threads. They are thin, widely spaced and with slight variations in strength on the juvenile spire. The adult spire is ornamented with about six spiral threads of unequal strength running either above and below the selenizone. Thin, sharp and dense growth threads and spiral threads form a finely reticulate ornament with fine granules at the intersection. Peripheral angulation of the base bearing a row of low and small nodes which are exposed on the spire just above the suture. Small and dense nodes mark the angulation of the juvenile whorls, which attenuate during the adult growth. Selenizone seemingly smooth. Growth lines slightly prosocline above the selenizone, opisthocline below it.

Remarks.—This species belongs to the genus Pleurotomaria Defrance, 1826 as indicated by the morphology of the juvenile spire which bears a nodose angulation and by the selenizone running on the outer whorl face. It differs from the other congeneric species described here in having strongly and almost evenly convex mature whorls and in the weakness of the nodose ornament.

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

Pleurotomaria cognata Chapuis and Dewalque, 1853 Fig. 11B, C .

1853 Pleurotomaria cognata sp. nov.; Chapuis and Dewalque 1853: 95–96, pl. 13: 1a, b.

?1936 Pleurotomaria intermedia Münster in Goldfuss; Joly 1936: 75, pl. 1: 1a–c.

Material.— Three specimens: MNHNL HE197−1 , MNHNL HE197−2 , MNHNL LI113 , Burmerange , Hettangian–Lower Sinemurian (exact stratigraphical level uncertain) .

Dimensions.—See Table 2.

Description.—Trochiform−gradate shell with slightly cyrtoconical outline. Early juvenile spire turbiniform and composed of strongly convex whorls. Post−juvenile spire gradate, with whorls having a well−marked and oblique ramp. Ramp flat to just slightly concave, narrower than the outer whorl face and delimited by an angulation which becomes progressively stronger during the adult growth. Outer whorl face feebly oblique and almost flat. Peripheral angulation swollen and relatively prominent, slightly exposed on the spire. Su−

doi:10.4202/app.2010.0098

tures impressed. Selenizone moderately wide, running slightly above the middle of the outer whorl face. Surface of selenizone concave on the early teleoconch whorls, becoming flat and then slightly convex during the adult growth. Base flat, with slightly convex surface, phaneromphalous. Umbilicus moderately wide and deep. Aperture subtrapezoidal, with discontinuous peristome on the parietal lip. Ornament reticulate by intersection of sharp spiral and collabral threads. Collabral threads closely spaced and extending from suture to suture. Ramp with three to four spiral threads. Angulation of the whorls marked by a cord−like spiral thread. One spiral thread runs on the surface between the angulation and the selenizone, one to two spiral threads below the selenizone. Ramp crossed by shallow and wide prosocline collabral riblets. They originate from prominent nodes on the angulation of the whorls and disappear before reaching the upper suture. Nodes evenly spaced, about twenty on the last whorl. Peripheral angulation marked by a strong and sharp thread on the juvenile whorls, bearing nodes on the adult whorls. They are smaller than those of the angulation of the whorls and extend slightly on the peripheral band of the base. Selenizone delimited by thin spiral threads. A strong lira appears along the mid−line of the selenizone from about the third whorl onward, becoming progressively more prominent during the adult growth. Lunulae sharp, well separated from each other and equally sized in the juvenile spire, becoming closely packed and irregularly sized on the adult spire. Base ornamented with a dozen, relatively strong spiral threads. Dense collabral threads occur between the spiral threads. Growth striae prosocline and prosocyrt above the selenizone, slightly opisthocline and prosocyrt below the selenizone, widely opisthocyrt on the base, becoming slightly prosocyrt near the umbilicus.

Remarks.—The specimens are rather well preserved but their stratigraphical position is uncertain. The sediment infilling the shells consists of a grey marl similar to that of the Elvingen marl (lowermost Hettangian) and of the Strassen marl and limestone (Lower Sinemurian). The type material of Pleurotomaria cognata Chapuis and Dewalque, 1853 comes from the Jamoigne Marl Formation (Hettangian) of Belgium ( Chapuis and Dewalque 1853). The specimens from Luxembourg differ from the type material only in having stronger and larger nodes.

The specimen figured by Joly (1936) as Pleurotomaria intermedia Münster in Goldfuss, 1844 differs from P. cognata in having stronger collabral riblets, less marked nodes, and the peripheral row of nodes ornamenting only the last whorl. The low number of available specimens prevents to establish whether or not these differences are due to intraspecific variation of P. cognata .

Pleurotomaria hettangiensis Terquem, 1855 differs from P. cognata in having a more prominent selenizone and in the presence of a much narrower umbilicus. Moreover, P. cognata is smaller and exhibits a more gradate spire due to the presence of a wider ramp. Pleurotomaria wanderbachi Terquem, 1855 differs from P. cognata in the larger size and in a more acute spire made by higher whorls. Moreover, the selenizone is more prominent and wider.

Stratigraphic and geographic range.—Early Hettangian, Chiny and Fontenoille ( Belgium); earliest Hettangian or Early Sinemurian, Burmerange ( Luxembourg).

Figures

Fig. 11. Pleurotomariid gastropods from Hettangian–Early Sinemurian, Early Jurassic of Grand−Duchy of Luxembourg. A. Pleurotomaria hennocquii Terquem, 1855, MNHNL BR881, dorsal (A1), lateral (A2), apical (A3) views, and detail of the ornament (A4), Brouch (Mersch, Grand−Duchy of Luxembourg), Late Hettangian (Schlotheimia angulata Zone, Schlotheimia complanata Subzone). B, C. Pleurotomaria cognata Chapuis and Dewalque, 1853. B – MNHNL HE197−1, apertural (B1), basal (B2), and dorsal (B3) views. C – MNHNL HE197−2, dorsal (C1) and basal (C3) views, Burmerange, exact stratigraphical level uncertain. D. Pleurotomaria sp., MNHNL GL344, dorsal (D1), apertural (D2), and basal (D3) views, Burmerange, exact stratigraphical level uncertain.