Laevitomaria fasciata ( Sowerby, 1818 ) Gatto & Monari & Szabó & Conti, 2015

Gatto, Roberto, Monari, Stefano, Szabó, János & Conti, Maria Alessandra, 2015, The Jurassic pleurotomarioidean gastropod Laevitomaria and its palaeobiogeographical history, Acta Palaeontologica Polonica 60 (1), pp. 217-233 : 224-225

Laevitomaria fasciata ( Sowerby, 1818) comb. nov.

Fig. 5.

1818 Trochus fasciatus ; Sowerby 1818: 37, pl. 220: 1.

non 1844 Pleurotomaria fasciata Sandberger ; Goldfuss 1844; 64, pl. 183: 1.

1873 Pleurotomaria fasciata Sowerby ; Tawney 1873: 51.

1895 Pleurotomaria fasciata Sowerby ; Hudleston 1895: 416, pl. 36: 3.

1884 Pleurotomaria fasciata Sw. ; Quenstedt 1884: 347, pl. 198: 37. non 1886 Pleurotomaria fasciata Sowerby ; Vacek 1886: 106, pl. 18: 2. 1907 Pleurotomaria fasciata Sowerby ; Sieberer 1907: 32, pl. 4: 5.

Material.— One specimen: MNHNL ZS508 , Piedmont (Longwy, Lorraine, eastern France), Early Bajocian .

Description.—The shell is conoidal-trochiform, subgradate, and it is composed of about 8–9 whorls. The last whorl makes up about two third of the height of the shell. The juvenile spire is slightly cyrtoconoid with moderately convex surface of the whorls. The adult whorls are distinctly convex and edged by a clearly impressed suture. They have a convex ramp which smoothly passes into the almost flat outer face. The suture runs on a roundedly angulated and slightly swollen periphery of the preceding whorl. The periphery becomes partially exposed on the last whorls. The selenizone is rather wide, its width being almost 15% of the whorl surface. It is situated slightly below mid-whorl and along the line of maximum convexity of the whorl surface. It is feebly concave on the early whorls, where it is edged by sharp marginal spiral threads, and becomes flat or very feebly convex during the growth. The base is almost flat, with a slightly convex surface and a narrow umbilicus. The aperture is subpentagonal-elliptical and wider than high.

The ornament of the juvenile shell is quite roughly reticulated, being composed of collabral riblets crossing spiral threads with small, flattish granules at the intersections. The surface of the early whorls above the selenizone bears about 6–8 spiral threads, among which the most adapical three are stronger and more distant to each other. The collabral riblets are coarse and slightly irregular in size. Below the selenizone, the early whorls are ornamented by 4–6 thin, subequally sized and evenly spaced spiral threads intersected by collabral riblets. These collabral riblets are finer and sharper than those present above the selenizone. The ornament of the early shell attenuates during growth. The spiral threads sculpturing the adult whorl surface above the selenizone become shallower and almost obscure and the collabral riblets gradually change in dense growth lines. In contrast, the spiral ornament below the selenizone is more persistent and slightly roughed by intersection with very thin, closely spaced, collabral riblets. The selenizone of the early whorls bears a median spiral thread and it is sculptured by well-marked and evenly spaced lunulae. On the adult shell, the lunulae become gradually finer and less pronounced. A further spiral thread on both sides of the median thread appears. These threads are shallower than the median one and slightly granulated by intersection with the low lunulae. The spiral ornament of the selenizone fades out and becomes obscure on the last whorls. The base is seemingly smooth. The growth lines are strongly prosocline and slightly prosocyrt above the selenizone, almost orthocline and feebly prosocyrt below it. On the base, the growth lines are widely opisthocyrt and become prosocyrt on the periumbilical area.

Remarks.—Among the material assigned by previous authors to Laevitomaria fasciata ( Sowerby, 1818) , the shell illustrated by Sieberer (1907) is the most similar to the specimen here described. The specimen figured by Hudleston (1895) differs in having a fine spiral ornament on the base. Its periphery is roundedly angulated whereas in the specimen here described it is slightly swollen. These differences are here considered as representing the intraspecific variation of L. fasciata .

A B 10 mm C D E 5 mm

The feebly swollen periphery and the attenuation of the ornament on the surface of the shell above the selenizone make the specimen here described closely reminiscent of Laevitomaria stoddarti ( Tawney 1873: 50, pl. 3: 5; Hudleston 1895: 418, pl. 36: 2). The latter taxon has a wider spire angle, a more distinctly swollen periphery, a deeper suture running below the periphery of the preceding whorl, and a consistently wider umbilicus.

Laevitomaria amyntas (d’Orbigny, 1850) differs from L. fasciata in having lower whorls with sharper periphery, and the last whorls subangulated at the selenizone. In L. amyntas the selenizone is more prominent and situated distinctly below the mid-whorl. Furthermore, the immature spire whorls are ornamented by a fine and regular network of spiral threads and collabral riblets whereas in L. fasciata , the reticulate sculpture of the early shell is coarser.

The specimen attributed by Vacek (1886) to Pleurotomaria fasciata ( Sowerby, 1818) and classified by Conti and Szabó (1989: 32, pl. 1: 7) as Leptomaria aff. fasciata differs from Sowerby’s (1818) species in having a definitely wider umbilicus and an early shell ornamented by a sharper and much more regular network of spiral threads and collabral riblets.

Stratigraphic and geographic range.—Early Bajocian, south-western England, Swabia (southern Germany), Lorraine (eastern France).