Laevitomaria cf. subplatyspira (d’Orbigny, 1850), MNHNL BU

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 : 225-226

Laevitomaria cf. subplatyspira (d’Orbigny, 1850)

Fig. 6 .

Material.— One specimen: MNHNL BU233 , Piedmont (Longwy, Lorraine, eastern France), Early Bajocian .

Description.—The adult shell is conoidal-trochiform. It is composed of moderately convex whorls separated by impressed sutures. The selenizone is flat and moderately wide, its width being almost 15% of the whorl surface. It is almost at mid-whorl on the spire and shifts slightly below it on the last whorls. The periphery is evidently angulated. The base is rather flat and phaneromphalous. Its surface is slightly convex. The umbilicus is moderately wide. The aperture has a subpentagonal section. Small shell remains indicate that the ornament of the adult shell is most probably very weak or absent. Some obscure spiral lines are visible below the selenizone. The base is seemingly smooth.

Remarks.—The material is represented only by a specimen lacking the apical spire and mostly preserved as inner mould. The few observable characters agree rather well with those described in Laevitomaria subplatyspira (d’Orbigny, 1850) . These characters concern the size of the whorls and its increment during the growth, a quite sharp periphery, a flat base, and a moderately wide and flat selenizone running almost at mid-whorl on the spire and shifting slightly below mid-whorl during the last growth. Remains of the shell seem to indicate that the ornament is very weak or absent on the last whorls and on the base, as it is the case in L. subplatyspira . However, due to its poor state of preservation the specimen is left in open nomenclature. Caution is also needed because of the uncertainty about the status of L. subplatyspira . As underlined by Fischer and Weber (1997: 187), the species was erected by d’Orbigny 1850: 269) based on the material on which Eudes-Deslongchamps (1849: 54, pl. 6: 2a–c) described Pleurotomaria fasciata var. platyspira . D’Orbigny (1850) changed the name in subplatyspira because the original name (platyspira) was already employed by him to raise Pleurotomaria debuchii var. platyspira Eudes-Deslongchamps, 1849 to species rank. The type material of L. subplatyspira is missing and no other specimen was found in the type locality ( Fischer and Weber 1997).

As remarked by Fischer and Weber (1997), L. subplatyspira differs from Laevitomaria fasciata ( Sowerby, 1818) in having less convex whorls, a sharper periphery and a less convex base. In L. fasciata the whorls are higher and the umbilicus is narrower. Laevitomaria amyntas (d’Orbigny, 1850) has a narrower and convex selenizone and its last whorls are angulated, whereas in L. subplatyspira they are evenly convex. Laevitomaria stoddarti ( Tawney, 1873) differs from L. subplatyspira in having a wider spire angle and more convex whorls and base. In L. stoddarti , the ornament is coarser and the periphery is swollen.

Figures

Fig. 6. Pleurotomarioidean gastropods. A. Laevitomaria cf. subplatyspira d’Orbigny, 1850) from the Early Bajocian of Piedmont (Longwy, Lorraine, eastern France), MNHNL BU233, in dorsal (A 1), basal (A 2), and apertural A 3) views. B. Laevitomaria subplatyspira (d’Orbigny, 1850) from the Early Bajocian, Bayeux (Calvados, northern France); figured by d’Orbigny 1856: 469, pl. 393: 1).