Laevitomaria amyntas (d’Orbigny, 1850) 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 : 222-224

Laevitomaria amyntas (d’Orbigny, 1850) comb. nov.

Fig. 4 .

1850 Pleurotomaria amyntas ; d’Orbigny 1850: 268.

1856 Pleurotomaria amyntas d’Orb. ; d’Orbigny 1856: 495, pl. 392: 6–10.

1873 Pleurotomaria amyntas d’Orbigny ; Tawney 1873: 41.

1895 Pleurotomaria (? Leptomaria ) amyntas d’Orbigny ; Hudleston 1895: 415, pl. 35: 12.

non 1919 Pleurotomaria (Leptomaria) amyntas d’Orbigny ; Cossmann

1919: 431, pl. 16: 6, 7. non 1937 Pleurotomaria amyntas ; Pchelincev 1937: 23, pl. 1: 20. 1997 Bathrotomaria amyntas (d’Orbigny) ; Fischer and Weber 1997:

187, pl. 33: 2a, b. 2011 Bathrotomaria amyntas (d’Orbigny) ? [sic!]; Gründel et al. 2011:

100, pl. 1: 11–15.

Material.— Five specimens: MNHNL ZS248, MNHNL MDB254 (Giele Botter, Differdange, south-western Luxembourg), MNHNL ZS314 (Rollesbierg, Differdange, south-western Luxembourg), MNHNL ZS494S1, MNHNL MDB275 (Rollesbierg or Giele Botter, Differdange area, south-western Luxembourg). Early Bajocian ( Hyperlioceras discites Zone – Witchellia laeviuscula Zone ).

Description.—The shell is conoidal to slightly cyrtoconoid, roundedly gradate, consisting of about ten whorls. The apical spire is feebly cyrtoconoid. The early whorls are convex and with a poorly defined, narrow ramp. The sutures are impressed. The convexity of whorls diminishes during growth. On the last whorl an obtuse shoulder develops on the lower third of the whorl surface. This shoulder separates a wide, feebly concave to convex ramp from a narrower, flat or slightly convex outer face. The periphery is distinctly angulated on the immature shell and becomes obtusely angulated on the adult shell. The selenizone of the early shell is concave and edged by sharp marginal spiral threads. It runs clearly below the maximum convexity of the whorl and rapidly becomes flat and then convex during growth. On the adult whorls the selenizone is slightly cord-like and moderately wide, its width being 12% of the whorl surface. It runs clearly below the mid-whorl and on the angulation of the last whorl. Its lower and upper edges correspond to sharp spiral striae. The base is rather flat and slightly convex. The umbilicus is deep and moderately wide, almost funnel-shaped in the fully adult shell. The aperture is subpentagonal. The slit extends less than one fourth of the last whorl length.

The ornament of the immature spire consists of a dense and quite regular network of fine spiral threads and collabral riblets. Collabral riblets are more distinct above the selenizone and disappear during the growth of the fourth teleoconch whorl. The spiral threads are evenly sized and densely spaced. They increase in number during growth. The third to fourth whorl bears 8–10 spiral threads above the selenizone and 4–5 spiral threads below it. On the seventh whorl there are about 15 spiral threads above the selenizone and 7–8 below it. The spiral ornament persists longer than the collabral sculpture. On the last whorls the spiral threads tend to disappear too, leaving only obscure lines that are more pronounced below the selenizone. On the earliest teleoconch whorls the selenizone is sculptured only by sharp and regularly spaced lunulae. Subsequently, a middle spiral thread appears. This thread rapidly strengthens so that the selenizone finally appears as a slightly raised cord crossed only by dense and very faint growth lines. The base bears numerous fine, almost obscure spiral threads. The growth lines are strongly prosocline and slightly prosocyrt above the selenizone and slightly opisthocline and prosocyrt below it. The base bears widely opisthocyrt growth lines becoming prosocyrt on the periumbilical area.

Remarks.—The specimens from Luxembourg differ from the holotype of Laevitomaria amyntas (d’Orbigny, 1850) in having a more obtuse adult spire angle and a less marked spiral ornament of the adult shell. The specimen identified by Hudleston (1895) as Pleurotomaria (? Leptomaria ) amyntas has a narrower spire angle, but the other characters correspond exactly to those of the specimen from Luxembourg. In particular, Hudleston (1895) described a fine reticulate ornament of the apical shell and the disappearance of the spiral ornament on the spire whorls and on the base during the adult growth. Tawney (1873) mentioned a specimen with a spire angle of 58°, i.e., intermediate between the holotype 54°) and the specimens from Luxembourg (62–66°).

Gründel et al. (2011) tentatively identified two specimens from Late Aalenian of southern Germany as L. amyntas . According to these authors, their material differs from the holotype in showing a weaker shoulder at the selenizone. In the fully adult specimens described here the shoulder appears during the growth of the last whorls. In one specimen ZS248) it becomes quite sharp on the last whorl and edges a slightly concave ramp. Tawney (1873) pointed out that one of his largest specimens exhibits a slightly concave ramp on the last whorl. However, in another specimen from Luxembourg MDB275), the shoulder is much less evident and the ramp is convex. These observations indicate that the sharpness of the shoulder and the shape of the ramp of the adult shell are rather variable in L. amyntas . The specimens from southern Germany fall within this range of variability.

Laevitomaria gyroplata ( Eudes-Deslongchamps, 1849) differs from L. amyntas in having a narrower spire angle 40–50°; Fischer and Weber 1997) and less convex whorls. Moreover, the umbilicus is absent to fissure-like. As underlined by Fischer and Weber (1997), the specimens from the Bajocian of Nievre ( France) assigned by Cossmann (1919) to Pleurotomaria (Leptomaria) amyntas d’Orbigny, 1850 do not belong to that species. They show characters more comparable with those of Bathrotomaria subreticulata (d’Orbigny, 1850) . The specimen described by Pchelincev (1937) as Pleurotomaria amyntas d’Orbigny , from the Aalenian of the Caucasus, is represented by an inner mould. The general morphology of the shell indicates that it belongs to Laevitomaria . This specimen differs from L. amyntas in having a more acute shell, less convex whorls and last whorl without angulation. Its poor state of preservation prevents further comparisons.

Stratigraphic and geographic range.—Undifferentiated Middle to Late Aalenian, Vendée (western France). Middle to Late Aalenian, south-western England. Late Aalenian, Baden-Württemberg (southern Germany). Early Bajocian

Hyperlioceras discites Zone – Witchellia laeviuscula Zone ), south-western Luxembourg.

Figures

Fig. 4. Pleurotomarioidean gastropod Laevitomaria amyntas (d’Orbigny, 1850) from the Early Bajocian (Hyperlioceras discites Zone–Witchellia laeviuscula Zone) of south-western Luxembourg. A. MNHNL ZS314, apertural (A1), dorsal (A2), and basal (A3) views, detail of the apical shell (A4); Differdange area (Rollesbierg). B. MNHNL ZS494S1, apertural (B1), dorsal (B2), and basal (B3) views; Differdange area (Rollesbierg or Giele Botter). C. MNHNL ZS248, dorsal (C1), lateral (C2), and apertural (C3) views, detail of the whorl surface (C4), and basal view (C5); Differdange area (Giele Botter). D. MNHNL MDB275, apertural (D1) and dorsal (D2) views; Differdange area (Rollesbierg or Giele Botter).