Ischnochiton nitidum, Dell'Angelo, Landau, Van Dingenen & Ceulemans, 2018

Ischnochiton nitidum View in CoL Dell’Angelo, Landau, Van Dingenen & Ceulemans, 2018

Fig. 67 View FIGURE 67

Ischnochiton nitidum View in CoL Dell’Angelo, Landau, Van Dingenen & Ceulemans, 2018b, p. 27, fig. 14; Dell’Angelo et al. 2020b, p. 52, tab. 9.

Type material. Holotype MNHN.F.A67095, tail valve, width 6.3 mm ( Figs 67A–C View FIGURE 67 ) . Paratypes: MNHN.F.A67096– A67101 (6 valves, Figs 67D–I View FIGURE 67 ); NHMW 2017/0108/0020–0025 (6 valves, Figs 67J–L View FIGURE 67 ); RGM.1008375–1008380 (6 valves) .

Type locality. Saint-Clément-de-la-Place ( France) .

Type stage. Miocene (Tortonian).

Material examined. Miocene (Tortonian): France: Saint-Clément-de-la-Place: type material plus 635 valves ( MNHN.F.A67102, NHMW 2017/0108/0026, RGM.1008352, RGM.1008381, RGM.1008432, BD 141; Sceaux d’Anjou, La Presselière: 5 valves ( BD 142 ). Maximum width of the valves: 6.5 / 9 / 7 mm.

Description. Head valve semicircular, posterior margin widely V-shaped, front slope straight. Intermediate valve broadly rectangular (W/L = 2.25–2.28), rounded in anterior profile, moderately elevated (H/W = 0.37–0.42), anterior and posterior margins almost straight, side margins rounded, apex hardly or not developed, lateral areas hardly raised. Tail valve semicircular (W/L = 1.42–1.60), anterior margin slightly convex, mucro central, not prominent, antemucronal slope straight or slightly convex, postmucronal slope almost straight or slightly concave.

Tegmentum smooth and glossy. HV, LA and PMA smooth with several concentric growth lines. CA and AMA smooth, with some concentric growth lines, continuing from LA and PMA, though less strongly, in some cases with presence of hardly perceptible, short longitudinal ridges in area near anterior margin.

Articulamentum with apophyses rounded, separated by wide jugal sinus, slit formula: 11 / 1 / 9–13, slits inequidistant, slit rays clearly visible, on intermediate valves (and even less accentuated on tail valves) there are no slit rays formed by individual holes, but characteristic band of rays, teeth short, upper side roughened.

Remarks. The fossil record of Ischnochiton nitidum Dell’Angelo, Landau, Van Dingenen & Ceulemans, 2018 is limited to the upper Miocene of the Ligerian Basin, France. The valves are large, but generally not well preserved; complete valves are scarce in the studied material. A narrower band of rays is sometimes visible in the articulamentum near the posterior margin of intermediate valves, giving the impression of the presence of a second slit in the insertion plate.

Comparisons. The smooth and glossy tegmentum of Ischnochiton nitidum is characteristic, and different from that of other extant species of Ischnochiton from the Mediterranean and east Atlantic, which are characterized by a more or less marked longitudinal and radial tegmental sculpture (e.g., the Ischnochiton spp. discussed here, see tab. 7, and I. obtusus Carpenter in Pilsbry, 1893 from Portugal) or by a microgranulose sculpture [e.g., I. cessaci (de Rochebrune, 1881) , I. goreensis Thiele, 1909 and I. nicklesi Kaas & Van Belle, 1990 , all from the West Africa]. Ischnochiton nitidum is superficially similar to the living arctic-boreal species Stenosemus albus ( Linnaeus, 1767) (see below), from which it differs by the tegmentum (uniformly microgranulose in S. albus ), the different shape of the tail valve, the profile of intermediate valves (subcarinate in S. albus ) and the width of the apophyses (greatly expanded, almost coalescing across the small jugal sinus in S. albus ).

Distribution. Upper Miocene: northeastern Atlantic (Tortonian): Ligerian Basin, France: Saint-Clément-de-la-Place, Sceaux d’Anjou Dell’Angelo et al. 2018b).