Acanthochitona andegavensis, Dell'Angelo, Landau, Van Dingenen & Ceulemans, 2018

Acanthochitona andegavensis View in CoL Dell’Angelo, Landau, Van Dingenen & Ceulemans, 2018

Fig. 132 View FIGURE 132

Acanthochitona andegavensis View in CoL Dell’Angelo, Landau, Van Dingenen & Ceulemans, 2018b, p. 44, fig. 22; Dell’Angelo et al. 2020b, p. 40, 53, tab. 9; Dell’Angelo et al. 2021b, p. 428, figs 162–165.

Acanthochitona fascicularis View in CoL [non Acanthochitona fascicularis ( Linnaeus, 1767) View in CoL ]; Dell’Angelo et al. 2016: p. 86, pl. 5, fig. 3.

Type material. Holotype: MNHN.F. A67148 View Materials , tail valve, width 5.2 mm, Figs 132E–F View FIGURE 132 . Paratypes: MNHN.F.A67149– A67154 View Materials (6 valves, Figs 132B–C, 132G View FIGURE 132 ), NHMW 2017/0108/0049–2017/0108/0054 (6 valves, Fig. 132D View FIGURE 132 ), RGM.1008416–1008421 (6 valves, Fig. 132A View FIGURE 132 ) .

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

Type stage. Miocene (Tortonian).

Material examined. Upper Miocene (Tortonian): Ligerian Basin, France: Beugnon: 6 valves ( RGM.1310173, RGM.1310177), Renauleau: 53 valves ( BD 156 , MNHN.F. A67156 View Materials , NHMW 2017/0108/0056), Saint-Clément-de-la-Place: type material plus 1178 valves ( BD 155 , MNHN.F. A67155 View Materials , NHMW 2017/0108/0055, RGM.1008358, RGM.1008391, RGM.1008422, RGM.1008438). Lower Pliocene : Italy (Zanclean): Borzoli : 8 valves ( BD 960 , MSNG, Fig. 132H View FIGURE 132 ). Upper Pliocene to upper Pleistocene : France: Bosq d’Aubigny: 7 valves ( RGM.1310181, RGM.1310187). Maximum width of the valves: 5.3 / 7.8 / 7.3 mm .

Description. Head valve semicircular, posterior margin straight, apex hardly developed. Intermediate valve trapezoidal (WT/LT = 1.02–1.26), semicarinate in anterior profile, moderately elevated (H/W = 0.43), jugal margin straight or slightly convex, lateral margins straight, weakly rounded near posterior margin, posterior margin slightly concave either side of well-developed apex, jugal area triangular, smooth, raised, with presence of marked growth lines. Tail valve elliptical/polygonal, with greatest diameter along longitudinal axis (WT/LT = 0.80–0.86), mucro in posterior position, jugal area smooth, raised, antemucronal and postmucronal slopes almost straight, forming angle of about 50° with antemucronal slope.

Tegmentum sculptured with irregularly arranged, ovate to elongate drop-shaped, flattened granules; several slightly more oblique longitudinal striae developed adjacent to JA. Granules widely separated from each other, of variable size, length 140–180 μm, slightly smaller on HV (ca. 110–120 μm). Each granule with one central megalaesthete and ca. 6–7 micraesthetes irregularly disposed.

Articulamentum well developed, with large, quadrangular apophyses. The insertion plates are prominent— strongly protruding on the intermediate valves—and expanded to completely surround the tail valve, except along the jugal margin. Slit formula: 5 / 1 / 2.

Remarks. The species is known only from the upper Miocene of France and Pliocene of Italy and France. Despite the large number of specimens found at Saint-Clément-de-la-Place, the valves are rarely complete and generally poorly preserved.

The jugal area is smooth, except for the presence of marked growth lines. Some fine longitudinal furrows are sometimes visible when the surface is particularly eroded. The shape of the intermediate valves is variable, most of the valves show a length approximately equal to the width of the tegmental area (e.g., Fig. 132B View FIGURE 132 , WT/LT = 1.02) or slightly less, others show a much greater width than length (up to WT/LT = 1.26). Also, the shape of the tail valve is variable, tending to almost hexagonal in some valves. The micraesthetes are difficult to see, due to poor preservation of tegmentum.

Comparisons. Acanthochitona andegavensis Dell’Angelo et al., 2018 is superficially similar to A. crinita ( Pennant, 1777) (see below).

Distribution. Upper Miocene (Tortonian): northeastern Atlantic: France: Beugnon, Renauleau, Saint-Clément-de-la-Place ( Dell’Angelo et al. 2018b). Lower Pliocene: central Mediterranean: Italy: Borzoli ( Dell’Angelo et al. 2021b). Upper Pliocene-lower Pleistocene: northeastern Atlantic: France: Bosq d’Aubigny ( Dell’Angelo et al. 2018b).