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

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

Fig. 133 View FIGURE 133

Acanthochitona chauvereauensis View in CoL Dell’Angelo, Landau, Van Dingenen & Ceulemans, 2018b, p. 46, fig. 23; Dell’Angelo et al. 2020b, p. 39, 53, tab. 9.

Type material. Holotype: MNHN.F. A67157 View Materials , tail valve, width 4 mm, Figs 133F–H View FIGURE 133 . Paratypes: MNHN. F. A67158 View Materials – A67163 View Materials (6 valves, Figs 133A–C View FIGURE 133 ) , NHMW 2017/0108/0057–2017/0108/0062 (6 valves, Figs 133D–E View FIGURE 133 ), RGM.1008423–1008425 (3 valves).

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

Type stage. Miocene (Tortonian).

Material examined. Upper Miocene (Tortonian): France: Renauleau : 12 valves ( BD 158 , MNHN.F. A67165 View Materials ), Saint-Clément-de-la-Place: type material plus 373 valves ( BD 157 , MNHN.F. A67164 View Materials , NHMW 2017/0108/0063, RGM.1008359, RGM.1008392, RGM.1008426, RGM.1008439), Sceaux d’Anjou: 4 valves ( RGM.1008445, RGM.1008448). Maximum width of the valves: 5.3 / 6.8 / 5.2 mm .

Description. Head valve semicircular, posterior margin straight, apex scarcely evident. Intermediate valve trapezoidal (WT/LT = 1.19–1.34), semicarinate in anterior profile, rather elevated (H/W = 0.48), jugal margin straight to slightly convex, lateral margins straight, weakly rounded near posterior margin, posterior margin slightly concave on both sides of pronounced apex, jugal area triangular, smooth, raised, with the presence of prominent growth lines. Tail valve elliptical/polygonal, with greatest diameter along longitudinal axis (WT/LT = 0.84–0.92), mucro in slightly posterior position, jugal area smooth, raised, antemucronal and postmucronal slopes almost straight, forming angle of about 40° with antemucronal slope.

Tegmentum sculptured with irregularly arranged, flattened roundish granules; granules densely packed, diameter 60–90 μm, with one central megalaesthete and up to 4–5 micraesthetes irregularly disposed.

Articulamentum well developed, apophyses quadrangular, insertion plates well developed, strongly protruding on intermediate valves, expanded and completely surround tail valve, except for jugal margin, slit formula 5 /1 / 2.

Remarks. The studied material is poorly preserved, with valves abraded and seldom complete. JA is smooth and has prominent growth lines, some fine longitudinal furrows are sometimes visible when the surface is particularly eroded. The micraesthetes are difficult to see, due to poor preservation of tegmentum.

Comparisons. At a first glance Acanthochitona chauvereauensis Dell’Angelo, Landau, Van Dingenen & Ceulemans, 2018 is closest to A. fascicularis ( Linnaeus, 1767) in having the tegmentum sculpture formed by roundish and densely packed granules. It differs from A. fascicularis by a smooth JA (longitudinally striated in A. fascicularis ), the different shape of the intermediate valves (more rectangular in A. fascicularis , with WT much greater than LT), the different shape of the tail valve (elliptical with WT> LT and with postmucronal slope strongly concave behind the mucro in A. fascicularis ) and in lacking a deep incision in the granules, typical for A. fascicularis (see Fischer & Renner 1979: figs 4–5; Bonfitto et al. 2011: fig. 1).

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