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

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

Fig. 137 View FIGURE 137

Acanthochitona globosa View in CoL Dell’Angelo, Landau, Van Dingenen & Ceulemans, 2018b, p. 42, fig. 21; Dell’Angelo et al. 2020b, p. 41, 53, tab. 9.

Type material. Holotype: RGM.1008383, tail valve, width 3.3 mm, Figs 137F–H View FIGURE 137 . Paratypes: MNHN.F.A67139– A67144 View Materials (6 valves), Figs 137A–D View FIGURE 137 , NHMW 2017/0108/0042–2017/0108/0047 (6 valves), RGM.1008384–1008389 (6 valves) .

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

Type stage. Miocene (Tortonian).

Material examined. Miocene (Tortonian): France: Renauleau : 12 valves ( BD 154 , MNHN.F. A67147 View Materials ,), Saint-Clément-de-la-Place: type material plus 884 valves ( BD 153 , MNHN.F. A67145 View Materials – A67146 View Materials , Fig. 137E View FIGURE 137 , NHMW 2017/0108/0048, RGM.1008357, RGM.1008390, RGM.1008415, RGM.1008437). Maximum width of the valves: 2.5 / 4.8 / 3.3 mm .

Description. Head valve semicircular, posterior margin straight, apex scarcely developed. Intermediate valve broadly rectangular, rounded in anterior profile, also longitudinally, rather elevated (H/W = 0.45), anterior margin almost straight or weakly convex, posterior margin slightly concave either side of strongly protruding apex, jugal area triangular, longitudinally striated, not raised. Tail valve elliptical, mucro slightly posterior, jugal area longitudinally striated, not raised, antemucronal and postmucronal slopes almost straight, forming an angle of about 55–60° with antemucronal slope.

Tegmentum sculptured with elongated flattened granules, irregularly arranged in radial striae starting from apex (on HV and intermediate valves) or from mucro on tail valve. Granules widely separated, of variable size, up to 250 μm in length on intermediate valves, slightly smaller on tail valve, with one central megalaesthete and many micraesthetes, 15 or more.

Articulamentum poorly preserved, apophyses quadrangular, narrow, insertion plates extending far over the tegmentum, slit formula 5 /1 / 2.

Remarks. The species is known from the Miocene (Tortonian) of France ( Dell’Angelo et al. 2018b). We analysed a large collection of valves, rarely complete and poorly preserved. The sculpture of the intermediate valves of this species is highly characteristic, with the radial striae of elongated flattened granules radiating from the apex, which continue from longitudinal striae on the jugal area without interruption, the jugal area is not raised and the profile of the valve is evenly rounded. The micraesthetes are difficult to see because of the poor preservation of tegmentum, although15 at least are visible on granules in one case ( Fig. 137D View FIGURE 137 ).

Comparisons. Acanthochitona globosa Dell’Angelo, Landau, Van Dingenen & Ceulemans, 2018 is superficially similar to A. oblonga ( Leloup, 1981) in having the tegmentum sculpture formed by elongated granules. Acanthochitona globosa differs from A. oblonga by the different shape of the intermediate and tail valves (profile semicarinate and with JA raised and distinctly separated from the other areas in A. oblonga ), the less developed articulamentum and the greater number of the micraesthetes in the pustules (15 or more in A. globosa vs. 6–9 in A. oblonga ).

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