Craspedochiton marsanensis

Craspedochiton marsanensis ( Dell’Angelo, Lesport, Cluzaud & Sosso, 2020) n. comb.

Fig. 146 View FIGURE 146

Pseudoacanthochitona marsanensis View in CoL Dell’Angelo, Lesport, Cluzaud & Sosso, 2020b, p. 41, fig. 27.

Type material. Holotype MZB 50583, tail valve, width 3.8 mm, Figs 146A–D View FIGURE 146 .

Type locality. Maureilhan ( France) .

Type stage. Miocene ( Lower Burdigalian) .

Material examined. Type material.

Description. Head and intermediate valves unknown. Tail valve broadly circular (WT/LT = 1.10), anterior margin strongly convex, jugal area triangular, not raised, mucro subcentral, not elevated and poorly developed, antemucronal slope slightly convex, postmucronal slope straight.

Tegmentum covered with small, roundish, elevated granules, irregularly disposed, only in jugal area giving impression of forming sinuous longitudinal striae.

Articulamentum thick, expanded around posterior margin, apophyses trapezoidal, posterior margin not uniform, with a large posterior callus, and seven slits, two deep and strong and five less developed located in between them.

Remarks. The species is known by a single tail valve, superficially similar to the genus Acanthochitona Gray, 1821 , but with important differences concerning JA and the articulamentum. The jugal area, not raised and in continuity with the pleural areas, has the same type of sculpture formed by small, roundish and elevated granules, but arranged more regularly, giving the impression of forming sinuous longitudinal striae ( Fig. 146B View FIGURE 146 ); the profile of the valve is evenly rounded. The articulamentum is expanded around the posterior margin, which is not uniform throughout, with two protrusions and strong slits that, based on their position, correspond to the incisions in the genus Acanthochitona . A further five weaker incisions (much less developed and irregular) are placed between the primary slits, partially closed by a large back callus, that delimit four small, sharp teeth ( Fig. 146C View FIGURE 146 ). The number of slits can therefore be considered 7 (2 + 5).

Šulc (1934: p. 16) described the new subgenus Pseudoacanthochiton for species characterized by tail valves similar to those of Acanthochitona , but with additional slits on the insertion plate. Considering that some tail valves of Acanthochitona with additional slits on the insertion plate had been reported [see above for A. fascicularis ( Linnaeus, 1767) ], we prefer to consider the genus Pseudoacanthochitona ( Šulc, 1934) as a synonym of Acanthochitona Gray, 1821 .

Comparisons. See Tab. 22 and 23 for a comparison with the Acanthochitona and Craspedochiton spp . considered in this study.

Distribution. Lower Miocene: northeastern Atlantic (Burdigalian): Aquitaine Basin, France: Maureilhan ( Dell’Angelo et al. 2020b).