Ischnochiton cluzaudi, Dell’Angelo & Sosso & Taviani, 2025

Ischnochiton cluzaudi sp. nov.

Fig. 64 View FIGURE 64

Ischnochiton korytnicensis View in CoL (non Ischnochiton korytnicensis Bałuk, 1971 View in CoL ); Dell’Angelo et al. 2018a, p. 23, fig. 5J–L; Dell’Angelo et al. 2020b, p. 5, fig. 2.

Type material. Holotype MSNG 62642 View Materials , intermediate valve, width 4.6 mm ( Figs 64D–F View FIGURE 64 ) . Paratype 1, MSNG 62643 View Materials , tail valve, width 3.5 mm ( Figs 64I–L View FIGURE 64 ) . Paratype 2, MHNBx 2024.2.1, head valve, width 3.6 mm ( Figs 64A–C View FIGURE 64 ).

Type locality. Gaas ( France) .

Type stage. Lower Oligocene (Rupelian) .

Etymology. The specific name honors Alain Cluzaud (Pessac, France), who collected part of the material here presented and greatly contributed to the knowledge of the malacofauna of the Aquitaine Basin.

Material examined. Lower Oligocene: France: Gaas : type material plus 66 valves ( AC, BD 529 , DA, JVC, Fig. 64G View FIGURE 64 , PR). Lower Miocene : France (Burdigalian): Carrière Vives : 3 valves ( BD 530 , JFL, Fig. 64H View FIGURE 64 ). Maximum width of the valves: 3.6 / 6.8 / 5.2 mm .

Diagnosis. Head valve semicircular, intermediate valves broadly rectangular, moderately elevated, rounded, without trace of apex, tail valve semicircular, mucro subcentral. Tegmentum sculptured with irregularly disposed granules, elevated and closely spaced in HV, LA, PMA, loosely longitudinally arranged, some granules joined together in CA, AMA, tegmentum fully covered by aesthetes, both on granules and between them. Articulamentum with apophyses rounded, slit formula: 11 / 2 / 9–10.

Description. Head valve semicircular, posterior margin widely V-shaped, slope straight. Intermediate valves broadly rectangular (W/L = 2.75–2.92), moderately elevated (H/W = 0.39–0.42), rounded in anterior profile, anterior margin almost straight on jugal side, side margins slightly rounded, posterior margin straight, without trace of apex, lateral areas poorly elevated. Tail valve semicircular (W/L = 1.46–1.78), anterior margin almost straight, mucro subcentral, antemucronal slope slightly convex, postmucronal slope slightly concave just under mucro.

HV, LA and PMA sculptured with closely spaced, elevated, irregularly distributed and irregularly shaped granules. CA and AMA sculptured with irregularly disposed granules, loosely longitudinally arranged, some granules joined together, others splitting in various areas, mainly near anterior margin and towards LA, more irregular in JA. Tegmentum fully covered by aesthetes, both on granules (a subcentral megalaesthete and up to 8–10 micraesthetes in PMA) and between them.

Articulamentum with apophyses rounded, jugal sinus wide, insertion lamina divided into rather large, irregular teeth, slit formula: 11 / 2 / 9–10.

Remarks. The fossil record of Ischnochiton cluzaudi sp. nov. is limited to the lower Oligocene and lower Miocene of the Aquitaine Basin ( France).

A lot of valves from the lower Oligocene and lower Miocene of the Aquitaine Basin has been attributed to Ischnochiton korytnicensis Bałuk, 1971 by Dell’Angelo et al. (2018a, 2020b), and really the two species ( I. korytnicensis and I. cluzaudi sp. nov.) are at first glance similar. Ischnochiton cluzaudi sp. nov. shows CA and AMA sculptured with granules irregularly longitudinally arranged, very different by the quite regular longitudinal and parallel ribs present in I. korytnicensis . This difference in the sculpture of CA and AMA is sufficient to separate the valves of I. cluzaudi sp. nov. from those of I. korytnicensis (compare Figs 70C View FIGURE 70 and 64D View FIGURE 64 ).

We illustrate a valve showing coalescence of the last two plates into one plate vii-viii ( Fig. 64G View FIGURE 64 ), rarely described in fossil valves [e.g., Dell’Angelo & Forli 1995b for two intermediate valves of Rhyssoplax saeniensis Laghi, 1984 from the Pliocene of Serre di Rapolano (Siena, Italy).

Comparisons. See Tabs 9, 10 for a comparison with the Ischnochiton spp. considered in the present study.

Distribution. Lower Oligocene: northeastern Atlantic (Rupelian): Aquitaine Basin, France: Gaas ( Dell’Angelo et al. 2020b; this study). Lower Miocene: northeastern Atlantic (Burdigalian): Aquitaine Basin, France: Carrière Vives ( Dell’Angelo et al. 2018a; this study).