Lepidochitona oligocaena ( Rolle, 1862 )

Lepidochitona oligocaena ( Rolle, 1862) View in CoL

Fig. 117 View FIGURE 117

Chiton oligocaenus Rolle, 1862, p. 213 , pl. 1, figs 9–14; Janssen 1978, p. 226; Dell’Angelo et al. 2018a, p. 11.

Lepidopleurus oligocaenicus ; de Rochebrune 1882, p. 57.

Ischnochiton oligocaenicus ; Fischer P.-H. 1957, p. 17.

Lepidochitona oligocaena View in CoL ; Van Belle 1981, p. 54; Dell’Angelo et al. 2011, p. 953; Dell’Angelo et al. 2015b, p. 365; Dell’Angelo et al. 2019b, p. 304, figs 3A, 4A–O; Dell’Angelo et al. 2020b, p. 13, figs 8–9.

Lepidopleurus daubrei (non Lepidopleurus daubrei de Rochebrune, 1882 ); Varone 2008, p. 156, figs 1–7 ( fide Dell’Angelo et al. 2020b).

Stenoplax monila Cherns & Schwabe 2019, p. 7 View in CoL , figs 5(I)–(X), (A′), (B′).

Type material. Syntypes: NHMW 1858/0018/0255, 47 valves, Figs 117A–D View FIGURE 117 ; NHMW 2011/0009/0006, 2 valves.

Type locality. Gaas , Larrat ( France) .

Type stage. Oligocene (Rupelian).

Material examined. Lower Oligocene (Rupelian): Gaas: type material ( syntypes) plus Benoist collection MHNBx [MHNBx 2014.14.32.1–2014.14.32.3)]; ZSM Mol 20060781 [12 valves (described as Stenoplax monila Cherns & Schwabe, 2019 )]; Rupelian levels: 208 valves ( BD 941 , Figs 117E–L View FIGURE 117 , MZB 50569–50570); Espibos : 45 valves ( AC, DA, PR); Lagouarde : 56 valves ( AC, DA); Larrat : 3,289 valves ( AC, Figs 117O–P View FIGURE 117 , DA, Figs 117M–N View FIGURE 117 , JCV, MHNBx 2020.2.1 – 2020.2.3). Maximum width of the valves: 3.8 / 5.5 / 4 mm .

Description. Head valve semicircular, some radial folds (6–8) may be present. Intermediate valve broadly rectangular (W/L = 2.38–3.44), rounded in anterior profile, moderately to highly elevated (H/W = 0.27–0.54), anterior margin almost straight or slightly sinuose, slightly concave in the jugum, side margins weakly rounded, posterior margin straight, apex not evident or just highlighted, lateral areas scarcely differentiated. Tail valve semicircular to triangular, W/L = 1.41–1.77, anterior margin slightly sinuose, slightly concave in jugum, mucro subcentral, antemucronal and postmucronal slopes almost straight, slightly concave behind mucro.

Tegmentum entirely covered by close-set, irregularly arranged granules, some coalescent, roundish in head valves (diameter ca 40–45 μm), more irregular and tending to rhomboidal in other valves (maximum length ca 50–68 μm). Each granule with 8–12 aesthetes of equal size, often one (or more) of them centrally located.

Articulamentum with rounded apophyses, separated by a wide jugal sinus, slit formula: 10–11 / 2 / 9–13, slits inequidistant, the second slit in intermediate valves lies close to posterior margin, slit rays clearly visible in head and intermediate valves, teeth short.

Remarks. A study of a large quantity of valves from Gaas was carried out by Dell’Angelo et al. (2020b), who also examined the valves preserved in the Benoist collection (MHNBx 2014.14.32.1–2014.14.32.3), attributed to Lepidochitona oligocaena ( Rolle, 1862) . In addition, these Authors examined the valves preserved at ZSM as Stenoplax monila Cherns & Schwabe (2019) , concluding that this species is a younger synonym of L. oligocaena .

Lepidochitona oligocaena is characterized by a remarkable morphological variability ( Dell’Angelo et al., 2020b). Some intermediate valves ( Figs 117M–N View FIGURE 117 ) are more elongated (W/L = 2.91–3.44) and lower in anterior profile (H/W = 0.27–0.32), but the sculpture is similar with the same rhomboidal granules of L. oligocaena with aesthetes of equal size, and these valves have been considered to fall within the range of variability of L. oligocaena . The same for some tail valves ( Figs 117O–P View FIGURE 117 ) more than semicircular or tending to a triangular shape, compare Figs 117J View FIGURE 117 and 113O View FIGURE 113 .

Comparisons. Lepidochitona oligocaena is characterized by the intermediate valves showing a posterior margin straight with the apex not evident or just highlighted, LA scarcely differentiated and the tegmentum covered by irregular subrhomboidal granules with aesthetes of equal size, not differentiated. The only other species of Lepidochitona from Gaas with similar characters is L. tessellata Dell’Angelo, Lesport, Cluzaud & Sosso, 2020, which however differs in having different and coarser sculpture on LA and PMA (see below).

Distribution. Lower Oligocene: northeastern Atlantic (Rupelian): Aquitaine Basin, France: Gaas ( Rolle, 1862; Dell’Angelo et al. 2019 b, 2020b).