Rhyssoplax saeniensis ( Laghi, 1984 )

Rhyssoplax saeniensis ( Laghi, 1984)

Fig. 95 View FIGURE 95

Chiton saeniensis Laghi, 1984, p. 556 View in CoL , pl. 1, figs 1–20; Bertarelli & Inzani 1985, p. 299; Zanaroli 1985, tab. 2; Beurlen & Richter 1988, p. 59, fig.; Dell’Angelo & Forli 1995b, p. 77, figs 1–3; Giani 1998, p. 116, pl. 43, fig. 11; Della Bella & Scarponi 2000, p. 68; Dell’Angelo et al. 2001a, p. 152, fig. 29; Dell’Angelo et al. 2003, p. 1193; Forli et al. 2003, p. 152; Chirli 2004, p. 15, pl. 5, figs 16–18, pl. 6, figs 1–6; Schwabe 2005, p. 101; Puchalski et al. 2008 (database: chiton fossil records); Dell’Angelo et al. 2013, p. 89; Dell’Angelo et al. 2016, p. 82; Cresti & Forli 2021, p. 192, 200; Forli & Guerrini 2022, p. 173, fig. 11.9.

Type material. Holotype MPM, intermediate valve figured by Laghi (1984: pl. 1, fig. 6a–b).

Type locality. Serre di Rapolano (“ Il Campino ”), Siena ( Tuscany, Italy) .

Type stage. Pliocene, “sabbie gialle senesi”.

Material examined. Pliocene: Italy: Tuscany: Cetona: 1 valve ( BD 819), Cisternino: 2 valves ( BD 820), Linari: 1 valve ( BD 821), Montaione: 1 valve ( BD 822), Orciano Pisano: 2 valves ( BD 823), Pietrafitta: 54 valves ( BD 824), Pietrafitta Melograni: 48 valves ( BD 825); Serre di Rapolano: 1500+ valves ( BD 826, Figs 95A–L View FIGURE 95 ), Villa Banfi: 2 valves ( BD 827); Umbria: Castel Viscardo: 1 valve ( BD 828), Corbara: 1 valve ( BD 829), Fabro Scalo: 2 valves ( BD 830). Maximum width of the valves: 15 / 20 / 14 mm.

Description. Head valve semicircular, front slope almost straight to faintly convex, posterior margin widely V-shaped. Intermediate valves broadly rectangular, elongated (W/L = 2.20–3.44), moderately elevated (H/W = 0.27–0.39), carinated in anterior profile, anterior margin straight to slightly convex, side margins slightly rounded, posterior margin almost straight, minute apex. Tail valve semicircular (W/L = 1.81–2.02), anterior margin almost straight, mucro subcentral, antemucronal slope straight, postmucronal slope almost straight to weakly concave.

Tegmentum smooth, crossed by several well marked growth lines, sometimes very light flat ribs are present, radial on HV, LA and PMA, longitudinal on CA and AMA. Aesthetes very dense, each megalaesthete accompanied by many micraesthetes.

Articulamentum with apophyses rounded, tending to trapezoidal in tail valve, jugal sinus narrow, provided with a concave, dentate plate, teeth minutely denticulate, slit formula 11 / 1 / 12 (a second slit ray lies close to posterior margin on intermediate valves), slits narrow, slit rays weekly marked.

Remarks. The fossil record of Rhyssoplax saeniensis ( Laghi, 1984) is limited to the Pliocene of Italy; the species is very abundant at the site of its first discovery (Serre di Rapolano), much less frequent (a few valves) in the other Pliocene localities from Tuscany and Umbria. Despite the large size, it is very difficult to find complete valves, they are almost always incomplete or abraded ( Fig. 95I View FIGURE 95 ), with the tegmentum often densely bored by endobionts ( Fig. 95D View FIGURE 95 ).

Comparisons. Laghi (1984) compared Rhyssoplax saeniensis with R. olivacea ( Spengler, 1797) , illustrating the main morphological differences between the two taxa ( Laghi 1984: fig. 1); this Author also highlighted the different sculpture of R. olivacea (not formed by real longitudinal grooves/ribs more or less evident as in R. saeniensis , but rather by the spaces between small folds on the tegmentum).

Distribution. Pliocene: central Mediterranean, Italy: Tuscany: Cetona, Cisternino, Montaione, Orciano Pisano, Pietrafitta, Pietrafitta Melograni, Serre di Rapolano, Linari, Villa Banfi ( Laghi 1984; Dell’Angelo et al. 2001a; Chirli 2004; this study); Umbria: Castel Viscardo, Corbara, Fabro Scalo (this study).