Rhyssoplax capecchii ( Chirli, 2004 )

Rhyssoplax capecchii ( Chirli, 2004)

Fig. 88 View FIGURE 88

Chiton capecchii Chirli, 2004, p. 12 , pl. 3, figs 17–18, pl. 4, figs 1–16; Schwabe 2005, p. 91; Puchalski et al. 2008 (database: chiton fossil records); Dell’Angelo et al. 2013, p. 80, 89; Dell’Angelo et al. 2016, p. 82; Brunetti & Cresti 2018, p. 17, 28, fig. 7; Brunetti & Cresti 2023, p. 10.

Type material. Holotype MZB 42947.

Type locality. Poggio alla Fame, Siena ( Tuscany, Italy) .

Type stage. Pliocene, “sabbie argillose grigie” (gray clayely sands).

Material examined. Pliocene: Italy: Tuscany: Bibbiano: 12 valves ( BD 646), Cetona: 2 valves ( BD 647), Pietrafitta: 60 valves ( BD 648), Pietrafitta Melograni: 46 valves ( BD 649, Fig. 88D View FIGURE 88 ), Poggio alla Fame: 20 valves ( BD 650, Figs 88A–C, 88E–L View FIGURE 88 ). Maximum width of the valves: 2.8–3.3 – 2.6 mm.

Description. Head valve semicircular, front slope slightly convex, posterior margin widely V-shaped. Intermediate valves broadly rectangular, elongated (W/L = 2.27–3.30), moderately elevated (H/W = 0.29–0.37), carinate in anterior profile, anterior margin straight, side margins almost straight, posterior margin straight, minute apex, lateral areas not raised and not separated from CA. Tail valve semicircular (W/L = 1.71–1.83), anterior margin straight, mucro in anterior position, antemucronal slope straight, postmucronal slope slightly concave.

Tegmentum smooth, crossed by few growth lines just highlighted (not in LA). Aesthetes very dense, each megalaesthete accompanied by many micraesthetes.

Articulamentum with apophyses tending to trapezoidal, jugal sinus large, slit formula 12–15 / 1 / 15–16 (a second slit ray lies close to posterior margin on intermediate valves), teeth short, irregular, finely pectinated, slits narrow, slit rays very finely indicated.

Remarks. The fossil record of Rhyssoplax capecchii ( Chirli, 2004) is thus far limited to the Pliocene of Tuscany ( Italy).

Chirli (2004) described the small valves of this species (up to 3.6 mm of width) and highlighted the similarity to Rhyssoplax saeniensis ( Laghi, 1984) ; in his opinion, R. capecchii differs from its smaller size, and other characteristics ( Chirli 2004), which could well be included in the known variability of shape and sculpture existing in growth series of Polyplacophora [see, for example, Dell’Angelo et al. 2012, for Stenosemus dolii ( Van Belle & Dell’Angelo, 1998) from Altavilla; Dell’Angelo et al. 2022, for Ischnochiton zbyi Dell’Angelo & Silva, 2003 from Vale de Freixo]. Laghi (1984) already highlighted such valve’s variability, in the original description of R. saeniensis a growth series of head, intermediate and tail valves ( Laghi 1977: pl. 1, figs 1–5, 7–13, 15–20, respectively). The main differences between Rhyssoplax capecchii and R. saeniensis stay, therefore, in the considerably smaller size (maximum valve’s width 3.3 mm for R. capecchii ) and the smooth tegmentum of R. capecchii , whereas R. saeniensis consistently display little-pronounced flat radial/longitudinal ribs.

Moreover, the two taxa apparently inhabited different habitats: R. saeniensis is more often associated with shallow water sandy-gravels ( Forli et al. 2003), whilst R. capecchii lived in sandy-clayey bottoms ( Chirli & Bogi 2002; Bogi & Chirli 2004). Despite the great quantity of valves of R. saeniensis examined from Serre di Rapolano (> 1500), the smallest valve found has a width of 5.1 mm. Further, the two species do not co-occur in the sites where they are most frequently found (Serre di Rapolano for R. saeniensis, Poggio alla Fame and Pietrafitta Melograni for R. capecchii ). We prefer at this stage to keep, therefore, the two taxa distinct.

Comparisons. Rhyssoplax capecchii differs from the other Rhyssoplax spp . mainly by the tegmentum entirely smooth.

Distribution. Pliocene: central Mediterranean, Italy: Tuscany: Bibbiano, Cetona, Pietrafitta, Pietrafitta Melograni, Poggio alla Fame ( Laghi 1984; Dell’Angelo et al. 2001a; Chirli 2004; this study).