Callochiton calcatus, Dell'Angelo & Palazzi, 1994

Callochiton calcatus View in CoL Dell’Angelo & Palazzi, 1994

Fig. 50 View FIGURE 50

Callochiton calcatus View in CoL Dell’Angelo & Palazzi, 1994 a, p. 15, figs 1–12, 14–19, 20B; Dell’Angelo & Giusti 1997, p. 52, fig. 7; Van Belle 1997, p. 50, pl. 41; Dell’Angelo et al. 1998a, p. 246, pl. 2, figs 5, 7; Dell’Angelo & Smriglio 1999, p. 131, pls 42–43, figs 64–66; Dell’Angelo et al. 2001a, p. 147, fig. 9; Chirli 2004, p. 8, pl. 2, figs 16–18; Kaas et al. 2006, p. 24, fig. 4, map 33; Puchalski et al. 2008 (database: chiton fossil records); Dell’Angelo et al. 2013, p. 84; Dell’Angelo et al. 2017, p. 248; Dell’Angelo et al. 2018b, p. 34; Forli & Guerrini 2022, fig. 11.18 (15).

Type material. Holotype MZB 11620, one specimen (length 4.5 mm). Paratypes: Dipartimento di Scienze della Terra of Milan University, reg.n. 7040–7044, 5 spm from Isole Pontine); private collections: AL (1 spm from Favignana Island), BD 4283 (1 spm from Isole Pontine), BD P50/4 (10 valves from Marettimo Island), BD P152/3 (2 valves from Northern Tyrrenian Sea), BD P157/2 (1 valve from Elba Island), EM (1 spm from Capo Passero), LB (1 spm from Trapani), LD A199 (1 spm from Mallorca Island), LD A208 (3 spm from Mallorca Island), A213 (2 spm from Mallorca Island), LD A256 (2 spm from Mallorca Island), LD A412 (1 spm from Formentera).

Type locality. Villasimius (Cagliari) ex nets at 80–100 m.

Material examined. Pliocene: Italy: Tuscany: Poggio alla Fame : 10 valves ( BD 573 , Figs 50A–G View FIGURE 50 ). Pleistocene : Italy: Tuscany: Capraia Island-Capo Corso -350/ 500 m: 62 valves ( BD 574 , Figs 50H–I View FIGURE 50 ); Calabria: Petti di Carrubbare : 8 valves ( BD 575 , Figs 50J–L View FIGURE 50 ); Sicily: Capo Milazzo: 2 valves ( BD 576 ), Casa Catarinicchia: 5 valves ( BD 577 ), Casa Parrino: 6 valves ( BD 578 ). Recent : Italy: type material from Elba Island (Grosseto), Isole Pontine (Latina), Villasimius (Cagliari), Capo Passero (Siracusa), Favignana Island (Trapani), Trapani ; Spain: type material from Formentera and Mallorca Island . Maximum width of the valves: 2.5 / 2.5 / 1.9 mm .

Description: Head valve semicircular, front slope convex. Intermediate valves broadly rectangular (W/L = 1.51–1.98), semicarinate in anterior profile, decidedly elevated (H/W = 0.50–0.75), anterior margin convex to sinuose, side margins rounded, posterior margin concave at both sides of strongly protruding, obtuse apex, lateral areas raised, quite evident. Tail valve semicircular/elliptical (W/L = 1.44–1.50), anterior margin convex, mucro in anterior position, not prominent, antemucronal slope almost straight, postmucronal slope slightly convex.

Tegmentum granulose, rough. HV, LA and PMA sculptured with fine granules, fuse into continuous lines radially, and with characteristic black dots, pigmented cusps of shell-eyes, irregularly distributed but with prevalence towards margins. CA and AMA sculptured with longitudinal lines of micraesthetes, with 6–8 conspicuous longitudinal grooves in front of diagonal ridge, grooves shortening towards jugum

Articulamentum with apophyses wide, short, regularly rounded, connected at jugum by a relatively long jugal plate, slit formula 15–16 / 2 / 12–13, teeth irregular, slit rays evident, with large pores, eaves coarsely porous.

Remarks. The fossil record of Callochiton calcatus Dell’Angelo & Palazzi, 1994 is limited to the Pliocene and Pleistocene of Italy.

This extant species is easily distinguished by the mucro in anterior position, the postmucronal slope convex noticeable dorsal and the noticeable elevation of its intermediate valves. We have attributed to Callochiton calcatus some valves from the Pliocene of Poggio alla Fame showing a slightly lower H/W ratio (0.50–0.56, Figs 50C–D View FIGURE 50 , vs. 0.65–0.72 reported in the original description of the species). The other characters are those typical of C. calcatus , e.g., the mucro’s position of the tail valve and the convexity of the postmucronal slope ( Figs 50G View FIGURE 50 ), and so we can consider the H/W ratio to be highly variable, in a range from 0.50 ( Fig. 50C View FIGURE 50 ) to 0.75 ( Fig. 50I View FIGURE 50 ).

Comparisons. Differences with Callochiton pouweri are discussed below.

Distribution. Pliocene: central Mediterranean, Italy: Poggio alla Fame ( Chirli 2004; this study). Pleistocene. central Mediterranean, Italy: Capraia Island-Capo Corso -350/ 500 m ( Dell’Angelo et al. 2001a), Capo Milazzo, Casa Catarinicchia, Casa Parrino, Petti di Carrubbare (this study). Recent. Mediterranean: random records from the Balearic Islands (Mallorca, Formentera) to the Tyrrhenian coasts of Italy, Sardinia and Sicily ( Dell’Angelo & Smriglio 1999), Strait of Sicily: Graham Bank (Dell’Angelo et al. 2008).