Callochiton septemvalvis ( Montagu, 1803 )

Callochiton septemvalvis ( Montagu, 1803) View in CoL

Fig. 53 View FIGURE 53

Chiton laevis Montagu, 1803, p. 2 ; Brogger 1900, p. 660; Antevs 1917, p. 396 –409, 416; Antevs 1928, p. 666 –677.

Chiton septemvalvis Montagu, 1803, p. 3 .

Callochiton laevis View in CoL ; Laghi 1977, p. 108, pl. 2, figs 14–18 ( partim); Zanaroli 1985, p. 79, partim

Callochiton septemvalvis septemvalvis View in CoL ; Kaas 1978, p. 73.

Callochiton septemvalvis View in CoL ; Dell’Angelo & Forli 1995b, p. 78; Dell’Angelo & Silva 2003, p. 11 ( partim); Dell’Angelo et al. 2004, p. 34 ( partim); Puchalski et al. (database: chiton fossil records); Strack 2010, p. 62, fig. 51; Dell’Angelo et al. 2016, p. 73, pl. 1, figs 1–4; Dell’Angelo et al. 2018b, p. 31; Dell’Angelo et al. 2020b, p. 52, tab. 9; Dell’Angelo et al. 2022, p. 12, figs 7.7–7.12.

non Callochiton septemvalvis View in CoL ; Dell’Angelo & Forli 1995a, p. 226, figs 10, 17; Dell’Angelo & Giusti 1997, p. 51, fig. 5; Dell’Angelo & Smriglio 1999, p. 125, pls 40–41, figs 55–63; Dell’Angelo et al. 2001a, p. 147, fig. 10; Forli et al. 2003, p. 152; Chirli 2004, p. 8, pl. 3, figs 1–4; Dell’Angelo et al. 2004, p. 34, pl. 3, figs 2, 5, partim; Garilli et al. 2005, p. 134, pl. 2, figs 7–10; Dell’Angelo & Vardala-Theodorou 2006, p. 326, 2 figs; Dell’Angelo et al. 2007b, p. 141; Koskeridou et al. 2009, p. 314, figs 8.3–8.4; Dell’Angelo et al. 2012, p. 60, fig. 4L; Dell’Angelo et al. 2013, p. 83, pl. 5, figs N–P; Ruman & Hudáčková, p. 160, figs 2.7, 2.8, 3.1 (= Callochiton doriae View in CoL , fide Dell’Angelo et al. 2016).

? Chiton laevis ; Manzoni 1868, p. 67; Seguenza 1874, p. 12; Tiberi 1877, p. 143, 147, 158; Socin 1941, p. 245.? Chiton laevis var.; Brugnone 1877, p. 18.

? Callochiton laevis View in CoL ; Leloup & Volz 1938, p. 49.

? Callochiton achatinus View in CoL ; Crovato & Taviani 1985, p. 292.

? Callochiton septemvalvis View in CoL ; Vardala-Theodorou & Nicolaidou 2007, p. 64; Albano & Sabelli 2011, p. 211.

Type material. Holotype NHMUK ( fide Kaas & Van Belle 1985b).

Type locality. Salcomb Bay ( England) .

Material examined. Miocene (Tortonian): Italy: Borelli: 1 valve ( BD 633, Figs 53F–H View FIGURE 53 ) Rio di Bocca d’Asino: 33 valves ( BD 634, MZB 32062–32063, PG), Montegibbio: 10 valves ( BD 635, MZB 32064, Fig. 53E View FIGURE 53 ). Pliocene: Portugal: Vale de Freixo: 269 valves ( GeoFCUL VFX.03.355, GeoFCUL VFX.03.337–338, RGM.1364009– 1364011, MNHN.F. A81986 View Materials , BD 241); Spain: Estepona: 2 valves ( BD 636); Italy: Poggio alla Fame: 1 valve ( BD 637, Figs 53A–C View FIGURE 53 ), Serre di Rapolano: 2 valves ( BD 638, Fig. 53D View FIGURE 53 ). Maximum width of the valves: 2 / 6.8 / 3.8 mm.

Description. Head valve semicircular, posterior margin widely V-shaped, front slope straight to slightly concave. Intermediate valves broadly rectangular, width about three times length (W/L = 2.86–3.16), carinate in anterior profile, moderately elevated (H/W = 0.35–0.46), anterior margin regularly convex or somewhat sinuate, side margins slightly rounded, posterior margin slightly concave at both sides of pronounced apex, lateral areas raised. Tail valve semicircular, anterior margin straight or slightly convex, central mucro small, hardly raised, antemucronal slope almost straight, postmucronal slope straight or very slightly concave directly behind mucro.

Tegmentum granulose, rough. HV, LA and PMA sculptured with fine microgranules, fuse into continuous radial lines, marked with a few concentric growth lines toward outer margins, and with characteristic black dots, pigmented cusps of shell-eyes, irregularly distributed in HV and PMA, in radiating rows in LA, except for a narrow strip along posterior margin. CA and AMA sculptured with regular longitudinal lines of micraesthetes interrupted only by megalaesthetes distributed normally in a six-row recurring sequence.

Articulamentum with apophyses wide, short, regularly rounded, connected at jugum by a lamina, insertion plates short, with many slits, slit formula 19–20 / 2–3 / 17–18, teeth irregular, slit rays well visible, eaves very porous.

Remarks. The fossil record of Callochiton septemvalvis ( Montagu, 1803) is limited to the Miocene (Tortonian) of Italy, and Pliocene of Italy, Spain and Portugal.

We follow Dell’Angelo et al. (2016) by attributing fossil valves of Callochiton without longitudinal grooves on the pleural areas to Callochiton septemvalvis , whilst those provided of longitudinal grooves, to Callochiton doriae ( Capellini, 1859) (synonym of Chiton euplaeae O.G. Costa, 1830, nomen dubium). We are unable to ascertain whether literature records of C. septemvalvis do all pertain to Montagu’s species, or if they encompass C. doriae . Thus, we report in the synonymy only those specimens that are safely ascribable to one or another taxon based upon the morphological details discussed above, while dubious cases are also reported in the bibliographic references, with the species preceded by a question mark.

The sculpturing of the dorsal surface was highlighted by Baxter & Jones (1984) and Carmona Zalvide et al. (2002). Although past authors did not fully appreciate the characteristics differentiating this species from C. doriae , we consider C. septemvalvis as absent from the present Mediterranean chiton fauna, although its fossil record in this basin leaves the question open.

Comparisons. This species is quite similar to Callochiton doriae (see above), from which it differs mainly by the absence of longitudinal grooves on CA and AA. Carmona Zalvide et al. (2002), in their study on modern Callochiton spp . of the Iberian peninsula, showed other differences regarding the maximum size of the living specimens ( 22 mm for C. septemvalvis vs. 18 mm for C. doriae ), the structure of the aesthetes, both as regards the maximum diameter of the megalaesthetes, almost double for C. septemvalvis (10 µm vs. 5.5 µm for C. doriae ), and for the average distance between the micraesthetes aligned on the longitudinal ribs (3 µm for C. septemvalvis vs. 16 µm for C. doriae ); the species differs also for the slit formula (19/2–3/17–18 for C. septemvalvis vs. 15–16/2/14–16 for C. doriae ), and for characters not evaluable in fossil material, i.e. the size of all three types of girdle’s (dorsal, marginal and ventral) spicules.

Distribution. Upper Miocene: Proto-Mediterranean Sea (Tortonian): Po Basin, N Italy: Rio di Bocca d’Asino, Montegibbio ( Laghi 1977; Dell’Angelo et al. 2016). Pliocene: North Atlantic: Netherlands ( Strack 2010); northeastern Atlantic, Mondego Basin, Portugal: Vale de Freixo ( Dell’Angelo & Silva 2003; Dell’Angelo et al. 2022); western Mediterranean, Estepona Basin, Spain: Estepona (Dell’Angelo et al. 2004); central Mediterranean, Italy: Bibbiano, Serre di Rapolano ( Dell’Angelo & Forli 1995a; this study). Upper Pliocene to upper Pleistocene: central Mediterranean, Greece: Rhode Island ( Koskeridou et al. 2009). Pleistocene: North Atlantic: Sweden and Norway ( Brogger 1901; Antevs 1917, 1928). Recent: Northeastern Atlantic Ocean, from Scandinavia: N. Norway N to 67° N ( Dons 1934; Hansson 1998) along the European coasts (McKay & Smith 1979) to Portugal and Spain ( Rolan Mosquera et al. 1990; Consolado Macedo et al. 1999; Urgorri et al. 2017) and the Canary Islands ( Hernández & Rolán 2011; Dell’Angelo & Smriglio 1999).