Callistochiton ( Allerychiton ) pachylasmae ( Monterosato, 1879 ex G. Seguenza ms)

Callistochiton ( Allerychiton) pachylasmae ( Monterosato, 1879 ex G. Seguenza ms) View in CoL

Fig. 85 View FIGURE 85

Chiton Pachylasmae ; Monterosato 1878a, p. 77; Tiberi 1879, p. l50 ( nomen nudum). Chiton Pachylasmae Monterosato, 1879, p. 24 ; Maluquer 1916, p. 213, 262. Callistochiton pachylasmae View in CoL ; Kaas & Van Belle 1980, p. 94; Dell’Angelo et al. 1998b, p. 139, fig. 1; Kaas & Van Belle 1998, p.

136; Crocetta et al. 2014, p. 201; Amati & Oliverio 2016, p. 55, figs 1, 2; Appolloni et al. 2018, p. 21, figs 1C–D. Callistochiton ( Allerychiton) pachylasmae View in CoL ; Dell’Angelo & Oliverio 1997, p. 145, figs 1–13; Dell’Angelo & Smriglio 1999, p.

121, pls 39, 39 [bis], fig. 54; Dell’Angelo et al. 2015a, p. 238; Dell’Angelo et al. 2018c, p. 216, figs 1–3. Callistochiton ( Allerychiton) pachyplasmae [sic]; Koukouras & Karachle 2005, p. 30. Callistochiton sp. Dell’Angelo et al. 1998b, p. 139, fig. 2; Dell’Angelo & Smriglio 2009, p. 124; Dell’Angelo et al. 2015a, p.

238 ( fide Dell’Angelo et al. 2018c).

Type material. Holotype: MCZR –M–12683/H, a specimen (3.4 x 1.3 mm) collected by G. Seguenza on Pachylasma giganteum ( Appolloni et al. 2018) .

Type locality. Strait of Messina (Italy) .

Material examined. Pleistocene: Italy: Calabria: Santa Maria di Catanzaro; 1 head valve, width 1.3 mm ( BD 571, Figs 85A–B View FIGURE 85 ); Gallina: 1 head valve, width 1.5 mm ( AV, Figs 85C–D View FIGURE 85 ), Pecoraro: 1 intermediate valve, width 1.6 mm ( BD 572, Figs 85G–H View FIGURE 85 ). Recent: Italy: Pantelleria ( SB), Fig. 80F View FIGURE 80 ; Greece: Corfu ( SB), Fig. 85E View FIGURE 85 .

Description. Head valve semicircular, posterior margin widely V-shaped, with a rounded apical notch. Intermediate valves broadly rectangular, elevated, semicarinate in anterior profile, posterior margin nearly straight and apex slightly prominent, lateral areas raised. Tail valve semicircular, mucro slightly posterior, not prominent, postmucronal slope slightly concave.

HV and LA sculptured with large wrinkled, elevated radial ribs, 6–8 in HV starting from edge of apparently smooth apical area, interstices coarsely granulated, one in LA, starting from apex, covered by scaly tubercles. PMA with one wrinkled radial rib on either side a little behind antemucronal-postmucronal boundary, sculptured with large tubercles irregularly disposed, appearing arranged along concentric lines. CA and AMA sculptured with large, irregularly disposed, close-set tubercles, surface between tubercles strongly coarse, tubercles irregularly roundish, larger diameter ca. 44–56 µm in CA, with up to three aesthetes in lower part and another one or two slightly larger in more central position. Tail valve with antemucronal area sculptured with large tubercles as in central area of intermediate valves,

Articulamentum perlaceous, apophyses small and triangular, intermediate valves with two deep lateral dimples near apical area, with a marked radial grove, insertion plate divided by one deep cut on either side in two smooth teeth, slit formula 6–8 / 1 /?.

Remarks. The name was made available by a description included in the paper of Monterosato on Polyplacophora (1879), but the manuscript name by Giuseppe Seguenza already appeared as a nomen nudum in Monterosato (1878a: 77) and Tiberi (1879: 150). Callistochiton pachylasmae ( Monterosato, 1879) is the type species of the subgenus Allerychiton Dell’Angelo & Oliverio, 1997 (by original designation), proposed to highlight the unusual sculpture of this species, whose tail valve is different from that of the head valve, and with a single radial rib present on the lateral areas of the intermediate valves, vs. 2 or more ribs present in the other Callistochiton species ( Kaas & Van Belle 1994).

Dell’Angelo et al. (2018c) reported the second documented record of C. pachylasmae from the Strait of Messina , of this rarely recorded species. These authors contributed to the knowledge of habitat and distribution of this enigmatic species.

The fossil record of Callistochiton pachylasmae is limited to the Pleistocene (Calabrian) of southern Italy, with two head valves from Gallina (Reggio Calabria) and Santa Maria di Catanzaro, determined by Dell’Angelo et al. (1998b) as Callistochiton sp. and C. pachylasmae , respectively, and an intermediate valve from Pecoraro.

Comparisons. Callistochiton pachylasmae has CA sculptured with large, irregularly disposed, close-set tubercles, and this is the main difference towards the other two Callistochiton species discussed in this work, both showing a sculpture of granulose longitudinal ribs (see Tab. 13).

Distribution. Pleistocene: central Mediterranean, southern Italy: Gallina, Pecoraro, Santa Maria di Catanzaro ( Dell’Angelo et al. 1998b; this paper). Recent: Atlantic Ocean: Morocco ( Kaas, 1991); Mediterranean Sea: Italy, Croatia, Greece ( Dell’Angelo et al. 2018c).