Belknapchiton alveolus (M. Sars MS, Lovén, 1846 )

Belknapchiton alveolus (M. Sars MS, Lovén, 1846) View in CoL

Fig. 37 View FIGURE 37

Chiton alveolus Sars ms in Lovén, 1846, p. 159.

Leptochiton alveolus View in CoL ; Kaas 1981, p. 223, figs 8, 10A–B; Wu & Okutani, 1984, p. 6, pl. 1, figs 3–4, pl. 3, figs 9–14, pl. 4, fig. 1, pl. 5, figs 1–4; Kaas & Van Belle 1985a, p. 36, fig. 14, map 6; Kaas & Van Belle 1988, p. 25; Dell’Angelo et al. 2013, p. 71, pl. 2, figs A–D; Dell’Angelo et al. 2015a, p. 228; Dell’Angelo et al. 2021b, p. 410, figs 30–37.

Lepidopleurus alveolus View in CoL ; Dell’Angelo & Smriglio 1999, p. 69.

Lepidopleurus ( Leptochiton) alveolus View in CoL ; Dell’Angelo & Bonfitto 2005, p. 5, figs 13–16.

? Leptochiton ( Leptochiton) alveolus View in CoL ; Squires & Goedert 1995, p. 49, fig. 3–6; Goedert & Kaler 1996, p. 67; Rigby & Goedert 1996, p. 900; Peckmann et al. 2002, p. 870; Dell’Angelo et al. 2011, p. 936; Schwabe & Sellanes 2010, tab. 3; Bertolaso et al. 2015, p. 9; Hybertsen & Kiel 2018, p. 763.

Belknapchiton alveolus View in CoL ; Sirenko et al. 2022, p. 103, 104, 115; Taviani et al. 2023, p. 7, fig. 4.

non Lepidopleurus alveolus View in CoL ; Van Belle 1975, p. 57, figs 1–2, 4 (= Leptochiton geronensis View in CoL , fide Kaas & Van Belle 1985a).

Type material. Lectotype NRS, type collection n. 104, one specimen in alcohol, the valves missing (designed by Kaas 1981: 223).

Type locality. Bohuslän ( Sweden) .

Material examined. Lower Pliocene: Italy: Borzoli: 6 valves ( BD 407, MSNG); Zinola: 1 valve ( MZB 45701, Figs 37A–B View FIGURE 37 ). Pleistocene, presumably last glacial: Italy: off Bari, cruise SE06: SE06-35: 1 valve; SE06-40: 18 valves ( Figs 37E–G View FIGURE 37 ); SE06-50: 56 valves ( Fig. 37H View FIGURE 37 ). Recent. Norway, Sula Ridge, dredged at 215 m ( MZB, Figs 37C–D View FIGURE 37 ). Maximum width of the valves: 4.3 / 5.7 / 3.5 mm.

Description. Valves elevate and arcuate. Head valve semicircular, posterior margin widely V-shaped, notched in middle, slope straight. Intermediate valves broadly rectangular (W/L = 1.78–2.01), arched in anterior profile, elevate (H/W = 0.46), anterior margin straight, side margins rounded, posterior margin almost straight, without a distinct apex, lateral area indiscernible from central area, with few concentric growth marks. Tail valve more than semicircular (W/L = 1.38–1.66), anterior margin almost straight in wide jugal area, mucro not prominent, posteriorly placed at ⅓ from posterior margin, postmucronal slope concave.

Tegmentum rough, surface densely covered with oval microgranules disposing in quincunx, granules (length 70–80 µm) with one posterior megalaesthete and 4–5 micresthetes upward.

Articulamentum without insertion laminae, with small apophyses, sharply triangular, widely separated.

Remarks. Belknapchiton alveolus (M. Sars MS, Lovén, 1846) was originally described from the west coast of Scandinavia (Bohuslän, Sweden, and Bergen, Norway), but the species was reported under a dozen different names from elsewhere, and was initially considered as the only cosmopolitan species occurring in three oceans ( Kaas & Van Belle 1985a). Recently Wu & Okutani (1984) have demonstrated that B. belknapi ( Dall, 1878) , which is closely related to B. alveolus , shows several differential characters which warrant a specific separation, and a study of the geographical distribution of the synonymous species indicates that B. alveolus seems to be restricted to the Atlantic Ocean, while B. belknapi is confined to the Pacific and lndian Oceans. A good illustration of the valves of Recent specimens of Leptochiton alveolus is given by Wu & Okutani (1984).

Many valves of Belknapchiton alveolus from Sea-bottom sampling at bathyal depths off the Apulian margin (southwestern Adriatic Sea, SE06 cruise) have recently been studied by Taviani et al. (2023). The studied material ( Figs 37E–H View FIGURE 37 ) fully agree with the attribution to B. alveolus , compare with the figures of recent valves from Japan ( Wu & Okutani 1984: pl. 3, figs 9–14; pl. 4, fig. 1) and a single intermediate valve from Sula Ridge, Norway, dredged at 215 m ( Figs 37C–D View FIGURE 37 ). The finding of B. alveolus from the SE06 cruise is quite significant. This taxon is now extinct in the Mediterranean basin, also if some older reports from this area were erroneously recorded, i.e. Vayssiere (1913: 32 « Napoli, Palermo, Dalmatia », probably on the basis of records from previous authors) and Van Belle (1975), two specimens collected by fishermen at 200-250 m from Gerona, Spain, but the identification was proven wrong, and they have been reclassified as Leptochiton geronensis Kaas & Van Belle, 1985 , by the same authors.

Specimens reported as “ Leptochiton alveolus ” from Eocene-Oligocene cold-seep limestones of the Olympic Peninsula, Washington ( Squires & Goedert 1995; Peckmann et al. 2002, Schwabe & Sellanes 2010) has not been considered in the distribution of B. alveolus , because their specific and even generic assignment remains an open problem ( Schwabe & Sellanes 2010; Hybertsen & Kiel 2018).

Comparisons. See Tab. 5 for a comparison with the species of the Leptochiton tavianii group considered in the present study.

Distribution. Lower Pliocene: Italy, Liguria: Borzoli, Zinola ( Dell’Angelo et al. 2013, 2021b). Pleistocene, presumably last glacial: Italy: off Bari ( Taviani et al. 2023). Recent: North Atlantic: all along the Norwegian and Swedish west coast (Kaas 1981; Hansson 1988), Iceland ( Sneli & Gudmundsson 2018), Bay of Biscay ( Kaas 1979), Spain, Galicia ( Rolan Mosquera et al. 1990; Urgorri et al. 2017), NW Portugal ( Consolado Macedo et al. 1999), Gulf of Maine and Gulf of St. Lawrence ( Wu & Okutani 1984).

Genus Parachiton Thiele, 1909

Type species. Lepidopleurus ( Parachiton) acuminatus Thiele, 1909 View in CoL by original designation.

Distribution. Parachiton is known from the Miocene to the Recent. To date there are 23 Recent species known, all from the Indo-West Pacific except for P. africanus ( Nierstrasz, 1906) from the Mediterranean Sea. The fossil record includes the lower Miocene of France ( Dell’Angelo et al. 2018a), the Middle Miocene of Paratethys ( Šulc 1934; Bałuk 1984; Ruman & Hudácková 2015), the Miocene to Pleistocene of Italy ( Dell’Angelo et al. 2013, 2015a, 2018a) and the Pleistocene deposits of the Red Sea ( Dell’Angelo et al. 2020a).

Remarks. Parachiton was originally established by Thiele (1909) as a subgenus of Lepidopleurus Risso, 1826 on the basis of its disproportionately large tail valve with subterminal mucro, and overall similarities of the other valves with species of Lepidopleurus . It is now considered to be a distinct genus ( Saito 1996; Sirenko 2006), also due to differences in the radula ( Saito 1996). The sculpture is characteristic, with rows of granules with 3 pores of aesthetes, arranged usually longitudinally in the central area of the intermediate valves and also in the antemucronal area of tail valve. The main morphological characters of the Parachiton spp . considered in the present study are reported in Tab. 6.

observations).