Boreochiton ruber ( Linnaeus, 1767 )

Boreochiton ruber ( Linnaeus, 1767) View in CoL

Fig. 130 View FIGURE 130

Chiton ruber Linnaeus, 1767, p. 1107 , no. 7;?Seguenza 1876, p. 264;? Brugnone 1877, p. 18;?Tiberi 1877, p. 147; Dodge 1952, p. 22.

Chiton ( Boreochiton) ruber ; Knipowitsch 1900, p. 42, 44.

Boreochiton ruber View in CoL ; Brogger 1901, p. 653; Hoel 1914, p. 33; Antevs 1917, p. 354 –367, 412, 416; Grønlie 1927, p. 10, 17, 26; Antevs 1928, p. 646, 656–662, 677; Sirenko 2000, p. 71; Sirenko 2016, p. 27; Dell’Angelo et al. 2018b, p. 41; Sirenko & Dell’Angelo 2023, p. 278, figs 1–2.

Tonicella rubra View in CoL ; Hägg 1951, p. 235, 236, 244; Ferreira 1982, p. 119, figs 69–73; Kaas & Van Belle 1985b, p. 136, fig. 63, map 26; Dell’Angelo & Giusti 1997, p. 55, figs 19–22; Dell’Angelo & Smriglio 1999, p. 205; Dell’Angelo et al. 2001a, p. 148, figs 17–18; Puchalski et al. 2008 (database chiton fossil records); Strack 2010, p. 63, figs 52–53.

Type material. The Linnean Society of London ( Dodge 1952) .

Type locality. “In Oceano Septentrionali instar Patellae affixa”.

Material examined. Pleistocene: Italy: Capraia Island-Capo Corso -350/ 500 m: 327 valves ( BD 957, Figs 130A–K View FIGURE 130 ), Gallina: 3 valves ( AV, BD 958, Fig. 130L View FIGURE 130 ). Maximum width of the valves: 2.6 / 3.8 / 3.3 mm.

Description. Head valve semicircular, posterior margin widely V-shaped, some very faint radial fold may be present in some valves. Intermediate valves broadly rectangular (W/L = 2.40–2.79), moderately elevated (H/W = 0.35–0.53), semicarinate in anterior profile, anterior margin straight, side margins rounded, posterior margin straight to slightly concave at both sides of the protruding apex, lateral areas little or not elevated, hardly perceptible. Tail valve elliptical (W/L = 2.00–2.26), anterior margin roughly convex, straight between apophyses, mucro in anterior position, somewhat swollen, antemucronal and postmucronal slopes straight, generally rather steep.

Tegmentum smooth to naked eye, sculptureless except for concentric growth lines, aesthetes very dense, each megalaesthete accompanied by many micraesthetes.

Articulamentum strongly developed, apophyses wide, broadly triangular with rounded top, more or less trapezoid in the tail valve, separated by a rather narrow, flat jugal sinus, insertion plates short, slit formula 8–10 / 1 / 7–12, teeth sharp, smooth, slit rays little indicated, eaves finely porous.

Remarks. This species has a long and rather complex nomenclatural history, summarized by Ferreira (1982) and Kaas & Van Belle (1985b), triggered the differing opinions of Jakovleva (1952) and Sirenko (1974a, 1974b, 1976) on the identity of “ Tonicella ” rubra inhabiting the Pacific Ocean.

Boreochiton ruber was seldom recorded from the Mediterranean ( Leloup 1945; Fredj 1974; Monterosato 1890), unconfirmed by later authors ( Dell’Angelo & Giusti 1997; Dell’Angelo & Smriglio 1999), so that is not included among the species living in the Mediterranean Sea ( Renda et al. 2022; Dell’Angelo et al. 2024). As fossil, it has been reported from the Pleistocene (Sicilian) of Messina (Seguenza 1876) and Ficarazzi ( Brugnone 1877; Tiberi 1877), but these records need to be confirmed. The species is ascertained from last glacial Pleistocene submerged assemblages between Capraia Island and Capo Corso (Ligurian Sea) at a depth of 350/ 500 m, where is common ( Dell’Angelo & Giusti 1997; Dell’Angelo et al. 2024). The comparison between these fossil plates and recent material did not reveal any differences (compare figs 1 and 2 in Sirenko & Dell’Angelo 2023). The material examined shows a certain variability, mainly as regards the dorsal elevation (H/W = 0.35–0.53) and the greater or lesser accentuation of the apex of the intermediate valves, however always well evident.

Comparisons. Boreochiton ruber ( Linnaeus, 1767) is very similar to Tonicella marmorea ( Fabricius, 1780) ; furthermore, their geographic and bathymetric ranges almost overlap. Loose valves of both genera are extremely difficult to be objectively assessed upon shell characters alone.

Distribution. Pleistocene: North Atlantic: Russia, Barents Sea ( Knipowitsch 1900), Spitsbergen ( Hoel 1914; Hägg 1951), Norway ( Brogger 1901), Sweden ( Antevs 1917, 1928), Netherlands ( Strack 2010); central Mediterranean, Italy: Capraia Island-Capo Corso -350/ 500 m ( Dell’Angelo & Giusti 1997; Dell’Angelo et al. 2001a); Gallina (this study). Recent: N. Atlantic, Europe ( Hansson 1998; Sneli & Gudmundsson 2018), Azores ( Avila & Sigwart 2013).

Genus Tonicella Carpenter, 1873

Type species. Chiton marmoreus Fabricius, 1780 View in CoL , by subsequent designation ( Dall 1878).

Distribution. Tonicella is known from the Miocene to the Recent. It occurs in the northern parts of the Pacific and Atlantic Oceans, and in the Arctic Ocean ( Sirenko & Dell’Angelo 2023). Its fossil record extends back to the Miocene in Japan ( Itoigawa et al. 1981 –1982), Pliocene-Pleistocene of North America ( Vendrasco et al. 2012 and references therein), Pleistocene of Europe ( Feyling-Hanssen 1955) and California, U.S.A. (interglacial deposits, Muhs et al. 2006, 2012).

Remarks. The valves in this genus are characterised by a largely smooth tegmental surface ornamented by tiny granules; the valves have weakly defined lateral areas ( Ferreira 1982). The smooth ornament is an important feature of Tonicella , although also found in some species of Lepidochitona Gray, 1821 . Tonicella likely originated in the North Pacific.

Six species from the Eocene-Oligocene of Europe, before attributed to the genus Tonicella Carpenter, 1873 , have been recently included in the genus Lepidochitona . The main characters of these species (with smooth tegmentum and pores of megalaesthetes and micraesthetes of the same size) are provided by Tab. 17 [for 5 species from the NE Atlantic area: L. adunca ( Dell’Angelo, Lesport, Cluzaud & Sosso, 2020), L. lira ( Cherns & Schwabe, 2019) , L. modesta ( Rolle, 1862) , L. nuda ( Dell’Angelo, Lesport, Cluzaud & Sosso, 2020) and L. redoniensis ( Dell’Angelo, Landau, Van Dingenen & Ceulemans, 2018)] and in Tab. 19 [for the only species from the N Europe: L. tenuissima (Sandberger, 1859) ]. The main characters of Tonicella marmorea ( Fabricius, 1780) are defined in Tab. 21.