Leptochiton asellus ( Gmelin, 1791 )

Leptochiton asellus ( Gmelin, 1791) View in CoL

Fig. 25 View FIGURE 25

Chiton asellus Chemnitz, 1785, p. 290 , pl. 96, fig. 816. Chiton asellus Gmelin, 1791 , n. 3206, no. 21; Kaas & Knudsen 1992, p. 83, figs 26, 26c. Chiton arcuarius Wood, 1842, p. 460 ; Bronn 1848, p. 291 ( fide Dell’Angelo & Palazzi 1989). Chiton rissoi [non Ischnochiton rissoi ( Payraudeau, 1826) View in CoL ]; Wood 1848, p. 186, pl. 20, figs 11a–11c; Morris 1854, p. 243; Wood

1872–1874, p. 95, pl. 20, fig. 11; Reid 1890, p. 261, tab. 3; Van Belle 1981, p. 61 ( fide Malatesta 1962; Marquet 1984). Chiton cinereus [non Lepidochitona cinerea ( Linnaeus, 1767) View in CoL ]; Jeffreys 1865, p. 219 ( fide Malatesta 1962). Lepidopleurus cinereus [non Lepidochitona cinerea ( Linnaeus, 1767) View in CoL ]; Brogger 1901, p. 660, pl. 16, figs 10a–b; Antevs 1917,

p. 354, 412, 417 ( fide Malatesta 1962). Lepidopleurus asellus ;? Antevs 1928, p. 554–555, 646, 678–689; Marquet 1984, p. 336, pl. 1, fig. 1. Lepidopleurus cf. asellus ; Bellomo & Sabelli 1995, p. 201. Lepidopleurus ( Leptochiton) asellus ; Malatesta 1962, p. 150, figs 5–6; Dell’Angelo & Palazzi 1989, p. 56, pl. 5; Dell’Angelo &

Smriglio 1999, p. 43, pls 8–9, figs 16–17; Dell’Angelo & Giusti 2000, p. 53, figs 1–4; Marquet 2002, p. 11, pl. 1, fig. 1. Leptochiton asellus ; Kaas 1981, p. 217, fig. 1–3; Strack 2010, p. 61, figs 46–49; Taviani et al. 2023, p. 3, fig. 2. Leptochiton ( L.) asellus ; Kaas & Van Belle 1985a, p. 39, fig. 15; Sturrock & Baxter 1993, p. 49, pls 1–6.

Type material: Holotype at the Zoological Museum of the University of Copenhagen, a specimen attached to a shell of Modiolus ( Kaas & Knudsen 1992: fig. 26).

Type locality: Telemark, Kragerö ( Norway) .

Material examined. Lower Pliocene: Belgium: Kallo: 3 valves ( BD 401, Figs 25G–J View FIGURE 25 ). Pleistocene, presumably last glacial: Italy: Capraia Island-Capo Corso -350/ 500 m: 14 valves ( BD 402, Figs 25A–F View FIGURE 25 ); off Bari, cruise SE06: SE06-10: 1 valve; SE06-13: 1 valve; SE06-18: 2 valves; SE06-19: 1 valve; SE06-22: 1 valve; SE06-24: 1 valve; SE06-25: 2 valves; SE06-35: 5 valves; SE06-40: 1 valve; SE06-48: 1 valve; SE06-50: 25 valves ( Figs 25K–L View FIGURE 25 ). Maximum width of the valves: 3.6 / 5.2 / 4.3 mm.

Description: Head valve almost semicircular. Intermediate valves broadly rectangular, length more than three times the width (W/L = 3.03–3.30), moderately elevated (H/W = 0.36–0.38), semicarinate in anterior profile, anterior margin straight, lateral margins slightly rounded, posterior margin slightly concave at both sides of not very pronounced but well visible apex, lateral areas little raised. Tail valve semicircular, width about twice length (W/L = 1.88–2.05), mucro slightly anterior, postmucronal slope almost straight or slightly concave.

Tegmentum rough, space between striae of granules reduced, growth lines often present in variable number. HV, LA and PMA sculptured with minute, low, roundish to oval granules arranged in radial series (HV 70–80, LA 15– 20). CA and AMA sculptured with roundish, united granules arranged in longitudinal striae (CA 70–80), displaying a regular quincuncial pattern by granules of neighboring rows, tending to bend slightly towards outer margins. Each granule with aesthetes of same size, one central and 2–4 irregularly arranged.

Articulamentum lacking insertion laminae, apophyses small, well separate, triangular but tending to become trapezoidal in tail valve.

Remarks: Chiton asellus Gmelin, 1791 was originally described and inadequately illustrated by Chemnitz (1785) based on a Norwegian specimen attached to a shell of Modiolus , “ex Museo Spengleriano”. This description is not valid, Chemnitz’s work not being strictly binomial. The taxon was validated by Gmelin (1791), who added nothing new to the description, and was later better defined by Spengler (1797). This species has a complicated taxonomic history, and a thorough discussion of the interpretations given by various authors was provided by Kaas (1981) and Kaas & Knudsen (1992).

The fossil records of this species are rather scarce and must be accepted with caution; historical records are doubtful and require confirmation. The intermediate valves reported from the lower Pliocene of Kallo ( Belgium) by ( Marquet 1984, 2002) show a more rounded anterior profile ( Fig. 25H View FIGURE 25 ), not quite semicarinate as they should be ( Fig. 25D View FIGURE 25 ), but all the other features of the plates and of the sculpture agree with those of Leptochiton asellus , so we confirm this attribution.

Many valves of Leptochiton asellus ( Gmelin, 1791) 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), and this finding is quite significant, extending the presence of this species in the Italian Pleistocene, hitherto known only for a few finds.

Comparisons. See Tab. 3 for a comparison with the Leptochiton spp. considered in the present study.

Distribution. Lower Pliocene: North Europe, Belgium: Kallo ( Marquet 1984, 2002); NE Atlantic: U.K. ( Wood 1842, 1848; Reid 1890; Malatesta 1962); Pleistocene: North Atlantic: Netherlands ( Strack 2010); central Mediterranean, Italy: Pezzo ( Bellomo & Sabelli 1995). Pleistocene, presumably last glacial: North Atlantic: Sweden and Norway ( Brogger 1901; Antevs 1917, 1928); central Mediterranean, Italy: Capraia Island-Capo Corso -350/ 500 m ( Dell’Angelo & Giusti 2000), cruise SE06, off Bari ( Taviani et al. 2023). Recent: North Europe: from Spitsbergen and the Barents Sea South along the Scandinavian coasts ( Dons 1934; Hansson 1998); lceland ( Sneli & Gudmundsson 2018) (and perhaps Greenland); NE Atlantic Ocean: all around the British Islands and Ireland ( Light & Baxter 1990; McKay & Smith 1979), coasts of France ( Van Belle 1972; Kaas 1979), S. to Spain ( Borja 1987; Rolan Mosquera et al. 1990; Urgorri et al. 2017) and Portugal ( Consolado Macedo et al. 1999) (Kaas 1981; Kaas & Van Belle 1985a). Mediterranean Sea: a single record from the Limnos Island, Greece ( Mifsud & Ovalis 2008).