Leptochiton geronensis Kaas & Van Belle, 1985

Leptochiton geronensis Kaas & Van Belle, 1985 View in CoL

Fig. 16 View FIGURE 16

Lepidopleurus alveolus View in CoL [non Belknapchiton alveolus View in CoL (M. Sars in Lovén, 1846)]; Van Belle 1975, p. 57, figs 1–2, 4 ( fide Kaas

& Van Belle 1985a). Leptochiton View in CoL ( L.) geronensis Kaas & Van Belle 1985a, p. 67 View in CoL , fig. 28, map 3. Leptochiton geronensis View in CoL ; Van Belle 1985, p. 127, pl. 36, figs 2–12; Dell’Angelo et al. 2013, p. 74. Lepidopleurus ( Leptochiton) cfr. geronensis View in CoL ; Dell’Angelo & Forli, 1995a, p. 224, fig. 16. Lepidopleurus ( Leptochiton) geronensis ; Dell’Angelo et al. 1998a, p. 241; Dell’Angelo & Smriglio 1999, p. 68, pl. 18, fig. 27;

Dell’Angelo et al. 2001a, p. 147, fig. 7.

Type material. Holotype IRSN IG-26.356 ; Paratype VB 2907a, a specimen disarticulated off Bagur ( Gerona, Spain), - 250 m, Figs 16C–H View FIGURE 16 .

Type locality. off Llansa ( Gerona, Spain), - 200 m.

Material examined: Pleistocene: Italy: Riparbella: 1 valve, width 2.5 mm ( BD 376, Figs 16A–B View FIGURE 16 ).

Description. Head valve semicircular, posterior margin almost straight. Intermediate valves broadly rectangular, semicarinate in anterior profile, moderately elevated (H/W = 0.37), anterior margin straight between the apophyses, somewhat backwardly directed towards the sides, side margins evenly rounded, posterior margin practically straight, apex inconspicuous, lateral areas hardly perceptible. Tail valve a little less than semicircular, width at least twice the length, mucro placed anteriorly and not prominent, postmucronal slope straight.

Tegmentum rough, space between striae of granules large. HV, LA, PMA sculptured with roundish, clearly separated granules arranged in quincuncial pattern, giving the impression of radial rows near outer sides. CA, AMA sculptured with roundish granules with 2 large stems in apical part, arranged in longitudinal series (CA ca 40). Each granule with 3 aesthetes aligned, 1 megalaesthete subcentral and 2 micraesthetes at extremities, in CA.

Articulamentum without insertion laminae, weakly developed, apophyses small, broadly triangular, trapezoid in tail valve, widely separated by a more or less flat sinus.

Remarks. Leptochiton geronensis Kaas & Van Belle 1985 was described on the basis of two specimens collected by fishers at Gerona ( Spain) from a white coral thanatocoenosis, at a depth of 200–250 m; those specimens were at first classified as Lepidopleurus alveolus (M. Sars in Lovén, 1846) by Van Belle (1975). This species is little known and scarcely recorded, and the specimens are often difficult to identify, not only because of their small size, but also because of their precarious condition of preservation, as they are mostly found in detritus and therefore coiled up, encrusted, sometimes with a barely visible tegmentum.

The fossil record of Leptochiton geronensis is limited to a single tail valve from the Pleistocene of Riparbella ( Dell’Angelo & Forli 1995a).

Comparisons. Lepidopleurus geronensis Kaas & Van Belle 1985 is similar to L. xanthus Kaas & Van Belle, 1990 , an Atlantic species recently recorded also in the Mediterranean from S. Lucia Bank ( Dell’Angelo & Smriglio 1999), a species not treated here because not known as a fossil. The tegmentum of both species presents the same characteristic sculpture of roundish granules well-separated from each other, but L. geronensis differs from L. xanthus by the anterior profile of the intermediate valves (rounded in L. xanthus , semicarinate in L. geronensis ), the postmucronal slope (slightly concave in L. xanthus , straight in L. geronensis ), and above all by the accessory plate of the second lateral tooth of the radula (monocuspidate in L. xanthus , bicuspidate in L. geronensis ) ( Dell’Angelo & Smriglio 1999).

Distribution: Pleistocene: central Mediterranean, Italy: Riparbella ( Dell’Angelo & Forli 1995a; Dell’Angelo et al. 2001a). Recent: Mediterranean Sea: Gerona, Spain (Kas & Van Belle 1985a), Tuscan Archipelago: Formiche di Grosseto (Della Bella & Dell’Angelo 1985), Messina Strait (Scuderi & Dell’Angelo 1997; Dell’Angelo et al. 1998a), and Ustica Island (Castriota et al. 2005).