Lepidopleurus virgifer (Sandberger, 1859)

Lepidopleurus virgifer (Sandberger, 1859) View in CoL

Fig. 8 View FIGURE 8

Chiton virgifer Sandberger, 1859 , pl. 14, figs 4, 4a–b; Sandberger 1860, pl. 20, figs 15, 15a; Sandberger 1861, p. 184; Koenen 1892, p. 974; Sacco 1897, p. 90; Wenz 1932, p. 14.

Gymnoplax virgifer ; de Rochebrune 1882, p. 59.

Lepidopleurus virgifer View in CoL ; Pompecki 1912, p. 356, fig. 3; Zittel 1924, p. 436, fig. 802; Šulc 1934, p. 3; Fischer 1957, p. 14; Malatesta 1962, p. 146; Sabelli & Spada 1971, p. 6; Laghi 1977, p. 98; Janssen 1978, p. 218, pl. 14, figs 3–10; Van Belle , 1981, p. 80; Gürs 1983, p. 57; Hocht 1986, p. 209; Gürs 1995, p. 20, pl. 1, figs 8–10; Dell’Angelo et al. 1999, p. 261; Studencka & Dulai 2010, p. 263; Dell’Angelo et al. 2011, p. 953; Dell’Angelo et al. 2015a, p. 225; Dell’Angelo et al. 2018a, p. 11, 16; Dell’Angelo et al. 2018b, p. 6, 11; Dell’Angelo et al. 2019b, p. 300, figs 1, 2A–O.

Lepidopleurus ( Lepidopleurus) virgifer View in CoL ; Dell’Angelo & Palazzi 1989, p. 50, pl. 3, figs 1–2, 5; pl. 4, figs 1–5, 10–14 [ partim, non pl. 3, figs 3–4; pl. 4, figs 6–9; pl. 22, figs 8–9 = Lepidopleurus benoisti (de Rochebrune, 1883) View in CoL ].

non Chiton virgifer ? juv.; Boettger 1869, p. 9, pl. 1, fig. 11a– 11g; Boettger 1870, p. 39, pl. 9, fig. 11a– 11g; Van Belle 1981, p. 80 [= Leptochiton maguntiacus (de Rochebrune, 1882) View in CoL partim and L. poirieri (de Rochebrune, 1882) View in CoL partim, fide Janssen 1978: pp. 219, 221].

Type material. Syntypes: NHMW 1862/0012/0047, 2 intermediate valves; NHMW 1863/0017/0062, 3 valves ( Figs 8B–D View FIGURE 8 ).

Type locality. Gienberg ( Germany) .

Type stage. Oligocene (Rupelian), Alzey Formation.

Material examined. Lower Oligocene: Germany, Mainz Basin : type material, plus Gienberg: 9 valves ( BD 333 , NHMW 1868 /0001/0776, NHMW 1868/0001/0777, Figs 8I–J View FIGURE 8 ); Steigerberg: 5 valves ( BD 334 , Figs 8A, 8EH, 8K–L View FIGURE 8 ). Maximum width of the valves: 6.1 / 9.7 / 6 mm .

Description. Valves solid. Head valve large, semicircular, widely open posteriorly. Intermediate valves wide, broadly rectangular, width more than three times the length (W/L = 3.13), rounded in anterior profile, moderately elevated (H/W = 0.39), anterior and posterior margins straight, side margins rounded, apex inconspicuous, lateral areas strongly raised. Tail valve semicircular (W/L = 1.72–1.87), elevated, anterior margin from almost straight to slightly convex in jugal area, mucro pronounced, in anterior-subcentral position, antemucronal slope convex, postmucronal slope concave just behind mucro.

Tegmentum very coarse, with a granulated-nodulose sculpture. HV, LA and PMA sculptured with numerous and strongly irregular branching or anastomosing radial chains of roundish granules, coarser in LA, more variable in PMA, intersected by some more or less evident concentric, terraced ribs. CA and AMA sculptured with longitudinal chains of roundish granules, united with each other, diameter up to 100 µm, more irregular and rougher, shaped by groups of granules branching longitudinally many times in CA, more regular in AMA with longitudinal chains of granules finer and less subject to split. Each granule with a central megalaesthete and a series of micraesthetes (normally 6–8) spaced along margin.

Articulamentum without insertion laminae, apophyses narrow, triangular in intermediate valves, larger and rounded in tail valves, widely projecting.

Remarks. The study of the syntype material hosted at NHMW ( Dell’Angelo et al. 2019b) revelead a certain variability in intermediate and tail valves (head valves were not present), not so obvious in the original description. Sandberger described and figured a tail valve “with an almost exactly hemicircular margin”, while the syntypes show an anterior margin from almost straight to slightly convex, and a more variable position of the mucro, not in such an anterior position as mentioned in the original description. The antemucronal and postmucronal slopes of the tail valve (never described and illustrated by Sandberger or Janssen 1978) also show certain variability, mainly in the more or less accentuated concavity of the postmucronal slope ( Dell’Angelo et al. 2019b: figs 2K, 2O).

Šulc (1934) considered large tail valves (width of 18 mm) of Lepidopleurus decoratus Reuss, 1860 from the Middle Miocene of Pötzleinsdorf ( Austria) to be close to L. virgifer (see above, check L. benoisti ).

Comparisons. Lepidopleurus virgifer (Sandberger, 1859) differs from the other Oligocene species largely by the very coarse granulated-nodulose sculpture, which is even visible in juvenile segments, especially in the central area of intermediate valves.

Distribution. Lower Oligocene: North Europe, Germany: Böseberg, Gienberg, Glimmerode, Steigerberg, Zeilstück, Würzmühle ( Janssen 1978; Hocht 1986; Dell’Angelo et al. 2019b; this study).

Genus Leptochiton Gray, 1847

Type species. Chiton cinereus Montagu, 1803 (misapplication of name), by subsequent designation ( Gray 1847a), not Linnaeus, 1767 (= Chiton asellus Gmelin, 1791 ).

Distribution. Leptochiton is known from the Triassic to the Recent, with a living worldwide distribution, most species from the northeastern Atlantic and the Mediterranean Sea. Fossils determined as Leptochiton have been found worldwide and Leptochiton is one of the most ancient genera of “modern” polyplacophorans, dating back to the Triassic ( Laghi 2005), the Jurassic of France, Germany, Poland, Russia and Siberia (Sirenko 2013), and possibly even to the lower Carboniferous (Sirenko 2013). The fossil record includes the Paleocene of Denmark ( Sigwart et al. 2006), the Eocene of Europe, Australia ( Dell’Angelo et al. 2011), Antarctica ( Lopez Cabrera & Olivero 2011), the upper Eocene-lower Oligocene of Washington, U.S.A. ( Dell’Angelo et al. 2011), the Oligocene of Europe ( France and Germany: Dell’Angelo et al. 2011, 2018a) and New Zealand ( Lee et al. 2014; Wu & Lee 2024), the Miocene of the Paratethys ( Šulc 1934; Bałuk 1971, 1984), the Miocene-Pleistocene of the Mediterranean Basin ( Dell’Angelo et al. 2013, 2016), Australia ( Cotton 1964) and New Zealand ( Beu & Maxwell 1990; Sutherland et al. 1995), the Pliocene-Pleistocene of California ( Vendrasco et al. 2012), and the Holocene of Japan ( Kuroda et al. 1980).

Remarks. Sigwart et al. (2011) attempted to elucidate the relationships of the earliest diverging living chitons of the order Lepidopleurida using molecular phylogenetic methods. A result of this work found that the large cosmopolitan genus Leptochiton is not monophyletic and is restricted to the North Atlantic and the Mediterranean Sea. However, there are several groups of deep-water leptochitonids with complexes of similar morphological features which inhabit low latitudes in the Pacific, Indian and Atlantic Oceans ( Sirenko 2015).

The genus Leptochiton is characterized by the lack of insertion laminae, the apophyses small and neatly separate, and the tegmentum uniformly granulated. For a better identification and differentiation, the examined species are reported, in the tables that summarize the main diagnostic characters, in three groups which highlight the different tegmentum sculpture:

Leptochiton cancellatus group: HV, LA and PMA with granules arranged in radial series; CA and AMA with granules arranged in longitudinal series. The 16 species belonging to this group are most uncommon or rare, fragile and seldom found in good conditions, so they are not easy to determine. The main morphological characters of the species of Leptochiton cancellatus group considered in the present study are reported in Tabs 2 and 3.

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Leptochiton cimicoides View in CoL group: HV, LA and PMA with granules arranged irregularly or quincuncially; CA and AMA with granules arranged in longitudinal series. The 6 species belonging to this group are most uncommon or rare, a few fossil valves have been attributed to them but are difficult to determine. The main morphological characters of the species belonging to the Leptochiton cimicoides View in CoL group treated here are reported in Tab. 4. We attribute all previous paleontological records of Leptochiton cimicoides ( Monterosato, 1879) View in CoL to either L. prudenzae sp. nov. or L. antondohrni Taviani, Sosso & Dell’Angelo, 2023 View in CoL and consider L. cimicoides View in CoL only as a modern taxon.

Leptochiton tavianii group: tegmentum fully covered with randomly or quincuncially arranged granules. The 6 species belonging to this group are scarcely found, but the valves are more solid, and some species are locally not uncommon. The main morphological characters of the species of Leptochiton tavianii group considered in the present study are reported in Tab. 5. Previously Leptochiton alveolus View in CoL (M. Sars in Lovén, 1846), a species recently transferred to the genus Belknapchiton Sirenko, Saito & Schwabe, 2022 View in CoL , also belonged to this group. Considering the affinities existing with the other species of the Leptochiton tavianii group, we also insert Belknapchiton alveolus View in CoL in Tab. 5, to better highlight the differences.

The genus Leptochiton View in CoL shows a great number of species treated in this work, 28 in all, and for a better identification and differentiation of the examined species, we have grouped them by source area.