Plesiodiceras valfinense

Plesiodiceras valfinense (Boehm, 1881) Figs 11, 12 v * 1881 Diceras valfinense n. sp. – Boehm: 160 [86], pl. 25 [9], fig. 2 a, b. v 1881 Diceras Münsteri Goldf. sp. – Boehm: 159 [95], pl. 25 [9], fig. 3 a, b. 1882 [Plesiodiceras] Valfinense, Boehm – Munier-Chalmas: 478. 1888 Diceras (Plesiodiceras) Valfinense, Boehm – Loriol: 258, pl. 28, figs 1–11. v 1941 Heterodiceras (Plesiodiceras) Münsteri Goldfuss – Dechaseaux: 35, pl. 4, figs 1–8, 10. 1969 Plesiodiceras muensteri (Goldfuss) – Dechaseaux et al.: N 779, fig. E 245.2 b, c. ? 1999 Plesiodiceras munsteri (Goldfuss, 1840) – Skelton: 86, pl. 3, figs 3–6. 2017 Plesiodiceras muensteri (Goldfuss, 1837) – Schneider et al.: 58, pl. 22, figs C, E, F. 2022 Plesiodiceras valfinense (Boehm 1881) – Schneider and Al-Mojel: 6, fig. 5 A – C. Diagnosis. Shell up to 60 mm high, with broadly sub-circular, ventrally faintly to distinctly trilobate commissure. Left valve short-conical in shape, expanding quickly during ontogeny. Right valve operculum-shaped. Both valves coiled, with 1.5 to 2 whorls clearly traceable. Right valve hinge with small knob-like tooth 3 a and prominent, obliquely triangular tooth 3 b. Left valve hinge with distinct, sharply arcuate tooth 2. Right valve adductor muscle scars situated on downward extensions of hinge plate. Left valve adductor muscle scars situated on elevated, inwardly inclined buttresses. Material. Syntypes of Diceras valfinense Boehm (1881) (Kimmeridgian limestones of Valfin lès Saint Claude, Departement Jura, France): double-valved specimen (not figured; BSPG 1871 VII 1004); left valve (pl. 25, fig. 2 a; BSPG 1871 VII 752), designated lectotype herein; bouquet of left valve and double-valved specimen (not figured; BSPG 1871 VII 1005); right valve (pl. 25, fig. 2 b, BSPG 1871 VII 1002), left valve (not figured; BSPG 1871 VII 1003). Originals and reference material of Diceras muensteri (Goldfuss, 1837) sensu Boehm (1881) (late Kimmeridgian; Kelheim Member, Frankenalb Formation; Kelheim area, Bavaria, Germany): left valve (pl. 25, fig. 3 a; BSPG 1872 XI 571); left valve (not figured; BSPG 1872 XI 573); left valve (not figured; BSPG 1872 XI 574) (?, specifically indeterminate); right valve (pl. 25, fig. 3 b; BSPG 1872 XI 572); internal mould of double-valved specimen (not figured; BSPG 1872 XI 575) (?, specifically indeterminate). Numerous specimens from Saal, of which a representative suite was selected for this study. Twenty-six double-valved specimens (BSPG 2016 XXI 15 a, b [valves isolated], 73, 1399, 1400, 1413, 1414 [two individuals], 1519; BSPG 2019 IV 4; BSPG 2020 CIII 2, 4, 5, 11, 15, 18, 19, 21, 25, 33 [overgrown by left valve], 34–40). Questionably, one internal mould of a double-valved specimen (BSPG 2016 XXI 1393). Nine left valves (BSPG 2016 XXI 1395, 1409–1411; BSPG 2020 CIII 1 [on branching coral], 12, 13, 16, 41). Fifty-one right valves (BSPG 2016 XXI 74, 1371–1385, 1387–1390, 1392, 1396 [overgrown by Hypelasma fritzlangi], 1412, 1415, 1521, 1524–1543; BSPG 2019 IV 13; BSPG 2020 CIII 3, 7–10). Measurements and figure numbers. Type locality and stratum. Late Kimmeridgian strata of Valfin lès Saint Claude, Departement Jura, France. Description. Small epidiceratid with up to 60 mm high shells. Outline of commissure broadly sub-circular, ventrally faintly to distinctly trilobate; the largest, central lobe tracing the position of the body cavity; the smallest, anterior lobe and the slightly larger posterior one tracing the outer margins of the adductor muscle scars. Left valve short-conical in shape, with high whorl expansion rate. Right valve small, shaped like an operculum, with umbo not extending above commissure. Both valves coiled, with 1.5 to 2 whorls clearly traceable. Right valve hinge with small knob-like tooth 3 a and prominent, broad, oblique, dorsally bending, obliquely triangular tooth 3 b, extending to or projecting slightly above commissure; rather sharply bounded lunate recess on its anterior side marking the extent of rather deep socket 2 ’. Left valve hinge with prominent, sharp-crested, elongate-arcuate tooth 2, flanked by small, pit-like socket 3 a’ ventrally, and large, deep, arcuate socket 3 b’ posterior-dorsally. Right valve adductor muscle scars situated on downward extensions of hinge plate; the anterior one inserting just below and anterior to tooth 3 a; the posterior one significantly larger, lunate in outline, extending downward from behind tooth 3 b. Left valve adductor muscle scars situated on elevated, inwardly inclined buttresses, more or less at the level of the hinge plate. Remarks. Plesiodiceras valfinense and Plesiodiceras? muensteri (Goldfuss, 1837), which is discussed below, are in the same size category and have similar overall proportions, including a reduced, operculum-like right valve. However, compared to P.? muensteri, P. valfinense gains much more quickly in diameter or, in gastropod terms, has a greater whorl expansion rate, resulting in a short-conical left valve. The umbonal area is rarely well-enough preserved to establish the exact number of whorls developed during ontogeny, but 1.5 to 2 whorls are distinct in the specimens of P. valfinense studied by us. In contrast, the lectotype of P.? muensteri shows less than 1.5 whorls. In specimens growing without space restrictions, the commissure is broadly equidistant in P. valfinense, with its dorsal part generally well rounded, with a slight incurvature near the umbo in most specimens. The remainder of the commissure is arranged in three faintly indicated lobes – a central, largest, ventral one tracing the body cavity, and anterior and posterior ones, which trace the adductor muscle scars – the latter usually the larger of the two. In P.? muensteri, the commissure is higher than long and markedly reniform; its shape is not determined by soft body anatomy. When Boehm (1881) compared Diceras muensteri and his new Diceras valfinense, he only discussed his own material from Kelheim, and specimens from Cerin [‘ Cirin’ in Boehm 1881] and Valfin lès Saint Claude in France; he made no direct reference to Goldfuss’ (1837) syntypes of Chama muensteri. Critically also, Goldfuss’ material comprises no single valves, which makes comparison of internal characters impossible. Boehm (1881) found that the distinguishing character of the specimens from Valfin was the presence of a ledge bounding the left-valve posterior adductor muscle scar anteriorly. However, he also noted that the precise shape of the hinge teeth, as well as the arrangement of buttresses and muscle scars in the specimens from Valfin varied greatly. Seven of Boehm’s (1881) specimens treated under the names Diceras valfinense and D. muensteri, including the four shells figured by him, are preserved in the collections at Munich. We regard these as conspecific, and the variability discussed by Boehm (1881) as intraspecific; the specimens are all assigned to Plesiodiceras valfinense here. Notably, an even greater range of morphologies in topotypic P. valfinense was illustrated by Loriol (1888). Diceras sanctaeverenae Thurmann, 1853 from the Oxfordian of the ‘ Ste Vérène limestone’ north of Solothurn, Switzerland, is based on an unpublished manuscript name coined by Gressly. The species was compared with Plesiodiceras? muensteri by Thurmann (1853) and regarded as distinct, but was later synonymised with P.? muensteri by Greppin (1870: 99). Boehm (1881) had no clear opinion on its validity. Given that the species is only documented by Thurmann’s sketchy drawings (Thurmann 1853; Thurmann and Étallon 1863), its status is impossible to assess without revisiting type or topotypic material, and we regard it as a nomen dubium. The drawings of Diceras monsbeliardense Contejean, 1860 (originally spelled monsbeliardensis; incorrect gender agreement), from the Kimmeridgian of Montbéliard, Departement Doubs, France, depict a double-valved specimen, likely a Plesiodiceras, with ornament of radial striae on both valves. With regard to its general morphology, i. e. a low valve expansion rate and reniform rather than trilobate commissure, this specimen rather resembles Plesiodiceras? muensteri than P. valfinense. However, Contejean’s (1860) figures are generally not photorealistic and should be interpreted with care, particularly also because no description of the species was provided. Diceras monsbeliardense is thus only tentatively assigned to Plesiodiceras? muensteri below. Pchelintsev (1959) described seven new species of Plesiodiceras from Oxfordian (‘ Rauracian’) strata of the Crimean Mountains (Yayla Mountains), Crimea, Ukraine. Pchelintsev’s (1959) species are mainly distinguished by subtle variations of their growth form, which we think are intraspecific. Six of his new species, Plesiodiceras subvalfinense, P. uzuntaschi, P. orthogonale, P. yailense, P. kuruuzense and P. koreizense are based on relatively well-preserved type material, consisting exclusively of articulated specimens, thus no internal characters are known. However, they all show the relatively slowly expanding left valves typical of Plesiodiceras? muensteri, together with a more or less distinctly reniform commissure. Some specimens preserve fine radial striae on both valves. A specimen of Plesiodiceras uzuntaschi was refigured by Yanin (1989: pl. 6, fig. 1). These six species are treated as junior subjective synonyms of P.? muensteri below. The holotype of the monotypic Plesiodiceras acutiforme Pchelintsev, 1959 is relatively poorly preserved and difficult to interpret; it is considered a nomen dubium at present. The single early Kimmeridgian specimen of Plesiodiceras aff. monsbeliardense (Contejean) figured by Pchelintsev (1959) needs revisiting to establish its identity. The four species of Plesiodiceras from the Pamir described by Pchelintsev (1960) are relatively poorly preserved and illustrated. Most specimens seem to represent only left valves, and no internal characters are known. The holotype by monotypy of Plesiodice