Rhyssoplax miocenica ( Michelotti, 1847 )

Rhyssoplax miocenica ( Michelotti, 1847)

Fig. 92 View FIGURE 92

Chiton miocenicus Michelotti, 1847, p. 132 , pl. 16, fig. 7; Baily 1859, p. 334; Baily 1860, p. 171; Monterosato 1879, p. 15; Seguenza 1879, p. 274; Sacco 1889, p. 69; Sacco 1897, p. 89, pl. 7, figs 8–20; Bellini 1905, p. 627, 629, 632, 648; Malatesta 1962, p. 162; Van Belle 1981, p. 50; Ferrero Mortara et al. 1984, p. 299; Dell’Angelo & Palazzi 1989, p. 55; Cavallo & Repetto 1992, p. 30, fig. 3b; Giani 1998, p. 116; Dell’Angelo & Smriglio 1999, p. 173; Forli et al. 1999, p. 113, pl. 1, figs 4–6; Dell’Angelo et al. 2001a, p. 152, fig. 27; Chirli 2004, p. 14, pl. 5, figs 11–15; Dell’Angelo et al. 2004, p. 39, pl. 5, fig. 5, pl. 7, figs 3, 7; Dell’Angelo et al. 2007a, p. 43; Dell’Angelo et al. 2013, p. 87, pl. 7, figs A–H; Dell’Angelo et al. 2016, p. 80, pl. 2, figs 16–19, pl. 3, figs 1–4; Forli & Guerrini 2022, fig. 11.18(4–5).

Lepidopleurus miocenicus ; de Rochebrune 1882, p. 65.

Rhyssoplax miocenicus ; Puchalski et al. 2008 (database: chiton fossil records); Dell’Angelo et al. 2020b, p. 52, tab. 9; Brunetti & Cresti 2023, p. 10, 28, pl. 1, fig. B.

Rhyssoplax miocenica ; Dell’Angelo et al. 2021b, p. 421, figs 106–113.

Chiton olivaceus View in CoL [non Rhyssoplax olivacea ( Spengler 1797) View in CoL ]; Laghi 1977, p. 109, pl. 2, fig. 13 ( partim); Laghi & Russo 1978, p. 272, pls 1–7; Zanaroli 1985, tab. 2.

Chiton ( Rhyssoplax) olivaceus View in CoL f. miocenicus ; Dell’Angelo et al. 1999, p. 271, pl. 4, figs 1, 3, 5–8.

Chiton saeniensis View in CoL [non Rhyssoplax saeniensis ( Laghi 1984) ]; Dell’Angelo & Forli 1995b, p. 78 ( partim).

non Chiton miocenicus ; d’Orbigny 1852, p. 94; Cossmann & Peyrot 1919, p. 32, pl. 2, figs 21 - 22 [= Lepidopleurus benoisti (de Rochebrune, 1882) View in CoL , fide Cossmann & Peyrot 1919].

non Chiton miocenicus Var. ; Seguenza 1876, p. 264 (= Rhyssoplax garillii sp. nov., fide this study).

Gymnoplax orbignyi de Rochebrune 1882, p. 65 , pl. 1, fig. 7; Van Belle 1981, p. 55; Dell’Angelo et al. 2015a, p. 224; Dell’Angelo et al. 2016, p. 82 ( fide Dell’Angelo et al. 2015a).

Chiton miocenicus ; Marinescu 1964, p. 181, pl. 3, fig. 2.

Type material. MGR ( fide Sacco 1897: pl. 7, fig. 11 “es. tip. fig.”).

Type locality. The Turin hills ( Piedmont, Italy) .

Type stage. Lower Miocene .

Material examined. Lower Miocene: Italy ( Burdigalian): Sciolze : 2 valves ( PG), Valle Ceppi: 23 valves ( PG). Middle Miocene : Italy ( Langhian): Albugnano: 43 valves ( MZB 32079 , PG). Upper Miocene : Italy ( Tortonian): Borelli: 406 valves ( BD 743 , MGPT PU 135296-135298 , Figs 92A–B, 92L View FIGURE 92 ), MZB 32081 , PG), Moncucco Torinese: 1 valve ( PG), Montegibbio: 78 valves ( BD 744 , MZB 32080 , Fig. 92E View FIGURE 92 , LB), Rio di Bocca d’Asino : 149 valves ( BD 745 , PG), Sardigliano: 2 valves ( BD 746 ), Vargo: 1 valve ( PG), Villa Monti: 11 valves ( BD 747 , PG). Lower Pliocene : Italy: Liguria: Borzoli: 396 valves ( BD 748 ), Bussana: 4 valves ( BD 749 ), Garlenda : 5 valves ( MP), Rio Sant’Antonino : 332 valves ( BD 750 , MP, PG, MZB 45743-45746, Figs 92F–H View FIGURE 92 ), Rio Torsero : 1 valve ( BD 751 ). Pliocene : Spain: Estepona : 630 valves ( BD 752 , Figs 92I–K View FIGURE 92 ) ; Italy: Piedmont: San Damiano: 1 valve ( BD 753 ); Tuscany: Guistrigona: 1 valve ( BD 754 ), Montenero : 163 valves ( BD 755 , Figs 92C–D View FIGURE 92 ), Villa Banfi: 4 valves ( BD 756 ). Maximum width of the valves: 14.5 / 23 / 17.5 mm .

Description. Head valve semicircular, front slope faintly convex, posterior margin widely V-shaped, with minute notch in the middle. Intermediate valves broadly rectangular (W/L = 2.33–3.33), moderately elevated (dorsal elevation 0.24–0.43), carinate in anterior profile, anterior margin straight to slightly convex, side margins truncated to weakly convex, posterior margin slightly concave at both sides of minute apex. Tail valve semicircular (W/ L = 1.58–1.95), anterior margin convex, mucro subcentral, not prominent, antemucronal slope slightly convex, postmucronal slope almost straight.

HV, LA and PMA sculptured with rather coarse, flattish radiating ribs (HV 30–45, LA 5–11), often bifurcating, more irregular, crossed by concentric growth lines. CA and AMA sculptured with 10–35 longitudinal ribs at both sides of the smooth, more or less triangular jugum, all reaching the anterior margin. Aesthetes very dense, each megalaesthete accompanied by many micraesthetes.

Articulamentum with apophyses wide, rounded, tending to trapezoidal in tail valve, jugal sinus narrow, provided with a concave, dentate plate, teeth short, minutely denticulate, slit formula 8–10 / 1 / 13–14, slits narrow, slit rays very finely indicated, eaves narrow, porous.

Remarks. Rhyssoplax miocenica ( Michelotti, 1847) closely resembles R. olivacea ( Spengler, 1797) , and was considered in the past either a synonym of the latter or a distinct species. It was described from the Miocene of the Turin hills; one intermediate valve was well figured ( Michelotti 1847: pl. 16, fig. 7). Many subsequent authors considered the species a synonym of R. olivacea (e.g., Laghi 1977; Laghi & Russo 1978). Others considered, on the contrary, R. miocenica a valid species (e.g., Monterosato1879); Sacco 1897).

Gymnoplax orbignyi de Rochebrune, 1882 was originally proposed as “ nomen substitutivum ” for Chiton subcajetanus d’Orbigny, 1852 , based on chiton valves from the Turin hills. A syntype of Gymnoplax orbignyi is preserved at MNHN ( Dell’Angelo et al. 2015a: pl. 2, figs 14–17), and examination of this material made it possible to consider Gymnoplax orbignyi a synonym of Rhyssoplax miocenica ( Dell’Angelo et al. 2015a).

It is difficult to evaluate statistically the morphological characteristics of the two species, because of the insufficient number of complete valves of R. miocenica to examine. An attempt has been made with intermediate valves of Rhyssoplax olivacea [56 valves, from Miocene (4), Pliocene (2), Pleistocene (24) and living (26)] and R. miocenica [22 valves, from Miocene (6) and Pliocene (16)]. Some parameters were measured on each valve (width, length and height), and the numbers of ribs (or folds) on central and lateral areas were counted. The relationship between the valve’s width and the width/length and height/width ratios are showed in Tables 14 /1 and 14/2.

This study allowed to confirm R. miocenica as a distinct species, different from R. olivacea , and to better characterize these two species.

Comparisons. This species is closest to Rhyssoplax olivacea ( Spengler, 1797) , from which it differs mainly for the sculpture of CA and AMA formed by real longitudinal ribs ( Fig. 92D View FIGURE 92 , not small folds as in R. olivacea ), and for several other characters, e.g., the greater number of longitudinal ribs on CA (10–35 on each side, vs. 6–16 folds in R. olivacea ), LA (5–10 vs. 2–6 in R. olivacea ), and the different shape of tail valves, with the antemucronal slope slightly convex (more convex in R. olivacea ) and the postmucronal slope almost straight (mainly concave close to the mucro in R. olivacea ).

Distribution. Lower Miocene: Proto-Mediterranean Sea (Burdigalian): N. Italy: Sciolze, Valle Ceppi ( Michelotti 1847; Dell’Angelo et al. 2016). Middle Miocene: Proto-Mediterranean Sea (Langhian): Po Basin: Albugnano ( Dell’Angelo et al. 2016). Upper Miocene: Proto-Mediterranean Sea (Tortonian-Messinian): Po Basin, N Italy: Borelli, Moncucco Torinese, Montegibbio, Rio di Bocca d’Asino, Sardigliano, Vargo, Villa Monti ( Laghi 1977; Dell’Angelo et al. 1999, 2016). Lower Pliocene: central Mediterranean, Italy: Liguria: Borzoli, Bussana, Garlenda, Rio S. Antonino, Rio Torsero ( Dell’Angelo et al. 2001a, 2013; Sosso & Dell’Angelo 2010). Pliocene: western Mediterranean, Estepona Basin, Spain (Dell’Angelo et al. 2004); central Mediterranean, Italy: Piedmont: San Damiano; Tuscany: Guistrigona, Montenero, Villa Banfi ( Chirli 2004; this study).