Rhyssoplax assurrecta

Rhyssoplax assurrecta Dell’Angelo, Landau, Van Dingenen & Ceulemans, 2018

Fig. 87 View FIGURE 87

Chiton sp. A View in CoL Dell’Angelo, Sosso, Prudenza & Bonfitto, 2013, p. 88, pl. 7, figs I–J.

Rhyssoplax assurrectum Dell’Angelo, Landau, Van Dingenen & Ceulemans, 2018b, p. 31, figs 7A–D; Dell’Angelo et al. 2018a, p. 31, figs 7A–D.

Rhyssoplax assurrecta ; Dell’Angelo et al. 2021b, p. 419, figs 98–101.

Type material. Holotype MNHN.F.A67124, intermediate valve, width 6.5 mm ( Figs 87E–H View FIGURE 87 ) . Paratypes: MNHN.F.A67125– A67128 View Materials (4 valves) ( Figs 87I–K View FIGURE 87 ); NHMW 2017/0108/0033–0036 (4 valves) ( Figs 87A–B View FIGURE 87 ); RGM.1008368–1008371 (4 valves) .

Type locality. Saint-Clément-de-la-Place ( France) .

Type stage. Miocene (Tortonian).

Material examined. Lower Miocene: France (Aquitanian): Maïnot: 1 valve (PR). France (Burdigalian): Carrière Vives : 1 valve ( BD 639 ) . Upper Miocene : France (Tortonian): Saint-Clément-de-la-Place: type material plus 230 valves ( MNHN.F. A67129 View Materials – A67130 View Materials , NHMW 2017/0108/0037, RGM.1008355, RGM.1008372, BD 151 ) ;

Sceaux d’Anjou, La Presselière: 1 valve ( RGM.1008450). France (Messinian?): Moulin-Pochas: 1 valve (PR). Lower Pliocene: Italy: Borzoli: 37 valves ( BD 640, MZB 45747); Bussana: 13 valves ( BD 641); Caranchi: 5 valves ( BD 642, MP); Rio S. Antonino: 4 valves ( BD 643, MP). Pliocene: Italy: Tuscany: Pietrafitta Melograni: 10 valves ( BD 644, Figs 87C–D, 87L View FIGURE 87 ), Poggio alla Fame: 12 valves ( BD 645, MP). Maximum width of the valves: 5.7 / 7.5 / 4.5 mm.

Description. Head valve semicircular, posterior margin markedly V-shaped, front slope straight.

Intermediate valves broadly rectangular (W/L = 1.90–2.35), carinate in anterior profile, elevated (H/W = 0.55– 0.71), anterior margin slightly convex, side margins truncated, posterior margin straight, interrupted by small apex, lateral areas raised, clearly defined. Tail valve more than semicircular (W/L = 1.38–1.63), anterior margin convex, mucro subcentral, antemucronal slope slightly convex, postmucronal slope almost straight or slightly concave, just behind mucro.

HV, LA and PMA smooth, concentric growth lines weakly developed, more distinct on head and tail valves. CA and AMA sculptured with up to 10–11 irregular, longitudinal ribs on both sides of broadly triangular, smooth jugum, 2–4 innermost ribs not reaching anterior margin. Aesthetes very dense, each megalaesthete accompanied by many micraesthetes.

Articulamentum with wide, rounded apophyses, teeth irregular, slit formula 8 / 1 / 10–12, slit rays conspicuous.

Remarks. This species has been attributed to the genus Rhyssoplax based on the morphological affinities with the morphological closer R. corallina ( Risso, 1826) . Valves reported from the Pliocene of Liguria as Chiton sp. A ( Dell’Angelo, Sosso, Prudenza & Bonfitto, 2013) have been later ascribed to R. assurrecta ( Dell’Angelo, Landau, Van Dingenen & Ceulemans, 2018).

Comparisons. Rhyssoplax assurrecta Dell’Angelo, Landau, Van Dingenen & Ceulemans, 2018 is very similar to R. corallina ( Risso, 1826) , from which it differs mainly by the more elevated intermediate valves, H/W = 0.55–0.71 vs. 0.30–0.47 in R. corallina . The elevated intermediate valves are also a constant character in juvenile specimens ( Dell’Angelo et al. 2018b, compare figs 19K–L and 18F). The head valves of Rhyssoplax assurrecta and R. corallina are similar, and can only be separated based on the higher elevation of the valves of R. assurrecta ( Dell’Angelo et al. 2018b, compare figs 19F and 18B).

Distribution. Lower Miocene: northeastern Atlantic: Aquitaine Basin, France: Maïnot (Aquitanian), Carrière Vives (Burdigalian) ( Dell’Angelo et al. 2018a). Upper Miocene: northeastern Atlantic: Ligerian Basin, France (Tortonian): Saint-Clément-de-la-Place, Sceaux d’Anjou ( Dell’Angelo et al. 2018b); Ligerian Basin, France (Messinian?): Moulin Pochas ( Dell’Angelo et al. 2018a). Pliocene: central Mediterranean, Italy: Liguria ( Dell’Angelo et al. 2013); Tuscany: Pietrafitta, Poggio alla Fame (this study).