Stenoplax veneta, Dell'Angelo & Palazzi, 1992

Stenoplax veneta View in CoL Dell’Angelo & Palazzi, 1992

Fig. 73 View FIGURE 73

Stenoplax veneta View in CoL Dell’Angelo & Palazzi, 1992: 27, figs 1–3; Schwabe 2005: 104; Puchalski et al. 2008 (database: chiton fossil records); Dell’Angelo et al. 2011: 941, 953; Dell’Angelo et al. 2012b: 27; Dell’Angelo et al. 2014: 51, pl. 1, figs 1–9; Dell’Angelo et al. 2015b, p. 49, figs 3A–L.

Type material. Holotype MZB 10301, tail valve. Paratype BD 3039, a head and 2 tail valves ( Figs 73A–D View FIGURE 73 ).

Type locality. Case Soghe , Vicenza ( Veneto, Italy) .

Type stage. Lower Oligocene (Rupelian) .

Material examined. Lower Oligocene: Italy: Case Soghe : type material plus 5 valves (2 intermediate and 3 tail, BD 543, Figs 73E–H View FIGURE 73 ). Maximum width of the valves: 2.8 / 3 / 4 mm .

Description. Head valve probably semicircular (only a fragment known). Intermediate valve broadly rectangular, anterior margin straight, posterior margin seems straight for the visible part, lateral area poorly defined, not distinctly separated by central area, only a sign of diagonal fold between lateral and central areas is present but hardly evidenced. Tail valve semielliptical (W/L = 1.24), depressed, anterior margin almost straight, mucro subcentral, not elevated, antemucronal slope straight or slightly convex, postmucronal slope almost straight just behind mucro.

Tegmentum fully covered by small granules, irregularly disposed and some coalescing in HV, LA and PMA, with marked concentric lines of growth visible on PMA, arranged in groups that seem obliquely directed in CA and AMA.

Articulamentum with trapezoidal apophyses in tail valve, separated by a straight sinus, teeth short and irregular, slit formula:? / 1? / 12–16.

Remarks. This is the sole described polyplacophoran species from the Oligocene of Italy. Originally it was described on four valves only (1 head fragment and 3 tail), and the morphology of the intermediate valves was unknown. New material found in recent years (2 intermediate and 3 tail valves) allowed to complete the description also for the intermediate valves and to better define some specific characters ( Dell’Angelo et al. 2015b).

Not all characters of the species match those of the genus Stenoplax Carpenter MS, Dall, 1879 . The lateral areas of Stenoplax spp . are generally rather elevated (Kaas & Van Belle 1987: 124; Vendrasco et al. 2012: 34) but are little raised and poorly defined in S. veneta Dell’Angelo & Palazzi, 1992. However, the same characteristic is seen in other Stenoplax species, e.g., S. marcusi ( Righi, 1971) .

Comparisons. The tegmentum fully covered by small granules of Stenoplax veneta is characteristic, and different from that of S. paviai Dell’Angelo, Giuntelli, Sosso & Zunino, 2014.

Distribution. Oligocene (Rupelian): N. Italy: Case Soghe ( Dell’Angelo & Palazzi 1992; Dell’Angelo et al. 2015b).

Genus Stenosemus von Middendorff, 1847

Type species. Chiton albus Linnaeus, 1767 , by subsequent designation ( Winckworth 1926).

Distribution. The genus Stenosemus is known from the Miocene up to Recent. Stenosemus is the most widely distributed genus of the order Chitonida , with as many as 24 living species described thus far, from the Arctic to Antarctic regions, including cold deep waters of tropical region, mainly at depths greater than 200 m down to 4572 m (Sirenko 2016, 2017). Its fossil record includes the lower Miocene of France ( Dell’Angelo et al. 2020b), the middle Miocene of Hungary ( Schwabe & Dulai 2024), the upper Miocene of France and Italy ( Dell’Angelo et al. 2015a, 2018a), the Pliocene of Spain and Italy (Dell’Angelo et al. 2004, 2012, 2013; this study), the Pleistocene of Italy ( Dell’Angelo & Giusti 1997; this study).

Remarks. The genus Stenosemus was considered by Kaas & Van Belle (1990) as a subgenus of Ischnochiton , differing mainly on the soft parts morphology (i.e. the dorsal girdle covered with conical scales in Stenosemus , with imbricating, generally striated scales in Ischnochiton ). However, the first who considered Stenosemus as a distinct genus was Ferreira (1981).

Regarding the Mediterranean basin, only one species, Stenosemus dolii ( Van Belle & Dell’Angelo, 1998), was considered up to now for the paleontological record of the Mediterranean basin, characterized by a great variability of valves’ morphology, especially regarding the greater or lesser graininess of the radial sculpture. In this respect, the Pliocene of Altavilla Milicia ( Sicily) provided abundant material attributed to Stenosemus dolii by Dell’Angelo et al. (2012). An in-depth reconsideration of such material suggests that two undescribed species are, in fact, present which we formalize here as S. juliuspisai sp. nov. and S. praedolii sp. nov.

The main morphological characteristics of the Stenosemus spp . considered in the present study are reported in Tab. 12.

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