Parachiton africanus ( Nierstrasz, 1906 )

Parachiton africanus ( Nierstrasz, 1906) View in CoL

Fig. 38 View FIGURE 38

Lepidopleurus africanus Nierstrasz, 1906, p. 155 View in CoL , figs 1–9.

Leptochiton ( Parachiton) africanus View in CoL ; Kaas 1977, p. 81, figs 1–5; Kaas & Van Belle 1985a, p. 163, fig. 75, map 26; Cesari 1987, p. 12, pl. 9, figs 1–6; pl. 10, figs 1–5.

Lepidopleurus africanus View in CoL ; Bałuk 1984, p. 286, pl. 1, figs 1–2; Crovato & Taviani 1985, p. 292.

Lepidopleurus ( Parachiton) africanus View in CoL ; Laghi et al. 1981, p. 1, pl. 1, figs 3–9; Dell’Angelo & Palazzi 1989, p. 80, pls 23–24; Dell’Angelo et al. 1998a, p. 242, pl. 1, figs 2–4, 6–7; Mancini 1998, p. 29, pl. 1, unnumbered fig.; Dell’Angelo & Smriglio 1999, p. 80, pls 23–24, figs 30–33; Mancini 1999, p. 20.

Lepidopleurus ( Parachiton) aff. africanus View in CoL ; Dell’Angelo et al. 2004, p. 29 ( partim, non pl. 2, figs 2, 6 = Parachiton statianus View in CoL , fide Dell’Angelo et al. 2015a).

Parachiton africanus View in CoL ; Dell’Angelo et al. 2015a, p. 230; Ruman & Hudácková 2015, p. 158, fig. 2.1; Dell’Angelo et al. 2018a, p. 19; Dell’Angelo et al. 2018b, p. 52; Dell’Angelo et al. 2020b, p. 52, tab. 9; Dulai & Katona 2024, p. 35, figs 8–9; Dulai 2025a, p. 6, figs 9–11; Dulai 2025b, p. 24, figs 3–4.

Lepidopleurus ( Parachiton) thielei Šulc, 1934, p. 6 View in CoL , pl.1, figs 4–5; Ashby & Cotton 1935, p. 389; Sieber 1959, p. 275; Kaas 1977, p. 84; Laghi et al. 1981, p. 4; Dell’Angelo & Palazzi 1989, p. 80.

Lepidopleurus thielei View in CoL ; Bałuk 1971, p. 454, pl. 1, fig. 8.

Leptochiton ( Parachiton) thielei ; Van Belle 1981, p. 76; Kaas & Van Belle 1985a, p. 165.

Type material. Holotype: RMNH, Leiden, no. reg. 2783, a specimen deprived of its end valves ( Kaas 1977) . Holotype of Parachiton thielei figured by Šulc 1934 ( Fig. 38I View FIGURE 38 ).

Type locality. Oran, Algeria .

Material examined. Middle Miocene: Central Paratethys: Austria: Steinabrunn: 1 valve ( NHMV 1933/0001/0042, Figs 38M–P View FIGURE 38 ; Hungary: Letkés: 1 valve ( BD 408). Pliocene: Spain: Estepona: 1 valve ( BD 409). Italy: Piedmont: Valle Andona: 1 valve ( BD 410); Emilia-Romagna: Cava di Campore: 3 valves ( BD 411). Pleistocene: Italy: Calabria: Archi S. Francesco: 9 valves ( BD 412, Figs 38A–F View FIGURE 38 ), Carrabbati: 1 valve ( BD 413), Gallina: 1 valve ( BD 414, Figs 38G–H View FIGURE 38 ), Petti di Carrubbare: 2 valves ( BD 415), Terreti: 3 valves ( BD 416); Sicily: Capo Milazzo: 1 valve ( BD 417). Maximum width of the valves: 5 / 7 / 5 mm.

Description. Head valve semicircular,posterior margin widely V-shaped.Intermediate valves broadly rectangular (W/L = 2.09–2.18), rounded in anterior profile, moderately elevated (H/W = 0.30–0.36), anterior margin straight between the apophyses, posterior margin straight, apex not indicated, lateral areas only discernible by difference in sculpture, marked with 6 or 7 faint concentric lines of growth. Tail valve from elliptical to almost triangular (W/L = 1.14–1.28), mucro somewhat swollen, rounded, posterior, at 7/8 of valve’s length, antemucronal slope slightly convex, postmucronal slope short, steep, straight, but for a little excavation directly behind the mucro.

Tegmentum finely granulose all over. HV and PMA sculptured with radial series of more or less oval granules, interrupted by weak growth lines.LA sculptured with granules quincuncially arranged,tending towards an arrangement in radial striae, interrupted by 6–7 concentric growth lines. CA and AMA sculptured with longitudinal series of granules (CA 45–55, AMA 38–50), with reduced space between striae, arranged side by side, with exception of jugal tract, where they are slightly convergent, on sides sometimes anastomosing, granules more clearly pronounced towards side margins.

Articulamentum weakly developed, apophyses small, more or less triangular, wide apart, jugal sinus broad, slightly convex or flat.

Remarks. Parachiton africanus ( Nierstrasz, 1906) is a rare Mediterranean species described upon a single specimen of small dimensions (length 7 mm, width 3 mm), from Oran, Algeria. Based upon a second specimen found after 70 years later at Gallipoli ( Puglia, Italy) compared with Nierstrasz’s holotype, Kaas (1977) validates the taxon, thus far apparently endemic to the Mediterranean.

The fossil species Lepidopleurus ( Parachiton) thielei was described by Šulc (1934) on the basis of a few intermediate and tail valves from the Miocene deposits of Steinabrunn in the Vienna Basin, and afterwards from Korytnica ( Poland) by Bałuk (1971, 1984). Kaas (1977) stated: “ in my opinion L. ( P.) africanus , if not conspecific with L. ( P.) thielei , must be regarded as a closely allied descendant of the latter ”). It was considered as a synonym of Parachiton africanus by several authors ( Laghi et al. 1981; Bałuk 1984; Kaas & Van Belle 1985a; Dell’Angelo & Palazzi 1989; Dell’Angelo et al. 2015a, 2018a). The only differences pointed out are those reported by Bałuk (1984: “ the differences concern only greater dimensions and an absence of regular interspaces between the concentric ridges on lateral areas of intermediate valves in L. thielei ”).

In order to explore whether there is ground to discriminate between the two taxa under scrutiny upon morphological arguments, we have analyzed in-depth the following features:

The tegmentum’s sculpture in CA and AMA. Nierstrasz (1906) does not report the number of longitudinal rows for Parachiton africanus , he says “at least 36” for intermediate valves, and “sculptured like the central area of the middle valves” for tail valve. It is difficult to count the number of longitudinal striae of granules in CA and AMA, the striae are mainly parallel and regular but sometimes anastomosing and often converging posteriorly on the jugum, and by no means as regular as Nierstrasz’s figures would suggest. Šulc reports ca. 50 longitudinal rows of granules for Parachiton thielei , parallel except on the jugal tract, where they converge (not in Šulc’s description, but evident in his fig. 4 of the holotype). In the valve at NHMW the longitudinal ribbing on the jugal tract is a little convergent indeed, and the number of longitudinal rows of granules agree with the 50 reported by Šulc. A study of all intermediate and tail fossil valves available and some recent ones allow us to better define the range of radial striae of granules in CA (45–55) and AMA (38–50), so also for this feature, a certain degree of variability seem acceptable.

The shape of tail valves. The shape of tail valves of Parachiton africanus is variable, from an elliptical, almost triangular profile, as in the holotype figured in Nierstraz (1906: figs 1, 6) to a more regularly rounded profile, as in the recent valves figured by Kaas (1977: figs 1, 6) and Dell’Angelo & Smriglio (1999: pl. 23, fig. F). The same variability is present in the fossil valves object of this study (compare Fig. 38E View FIGURE 38 and Fig. 38G View FIGURE 38 ). The same variability can be observed in Parachiton thielei . Šulc figured two tail valves ( Figs 38I–J View FIGURE 38 ), of which the first, larger (W = 3.8 mm), designated as holotype, shows an elliptical, almost triangular profile, while the second one, smaller, is more regularly rounded, which could suggest a range of variability of the tail valve’s shape, as already suggested by Šulc (1934: “ On the basis of these tail valves it can be deduced that in immature individuals they do not have such an elliptical shape, but a more circular one ”). However, the tail valve present in the Šulc collection at NHMW ( Figs 38M–P View FIGURE 38 ) has the margin of PMA more regularly rounded and has a wide ( 3 mm) more like that of the holotype ( 3.8 mm), not in full agreement with the previously hypothesized range of variability of the tail valve profile.

The possibility P. thielei being the ancestor of the recent P. africanus is still possible, or even likely. However, we did not find robust morphological evidence to separate the two species and this option would rest solely on biogeographic and paleoclimatic arguments. P. thielei inhabited subtropical contexts of the European Neogene vs. the temperate habitats of the Pleistocene to Recent P. africanus . Therefore, in consideration of the high degree of variability found on the examined material, we prefer to consider Parachiton thielei as a synonym of P. africanus .

Comparisons. See Tab. 6 for a comparison with the Parachiton spp . considered in the present study.

Distribution. Middle Miocene: Central Paratethys (Langhian-Serravallian): Austria: Steinabrunn ( Šulc 1934; this study); Slovakia: Rohožník ( Ruman & Hudácková 2015); Poland: Korytnica ( Bałuk 1971, 1984); Hungary: Devecser,Letkés¸Várpalota ( Dulai&Katona2024; Dulai2025 a,2025b; this study). Pliocene: western Mediterranean, Estepona Basin, Spain: Estepona (Dell’Angelo et al. 2004); central Mediterranean, Italy: Valle Andona ( Laghi et al. 1981), Castell’Arquato ( Dell’Angelo & Palazzi 1989), Cava di Campore (this study), Colle Sabbaco ( Mancini 1998, 1999). Pleistocene: central Mediterranean, S. Italy: Arangea ( Dell’Angelo & Palazzi 1989), Archi ( Crovato & Taviani 1985), Terreti ( Dell’Angelo & Smriglio 1999), Carrabbati, Gallina, Petti di Carrubbare, Capo Milazzo (this study). Recent: Northern part of the Atlantic coast of Morocco ( Kaas et al. 2006), and the Mediterranean Sea: Spain (Costa Brava), France ( Corsica), Italy: Gulf of Taranto, Strait of Sicily, Pantelleria island ( Dell’Angelo et al. 1998a), Turkey ( Ozturk et al. 2014), Lebanon ( Crocetta et al. 2014), Algeria: Oran ( Dell’Angelo & Smriglio 1999).