Ischnochiton krohi, Dell’Angelo & Sosso & Taviani, 2025

Ischnochiton krohi sp. nov.

Fig. 71 View FIGURE 71

Type material. Holotype: MSNG 62647 View Materials , intermediate valve, width 4.8 mm ( Figs 71B–D View FIGURE 71 ) . Paratype 1: MSNG 62648 View Materials , head valve, width 3.5 mm ( Fig. 71A View FIGURE 71 ) . Paratype 2: MSNG 62649 View Materials , tail valve, width 4 mm ( Figs 71I–L View FIGURE 71 ) . Paratype 3: MNHN.F.A98473, intermediate valve, width 3 mm. Paratype 4: NHMW xxx, intermediate valve, width 3.3 mm ( Figs 71E–G View FIGURE 71 ). Paratype 5: NHMW 2010/0256/0005 (as I. rudolticensis ), intermediate valve, width 2.7 mm ( Figs 71M–P View FIGURE 71 ) from Lapugy ( Romania) .

Type locality. Varovtsi ( Ukraine) .

Type stage. Middle Miocene.

Etymology. The name honors Andreas Kroh (NHMW), in recognition of his many paleontological contributions on European Cenozoic marine invertebrates.

Material examined. Middle Miocene: Eastern Paratethys : Ukraine: Varovtsi: type material plus 5 valves ( BD 252 , Fig. 71H View FIGURE 71 , BD 253 ). Maximum width of the valves: 3.5 / 4.8 / 4 mm .

Diagnosis. Head valve semicircular, intermediate valves broadly rectangular, width about three times length, moderately elevated, rounded, without trace of apex, tail valve semi-circular, mucro subcentral, not prominent. Tegmentum sculptured with large and irregular granules, seeming radial striae in HV, LA, PMA, forming a network in CA, AMA, tegmentum fully covered by aesthetes, both on nodular elevations and between them. Articulamentum with apophyses rounded, slit formula: - / 2 / 11.

Description. Head valve semicircular, posterior margin widely V-shaped. Intermediate valves broadly rectangular, width about three times length (W/L = 2.82–3.01), moderately elevated (H/W = 0.34), rounded in anterior profile, anterior margin almost straight, side margins slightly rounded, posterior margin straight, without trace of apex, lateral areas not elevated. Tail valve semicircular, W/L = 1.68–1.82, anterior margin straight, mucro subcentral, not prominent, antemucronal slope straight to slightly convex, postmucronal slope slightly concave.

HV, LA and PMA sculptured with large and irregular granules, very close to each other, randomly oriented, aligned along growth lines towards margins, but giving the impression to form radial striae of granules, especially in HV and PMA. CA and AMA sculptured with irregular elongated granules, slightly overlapping each other, less evident in JA, arranged randomly but giving the impression of forming a network. Tegmentum fully covered by aesthetes, both on nodular elevations (without a clearly recognizable structure of megalaesthetes and micraesthetes) and between them.

Articulamentum with apophyses rounded, trapezoidal in tail valve, jugal sinus wide, insertion lamina divided into irregular teeth, slit formula: - / 2 / 11.

Remarks. The fossil record of Ischnochiton krohi sp. nov. is limited to the Middle Miocene (Badenian) of Paratethys ( Romania and Ukraine). The material is scarce, a few valves but quite well preserved.

A tail valve ( Fig. 71H View FIGURE 71 ) shows a slightly different shape, more circular that those of the other tail valves examined, decidedly semicircular (e.g., Fig. 71I View FIGURE 71 ), but we consider that it can be included in the intraspecific variability of Ischnochiton krohi sp. nov.

Comparisons. The more similar species is Ischnochiton ligusticus Dell’Angelo, Sosso, Prudenza & Bonfitto, 2013, from which I. krohi sp. nov. differs mainly by the different sculpture, consisting of very irregular granules, arranged in segments of various size and shape, slightly overlapping each other, forming rugosities in I. ligusticus , while the granules are more regular in I. krohi sp. nov., giving the impression to be arranged in radial striae in HV and PMA. Also, the stratigraphic distribution is different (Middle Miocene of Paratethys for I. krohi sp. nov. vs. upper Miocene to lower Pliocene of Italy for I. ligusticus ), and the number of slits in intermediate valves, 2 in I. krohi sp. nov. vs. 1 in I. ligusticus .

Distribution. Middle Miocene: Central Paratethys (Langhian-Serravallian): Romania: Lapugy; Eastern Paratethys: Ukraine: Varovtsi (this study).

Genus Stenoplax Carpenter MS, Dall, 1879

Type species. Chiton limaciformis Sowerby in Broderip & Sowerby, 1832, by original designation.

Distribution. Stenoplax is known from the Eocene to the Recent, with a living distribution mostly in temperate or tropical regions of the central and eastern Pacific (Hawaii, and from Canada to Peru), Indo–Pacific (from Japan to India), Indian Ocean ( Madagascar), East Atlantic Ocean (Gulf of Guinea), West Atlantic Ocean (from Florida to Brazil including the Caribbean) ( Kaas & Van Belle 1988). The fossil record extends back to the middle Eocene in Europe [ U.K. ( Wrigley 1943; Cherns & Schwabe 2019) and Ukraine ( Bielokrys 1999)] and the upper Eocene or lower Oligocene in Washington, U.S.A. ( Puchalski et al. 2008; Dell’Angelo et al. 2011 a), the lower Oligocene in Italy ( Dell’Angelo & Palazzi 1992; Dell’Angelo et al. 2015a), the Miocene in Italy ( Dell’Angelo et al. 2014) and France ( Dell’Angelo et al. 2018a), and the Pliocene-Pleistocene of California, U.S.A. ( Berry 1922; Vendrasco et al. 2012 and the references listed therein) and Japan ( Itoigawa et al. 1976, 1978).

Remarks. Stenoplax is a distinctive genus whose members are markedly elongate and whose intermediate valves have prominent sutural laminae and generally raised lateral areas. The depressed and much more elongated (relative to other valves) tail valve is diagnostic for this genus. The main morphological characters of the Stenoplax spp . considered in the present study are reported in Tab. 11, together with the species of the genus Connexochiton Kaas, 1979 .

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