Stenosemus radiatus, Dell'Angelo, Lesport, Cluzaud & Sosso, 2020

Stenosemus radiatus View in CoL Dell’Angelo, Lesport, Cluzaud & Sosso, 2020

Fig. 78 View FIGURE 78

Stenosemus radiatus View in CoL Dell’Angelo, Lesport, Cluzaud & Sosso, 2020b, p. 6, fig. 3; Schwabe & Dulai 2024, p. 58, figs 4–7.

Type material. Holotype MZB 50568, intermediate valve, width 5.4 mm, from Carrière Vives ( France, Figs 78C– E View FIGURE 78 ) . Paratype MHNBx 2020.6.1, intermediate valve, from Lahitet ( France) .

Type locality. Meilhan ( France) .

Type stage. Miocene (Burdigalian).

Material examined. Lower Miocene (Burdigalian): France: type material plus Carrière Vives: 7 valves ( AC, Figs 78F–H View FIGURE 78 , BD 565, Figs 78A–B View FIGURE 78 ). Maximum width of the valves: 4.5 / 5.4 / 4.5 mm .

Description. Head valve semicircular, posterior margin widely V-shaped, slope almost straight. Intermediate valves broadly rectangular, width more than three times length, W/L = 3.00–3.71 ( 3.29 in holotype), semicarinate in anterior profile, moderately elevated (H/W = 0.25–0.41), anterior margin almost straight or slightly sinuose, side margins fairly straight, posterior margin almost straight at both sides of a not very prominent apex, lateral areas scarcely differentiated. Tail valve semicircular, anterior margin convex, mucro in anterior position, antemucronal slope slightly convex, postmucronal slope concave.

Tegmentum rough. HV, LA and PMA sculptured with large and rhomboidal granules randomly arranged. CA and AMA sculptured with small and roundish granules randomly arranged.

Articulamentum with large apophyses, slit rays numerous (more than 30 in head and tail valves, 4–5 in intermediate valves) and well evident in all valves, eaves spongy.

Remarks. The fossil record of Stenosemus radiatus Dell’Angelo, Lesport, Cluzaud & Sosso, 2020 is limited to the lower Miocene of northeastern Atlantic and to the Middle Miocene of Paratethys. Stenosemus radiatus has two unusual characteristics, i.e., the intermediate valves being more than three times as wide as long, and the presence of a great number of slits in the insertion laminae. The slits are not always evident because of the worn condition of the valves, but the articulamentum displays numerous slit rays, and consequently an equal number of incisions on the insertion lamina should be expected.

Also the generic attribution has been difficult, many extant species with a tegmentum entirely covered by granules arranged randomly are attributed to the genera Ischnochiton Gray, 1847 , Stenosemus von Middendorff, 1847 or Lepidochitona Gray, 1821 , mainly on the basis of the soft parts. Dell’Angelo et al. (2020b) attributed the studied material to the genus Stenosemus , following Ferreira (1981) who considered the great number of slits in the insertion laminae as a character typical for this genus.

Comparisons. Stenosemus radiatus differs from the other species of Stenosemus here described by the different sculpture (CA and AMA granulose in S. radiatus vs. longitudinal riblets in the other Stenosemus spp .), the shape of the intermediate valves and the higher number of slits (see Tab. 12).

Distribution. Lower Miocene: northeastern Atlantic (Burdigalian): Aquitaine Basin, France: Carrière Vives, Saint-Jean-de-Marsacq ( Dell’Angelo et al. 2020b). Middle Miocene: Central Paratethys (Langhian-Serravallian): Hungary: Borsodbóta ( Schwabe & Dulai 2024).