Onithochiton, Gray, 1847

Dell’Angelo, Bruno, Landau, Bernard M., Sosso, Maurizio & Taviani, Marco, 2020, Late Pleistocene Red Sea Mollusca: 1. Polyplacophora, Zootaxa 4772 (3), pp. 401-449 : 427-428

“ Onithochiton ” vandingeneni n. sp.

( Fig. 16 )

Type material. Holotype MZB 50552, 1 intermediate valve, width 1.5 mm, Figs 16 A–E.

Type locality. Gulf of Aqaba , Saudi Arabia (Ash Shaykh Humayd: St. 8) .

Recent material examined. Zabargad Island (13/30): 1 intermediate valve, width 3.6 mm ( Figs 16 F–I) ( MZB 50553).

Type stage. Late Pleistocene, last interglacial MIS5e.

Etymology. In honour of Frank Van Dingenen who contributed generously a substantial part of the material here studied.

Diagnosis. Intermediate valve rectangular, moderately elevated, posterior margin beaked, lateral areas weakly raised, with a diagonal line just outlined. Tegmentum smooth to naked eye, concentric growth lines vaguely indicat- ed on lateral areas, 3–4 ocelli on each side present on anterior half of lateral areas. Articulamentum with apophyses semicircular, one slit.

Description. Intermediate valve rectangular, moderately elevated (H/W = 0.30), rounded in anterior profile, anterior margin slightly concave, lateral margins rounded, posterior margin beaked, straight to concave at both sides of protruding apex, lateral areas weakly raised, with a diagonal line just outlined. Tegmentum microscopically punctate, smooth to naked eye, concentric growth lines vaguely indicated on lateral areas, extending (mostly eroded) over central area, 3–4 ocelli on either side (diameter ca 24 µm) present on anterior half of lateral areas.

Articulamentum with apophyses semicircular, one slit.

Remarks. This unique intermediate valve could be attributed to any one of the three genera included in the subfamily Toniciinae Pilsbry, 1893 ( Tonicia Gray, 1847 , Lucilina Dall, 1882 , and Onithochiton Gray, 1847 ), as generic attribution within the subfamily is mainly based on the characters of the tail valve ( Kaas et al. 2006). Unfortunately, tail valves are not present in the studied material. Thus far, the genus Tonicia has not been reported from the Red Sea (see above), and we provisionally attribute our material to Onithochiton , pending further material.

Specific attribution is also problematic. The single valve is quite different from that of the other species discussed herein under Lucilina (see above). It is also quite different from the only species of Onithochiton living in the Red Sea, O. erythraeus Thiele, 1909 (known also from Yemen, Socotra Isl., Oman and the Arabian Gulf), characterised by the central area sculptured with numerous fine, longitudinal, forwardly converging grooves ( Kaas et al. 2006).

Two further species of Onithochiton are known from the Indian Ocean, O. literatus ( Krauss, 1848) (from Mozambique and S. Africa] and O. maillardi ( Deshayes, 1863) (from southern Madagascar, and Mascarene, Rodriguez and Seychelles islands) ( Kaas et al. 2006), both characterised by a well defined tegmentum sculpture (not smooth as in “ O.” vandingeneni n. sp.).

A single intermediate valve very similar to “ Onithochiton ” vandingeneni n. sp. is present in the Recent material in study from the Red Sea (Zabargad Island, st. 09/06) ( Fig. 16 F–I). This valve is larger (width 3.6 mm, more than double the studied fossil valve). However, we consider that the differences found (the presence of rugosities in the lateral areas, and the greater number of ocelli of largest size, up to 55 µm vs 24 µm in the studied fossil valve) are likely to fall within the intraspecific variability of “O.” vandingeneni n. sp.

Distribution. Late Pleistocene: Saudi Arabia, Gulf of Aqaba (Ash Shaykh Humayd: this study). Present-day: Red Sea, Zabargad island.

Figures

FIGURE 16. “Onithochiton” vandingeneni n. sp. A–E. Saudi Arabia, Gulf of Aqaba, Ash Shaykh Humayd (St. 8), Late Pleistocene (last interglacial MIS5e), Holotype, MZB 50552, intermediate valve, width 1.5 mm, dorsal view (A), detail of tegmentum surface of pleural-lateral area (B), ventral (C), frontal (D) and posterior (E) views. F–I. Present-day, Zabargad Island, st. 13/30, MZB 50553, intermediate valve, width 3.6 mm, dorsal view (F), detail of tegmentum surface of pleural area (G), of lateral areas (H) and frontal view (I). Scale bar: 100 µm (B, H); 50 µm (G).