Ischnochiton yerburyi (E.A. Smith, 1891 )

Dell’Angelo, Bruno, Landau, Bernard M., Sosso, Maurizio & Taviani, Marco, 2020, Late Pleistocene Red Sea Mollusca: 1. Polyplacophora, Zootaxa 4772 (3), pp. 401-449 : 408-409

Ischnochiton yerburyi (E.A. Smith, 1891)

( Fig. 4 )

Chiton (Ischnochiton) yerburyi E.A. Smith 1891, p. 420 , pl. 33, fig. 6; Pilsbry 1892, p. 101, pl. 20, fig. 11.

Ischnochiton yerburyi ; Leloup 1960, p. 35, fig. 5; Ferreira 1983, p. 251, figs 1–2; Strack 1993, p. 9, pl. 3, fig. 2, pl. 7, fig. 3; Bosch et al. 1995, p. 189, fig. sp. 879; Kaas & Van Belle 1998, p. 203; Dekker & Orlin 2000, p. 7; Slieker 2000, p. 46, fig. 9; Abubakr 2004, p. 73; Callea et al. 2005, p. 93; Dell’Angelo et al. 2011b, p. 25; Mohammadian 2011, p. 211, fig. p. 212; Al-Yamani et al. 2012, p. 120, pl. 83; Blatterer 2019, p. 49, pl. 2, fig. 5 a-b.

Ischnochiton yerburi (sic); Leloup 1980, p. 10; Vine 1986, p. 125.

Ischnochiton (Ischnochiton) yerburyi ; Kaas 1986, p. 11, fig. 8; Anseeuw & Terryn 2004, p.7, figs 16–23, 38–39; Dinapoli & Janssen 2009, p. 23, pl. 12a–d; Hekmatara & Heidary Baladehi 2011, p. 1, fig. 2.

Ischnochiton (I.) yerburyi ; Kaas & Van Belle 1988, p. 116, figs 2–7; Kaas & Van Belle 1990, p. 124, fig. 53, map 21; Dell’Angelo et al. 2004, p. 51, pl. 3, figs 4–7.

Ischnochiton (Simplischnochiton) yerburyi ; Van Belle & Wranik 1991, p. 368, fig. 13; Van Belle 1994, p. 2.

Type material. BMNH 1888.4.9.345.

Type locality. Aden .

Material examined. Saudi Arabia, Gulf of Aqaba (Ash Shaykh Humayd: this study): St. 8: 8 valves (1 head and 7 intermediate) ( MZB 50589); St. 9: 4 valves (1 head, 2 intermediate and 1 tail), Figs 4 G–I ( MZB 50527). Egypt, Hurghada: St. 13: 6 intermediate valves ( BD 181); St. 14: 14 valves (1 head, 11 intermediate and 2 tail), Figs 4 A–F ( BD 182; MZB 50526; RGM.1356850); St. 14bis: 2 intermediate valves ( BD 183); St. 16: 7 valves (1 head, 5 intermediate and 1 tail) ( BD 184). Maximum width: 5 / 6.3 / 5.2 mm.

Description. Head valve semi-circular, posterior margin widely V-shaped, front slope straight.

Intermediate valve rectangular, L/W = 0.36–0.38, front margin sinuous, moderately elevated (H/W = 0.39– 0.44), anterior profile subcarinated, side margins rounded, posterior margin almost straight, apex hardly or not indicated, lateral areas slightly raised, poorly defined.

Tail valve semicircular, L/W = 0.58, front margin weakly convex in jugal part, mucro subcentral, not elevated, antemucronal slope slightly convex, postmucronal slope weakly concave directly behind mucro.

Tegmentum coarsely reticulated by a thimble-like sculpture, pits squarish to subromboidal, network finer on the jugal part.

Articulamentum with rounded apophyses, jugal sinus straight, relatively wide, insertion plates short, slit formula 11–13/1/11–12, slits deep, teeth inequidistant, smooth.

Remarks. A detailed description of this species was given by Kaas & Van Belle (1990). Ferreira (1983) synonymised four other nominal Ischnochiton species from the Indian Ocean with Ischnochiton yerburyi , all having in common a thimble-like sculptured tegmentum. After a thorough examination of the types, Kaas & Van Belle (1990) concluded that at least two species could be distinguished, I. yerburyi with a strong thimble-like sculpture that is apparent even on the head and tail valves, and I. sansibarensis Thiele, 1909 with a weaker net-like sculpture, in which the head and tail valves and lateral areas are evenly granulose-punctate. Other differences in the dorsal girdle scales and in the radula require further investigation ( Kaas & Van Belle 1990; Anseeuw & Terryn 2004).

We attribute our material to I. yerburyi , despite the valves having a weaker sculpture and the front margin of the figured tail valve ( Fig. 4G ) being convex, as reported by Thiele (1909: pl. 8, fig. 27) and Kaas & Van Belle (1990: fig. 55/2) for I. sansibarensis . The tail valve figured has 12 slits in the articulamentum ( Fig. 4H ), which agrees with the number reported by Kaas & Van Belle for I. yerburyi (11–13, vs 8–10 for I. sansibarensis ). Moreover, the sculpture of antemucronal and postmucronal areas seems similar (not granulose-punctate as in I. sansibarensis ), and agrees with the tail valves figured by Anseeuw & Terryn (2004: fig. 20) and Dinapoli & Janssen (2009: pl. 12d) for I. yerburyi .

This is the first report of this species as a fossil.

Distribution. Late Pleistocene: Saudi Arabia, Gulf of Aqaba (Ash Shaykh Humayd: this study); Egypt (Hurghada: this study). Present-day: Indian Ocean, from the north-western Red Sea to the Maldives; African coast from Somalia to Zanzibar, Socotra island to Madagascar, and the Northern Red Sea to Mozambique; Arabian Sea: Pakistan to Yemen ( Dinapoli & Janssen 2009; Hekmatara & Heidary Baladehi 2011; Blatterer 2019).

Figures

FIGURE 4. Ischnochiton yerburyi (E.A. Smith, 1891). A–F. Egypt, Hurghada (St. 14), Late Pleistocene (last interglacial MIS5e). A–C. MZB 50526a, head valve, width 4.8 mm, dorsal, ventral and lateral views. D–F. MZB 50526b, intermediate valve, width 4.8 mm, dorsal, ventral and frontal views. G–I. Saudi Arabia, Gulf of Aqaba, Ash Shaykh Humayd, Late Pleistocene (last interglacial MIS5e) (St. 9), MZB 50527, tail valve, width 5.2 mm, dorsal, ventral and lateral views.