Acanthochitona penicillata ( Deshayes, 1863 )

Dell’Angelo, Bruno, Landau, Bernard M., Sosso, Maurizio & Taviani, Marco, 2020, Late Pleistocene Red Sea Mollusca: 1. Polyplacophora, Zootaxa 4772 (3), pp. 401-449 : 430-431

Acanthochitona penicillata ( Deshayes, 1863)

( Fig. 18 )

Chiton penicillatus Deshayes 1863, p. 41 , pl. 6, figs 8, 10.

Acanthochites penicillatus ; Lamy 1938, p. 88.

Chiton (Acanthochiton) penicillatus ; Moazzo 1939, p. 218.

Achantochiton (sic) (Achantochiton) penicillatus ; Selli 1944, p. 6.

Acanthochiton penicillatus ; Leloup 1952, p.: 9, text-fig. 4, pl. 1, fig. 3, pl. 2, fig. 5; Leloup 1960, p. 32; Leloup 1981, p. 3; Vine 1986, p. 126.

Acanthochitona cf. A. penicillata ; Ferreira 1983, p. 284, fig. 32.

Acanthochitona penicillata ; Kaas 1985, p. 339; Kaas 1986, p. 18; Van Belle & Wranik 1991, p. 379, fig. 25; Strack 1993, p. 23, pl. 5, fig. 6; Kaas 1996, p. 372; Schwabe 1997, p. 28, unnumbered figs at p. 27; Drivas & Jay 1998, p. 33, fig. sp. 11; Kaas & Van Belle 1998, p. 141; Dekker & Orlin 2000, p. 7; Slieker 2000, p. 52, fig.37; Abubakr 2004, p. 73; Anseeuw & Terryn 2004, p. 17, figs 56–57; Dell’Angelo et al. 2004, p. 56, pl. 5, figs 10–16; Dekker & Gemert 2008, p. 124; Dinapoli & Janssen 2009, p. 27, pl. 14 a–d; Dell’Angelo et al. 2010a, p. 19, figs 7A–D, 12K; Dell’Angelo et al. 2010b, p. 39, fig. 4F; Sirenko & Schwabe 2011, p. 119; Blatterer 2019, p. 56, pl. 5, fig. 13 a–g.

Type material. Unascertained (fide Ferreira 1983). Type locality. “ Ile de la Réunion ” (21°06’ S, 55°36’E) .

Material examined. Saudi Arabia, Gulf of Aqaba (Ash Shaykh Humayd: this study): St. 8: 7 valves (2 head, 4 intermediate and 1 tail) ( MZB 60262); St. 9: 3 intermediate valves ( MZB 60263). Egypt, Hurghada: St. 12: 18 valves (2 head, 14 intermediate and 2 tail) ( BD 219); St. 13: 114 valves (19 head, 82 intermediate and 13 tail), Figs 18 D–I ( BD 220; MZB 50556; RGM.1356861); St. 14: 21 valves (2 head and 19 intermediate) ( BD 221); St. 14bis: 11 valves (1 head, 7 intermediate and 3 tail), Figs 18 A–C ( BD 222; MZB 50555); St. 16: 51 valves (1 head, 40 intermediate and 10 tail) ( BD 223); St. 17: 2 valves (1 head and 1 intermediate) ( BD 224). Maximum width: 3.3 / 5 / 4.4 mm.

Description. Head valve semicircular, posterior margin almost straight, apex hardly developed.

Intermediate valve subtrapezoidal, subcarinate in anterior profile, jugal margin straight, side margins rounded, posterior margin slightly concave either side of well developed apex, jugal area triangular, little raised, smooth, except for discrete transversal growth rugae, often showing texture of parallel longitudinal striae.

Tail valve almost circular, mucro subcentral, jugal area smooth, raised, antemucronal and postmucronal slopes almost straight, forming angle of about 126°.

Tegmentum densely covered with drop-shaped granules, variable in form, arranged in rows radiating from apex, jugal area smooth.

Articulamentum well developed, apophyses large, quadrangular, insertion plates well developed, strongly protruding on intermediate valves, expanded and completely surrounding tail valve, except for jugal margin, slit formula 5/1/2.

Remarks. The intermediate valves present in our material are keeled, slightly beaked, with a well defined, smooth jugal area, slightly elevated, broader in the front than in the back of the valve, smooth, except for growth lines, and with lateropleural areas bearing rows of roundish/oval granules, radiating from the apex.

Although it is difficult to reliably identify this species-group, we attribute our material to A. penicillata , mainly on the basis of the shape of the valves and the smooth jugal area (except for discrete transversal growth rugae). Deshayes’ (1863) original description of Chiton penicillatus is imprecise ( Ferreira 1983) and a potential source of confusion. Compared with specimens from Madagascar ( Dell’Angelo et al. 2004: pl. 5, figs 10–12) and from the Maldives ( Dell’Angelo et al. 2010a: figs 7A–C), the Red Sea valves have less drop-shaped, more rounded pleural granules. However, in his description of specimens from the Red Sea chitons, Strack (1993: 24) noted: “ tegmentum densely covered with pyriform or drop-shaped granules, variable in form, from almost round (seldom) to elongate pyriform, and in size ”.

The Red Sea material at hand has also been compared with other western Indian Ocean species. Many of them have the jugal area more or less longitudinally ribbed: A. mahensis Winckworth, 1927 , known from Red Sea, India, Seychelles and Andaman Islands ( Anseeuw & Terryn 2004); A. jugotenuis Kaas, 1979 known from Mozambique ( Kaas 1979); A. quincunx Leloup, 1981 known from Madagascar ( Leloup 1981); A. limbata Kaas, 1986 known from Madagascar ( Kaas 1986; Dell’Angelo et al. 2004); A. woodwardi Kaas & Van Belle, 1988 known from Kuwait, Qatar and Socotra Island ( Dinapoli & Janssen 2009); Leptoplax curvisetosa ( Leloup, 1960) known from Red Sea, Madagascar, Glorieuses Islands and Socotra Island ( Strack 1993; Dinapoli & Janssen 2009).

Acanthochitona mastalleri Strack 1989 , known from Red Sea, Oman and Kenya, is separated from its congeners in having the tegmentum reduced in lateropleural areas (see below).

Distribution. Late Pleistocene: Eritrea, Massawa; Saudi Arabia, Gulf of Aqaba (Ash Shaykh Humayd: this study); Egypt, Hurghada ( Selli 1944; this study). Present-day: Indian Ocean: Western Indian Ocean and the Northern Red Sea to Madagascar, Seychelles, Réunion and Mauritius; Arabian Sea: Yemen, Socotra Is. ( Dinapoli & Janssen 2009; Blatterer 2019).

Figures

FIGURE 18. Acanthochitona penicillata (Deshayes, 1863). A–I. Egypt, Hurghada, Late Pleistocene (last interglacial MIS5e). A–C. MZB 50555, head valve (St. 14bis), width 3.3 mm, dorsal, ventral and lateral views. D–F. MZB 50556a, intermediate valve (St. 13), width 5 mm, dorsal, ventral and frontal views. G–I. MZB 50556b, tail valve (BL), width 4.4 mm, dorsal, ventral and lateral views.