Lucilina confusa, Dell’Angelo & Landau & Sosso & Taviani, 2020

Dell’Angelo, Bruno, Landau, Bernard M., Sosso, Maurizio & Taviani, Marco, 2020, Late Pleistocene Red Sea Mollusca: 1. Polyplacophora, Zootaxa 4772 (3), pp. 401-449 : 419-422

Lucilina confusa n. sp.

( Fig. 11 , 12 )

Type material. The type material is reported in Tab. 2.

Other material. Saudi Arabia, Gulf of Aqaba (Ash Shaykh Humayd): St. 8: 12 valves (2 head and 10 intermediate) ( MZB 60258); St. 9: 20 valves (3 head and 17 intermediate) ( MZB 60259). Egypt, Hurghada: St. 13: 33 valves (2 head, 24 intermediate and 7 tail), Figs 11 M–N ( BD 205; MZB 50543); St. 14: 33 valves (5 head, 21 intermediate and 7 tail) ( BD 206; RGM.1356858); St. 14bis: 18 valves (1 head, 11 intermediate and 6 tail) ( BD 207); St. 15: 3 valves (1 head, 1 intermediate and 1 tail) ( BD 208); St. 16: 8 valves (2 head, 3 intermediate and 3 tail ( BD 209).

Recent material examined. Zabargad island (13/30): 42 valves (5 head, 28 intermediate and 9 tail) ( Figs 12 A- I) ( MZB 50544). Sudan, Sha’ab Rum Reef (P078): 14 valves (2 head, 10 intermediate and 2 tail) ( BD 210). Egypt, Gulf of Aqaba (Blue Hole) (P447A): 3 intermediate valves ( BD 211).

Maximum width: 4.4 / 7.5 / 7.2 mm.

Type locality. Saudi Arabia, Gulf of Aqaba (Ash Shaykh Humayd: this study): St. 9 .

Type stage. Late Pleistocene, last interglacial (MIS5e).

Etymology. The name indicates the fact that a rather common species in this region has been for long confused with morphologically close taxa.

Diagnosis. Head valve semicircular; intermediate valve rectangular, moderately elevated, anterior profile subcarinated, posterior margin slightly concave at both sides of prominent apex, lateral areas not raised; tail valve semicircular, mucro small, raised, in posterior position, antemucronal slope almost straight, postmucronal slope steep, straight to weakly convex. Tegmentum sculptured with irregular radial striae of roundish granules on head valve, lateral areas of intermediate valves and postmucronal area of tail valve; central area of intermediate valves and postmucronal area of tail valve sculptured with irregular and well-separated longitudinal striae of rounded to oval granules.Articulamentum white, apophyses well developed, rounded, jugal sinus wide, slit formula 8/1/9, teeth relatively long, finely grooved on upper side, denticulated on outer edge.

Description. Head valve (based on Paratype 1, Fig. 11F ) semicircular, front slope slightly convex, posterior margin widely V-shaped, tegmentum sculptured with ca 34 irregularly radial striae of roundish, elevated granules (diameter ca 60–75 µm), up to 10–12 granules per stria; striae do not start from apex, but from small, smooth circular area around apex, ocelli minute (diameter ca 40 µm), scarcely visible, aligned in interstices of radial striae of granules.

Intermediate valve (based on Paratype 2, Fig. 11J ) rectangular, L/W = 0.38, anterior margin sinuose, slightly concave in wide central part, convex at pleurae, moderately elevated (H/W = 0.36), anterior profile subcarinated, side margins rounded, posterior margin slightly concave at both sides of prominent apex, lateral areas not raised, sculptured like head valve with 7–8 irregular radial striae of granules, ocelli numerous in anterior third of valve, pleural areas with similar granules arranged in ca 20–22 longitudinal, irregular and well-separated striae on each side, slightly oblique near jugum and in opposite direction towards side margins, granules more irregular, rounded to oval, diameter up to 100 µm, jugal area smooth, triangular, irregularly defined.

Tail valve (based on Holotype, Fig. 11A ) semicircular, length ca half width (L/W = 0.51), front margin almost straight, mucro small, raised, in posterior position, antemucronal slope almost straight, postmucronal slope steep, straight to weakly convex, antemucronal area sculptured with ca 16 longitudinal striae of roundish-oval, more elevated granules on each side, with granule diameter up to 70 µm, jugal area smooth, irregularly defined, postmucronal area with sculpture like head valve with ca 40 radial striae of granules, ocelli randomly dispersed in postmucronal area.

Articulamentum white, apophyses well developed, rounded, jugal sinus wide, slit formula 8/1/9, slit rays faintly visible in head and intermediate valves, teeth relatively long, those of tail valve slightly forwardly directed, all teeth finely grooved on upper side, denticulated on outer edge, eaves narrow, solid.

Variability. The species show a great variability in the sculpture of the valves as well as other characters.

Head valve: the radial striae of roundish, elevated granules, can be more irregularly defined, large and widely spaced as in the figured valve from Egypt, Hurghada ( Fig. 11M , width 3.3 mm, ca 25 radial striae, diameter up to 90–100 µm, ca 6–7 granules for each radial stria). The number of more or less irregular radial striae of granules ranges between 25 and 35, and the number of slits in the articulamentum ranges between six and eight.

Intermediate valves: the number and the orientation of longitudinal striae of granules in the pleural areas is variable, as can be seen in the figured valve from Egypt, Hurghada ( Fig. 11N , width 5 mm, ca 16 longitudinal striae on each side, striae more regular and parallel). Also the radial striae of granules in lateral areas are generally irregularly defined, in some valves are more numerous and well defined (as in Fig. 11N ). L/W = 0.36–0.41; H/W = 0.33–0.37. The jugal area is usually triangular and smooth, only in very few valves the striae of granules cover partially the jugal area, leaving a very narrow, wedge shaped, smooth band. The ocelli are scarcely visible, not black, and can radially cover up to half of the lateral area.

Tail valve: the sculpture shows the same variability as in intermediate valves, also for these valves the striae of granules can partially cover the jugal area ( Fig. 11O ).The number of slits ranges between 9 and 12.

Remarks. Several species of Lucilina are known from the Indian Ocean ( Kaas et al. 2006), but only two from the Red Sea, L. sueziensis ( Reeve, 1847) and L. perligera Thiele, 1909 (see above) ( Anseeuw & Terryn 2004). Lucilina perligera is similar to Lucilina confusa n. sp., from which it differs mainly by the tegmentum sculptured with large, elevated, roundish, clearly separated, quincuncially arranged, granules in the head valve, lateral areas of intermediate valves, and postmucronal area of the tail valve, while in pleural and antemucronal areas the granules are vaguely arranged in curved, forwardly converging rows, also extending over the jugum. The granules are larger, and wider spaced than those of L. confusa n. sp.

Two other Lucilina species are present in the studied material and have a similar sculpture with longitudinal and radial striae of granules: Lucilina aegyptiaca n. sp. and Lucilina colantonii n. sp. (see below). The main characters of the intermediate valves of these species and of L. sueziensis (based on the description of Kaas et al. 2006 and on personal observations) are presented in Tab. 3.

Numerous valves of Lucilina confusa n. sp. are present in the extant material from the Red Sea expeditions (currently under study, unpublished), mixed in samples initially labeled as L. sueziensis and L. perligera . Some of these valves are illustrated for comparison ( Figs 12 A–I). No significant differences were found between the fossil and extant valves described in this study. The figured head valve ( Figs 12 A–C) has ca 35 irregular radial striae of granules, with striae of ocelli in the interstices (diameter ca 40 µm), eight slits, and falls within the range of variability indicated above. Similarly, the intermediate valve ( Figs 12 D–F) and the figured tail valve ( Figs 12 G–I) fall within the range of variability indicated above. The longitudinal striae of granules in intermediate valves partially extends over the jugal area ( Fig. 12D ), only a small part near the anterior margin is smooth.

Distribution. Late Pleistocene: Saudi Arabia, Gulf of Aqaba (Ash Shaykh Humayd: this study); Egypt (Hurghada: this study). Present-day: Red Sea: Zabargad island; Sudan (Shab Runi Reef); Egypt, Gulf of Aqaba (Blue Hole).

Figures

FIGURE 11. Lucilina confusa n. sp. A–L, O. Saudi Arabia, Gulf of Aqaba, Ash Shaykh Humayd (St. 9), Late Pleistocene (last interglacial MIS5e). A–E. Holotype, MZB 50539, tail valve, width 5 mm, dorsal view, detail of tegmentum surface of antemucronal area, ventral, lateral and posterior views. F–I. Paratype 1, MZB 50540, head valve, width 3.7 mm, dorsal view, detail of tegmentum surface, ventral and posterior views. J–L. Paratype 2, MZB 50541, intermediate valve, width 5.6 mm, dorsal view, detail of tegmentum surface of pleural area and frontal view. O. Paratype 3, MZB 50542, tail valve, width 7.2 mm, dorsal view. M–N. Egypt, Hurghada (St. 13), Late Pleistocene (last interglacial MIS5e). M. MZB 50543a, head valve, width 3.3 mm, dorsal view. N. MZB 50543b, intermediate valve, width 5 mm, dorsal view. Scale bars: 100 µm (B, G, K).

FIGURE 12. Lucilina confusa n. sp., Egypt, Hurghada, present-day (13/30). A-C. Head valve, MZB 50544a, width 3.7 mm, dorsal view, detail of tegmentum surface and lateral view. D-F. Intermediate valve, MZB 50544b, width 4.5 mm, dorsal view, detail of tegmentum surface and lateral view. G-H. Tail valve, MZB 50544c, width 3.6 mm, dorsal view and detail of tegmentum surface. I. Tail valve, MZB 50544d, width 3.3 mm, lateral view. Scale bars: 100 µm (B, E, H).