Serrata stylaster Boyer 2008, comb. nov.

Serrata stylaster (Boyer, 2001) comb. nov.

Fig. 40

Dentimargo stylaster Boyer, 2001: 159-160.

TYPE MATERIAL. — Holotype (dd) MNHN 20748 and 2 paratypes (dd) MNHN 20749-20750.

TYPE LOCALITY. — Banc Stylaster, guyots nord de la Ride de Norfolk, 23°38’S, 167°43’E, 430-452 m [SMIB 8: stn DW 167].

MATERIAL EXAMINED. — Norfolk Ridge. SMIB 8: stn DW 167, 23°38’S, 167°43’E, 430-452 m, 2 dd (holotype, Fig. 40, 1 paratype MNHN 20749); stn DW 169, 23°37’S, 167°42’E, 447-450 m, 1 dd (paratype MNHN 20750).

DISTRIBUTION. — Stylaster Bank, northern Norfolk Ridge, shells in 447-450 m.

DESCRIPTION. — Shell ovate biconical, solid, opaque. Protoconch paucispiral, wide, bulging, smooth. Spire tall, subconical, inflated, apex blunt, whorls weakly convex to flat. Aperture widening moderately over lower part, very oblique, narrowed anteriorly. Base tapering. Outer lip thin over lower part, thicker and reflected over upper part, slightly oblique, slightly sinuous. Shoulder slightly sloping, rounded, lower part of outer edge oblique, nearly straight, weakly convex in the centre, outer margin moderately thickened, rounded, not stepped, inner edge not reflected, weakly concave, bearing 10 faint unequal denticles, 2 strongest ones are the first one, situated deep within the aperture along the siphonal canal, and the penultimate one, on the posterior part of the inner labrum. Four thin columellar plaits, anterior one very thin, fragile, very oblique, second and third ones thin, oblique, fourth one shorter, less oblique.

Colour porcellaneous white, spire pale golden, sutural zone chalk white.

Dimensions: 4.70 x 2.35 mm.

Radula unknown.

REMARKS. — Serrata stylaster was described in the genus Dentimargo, referring to its biconical outline and the shape of its denticulate outer lip. Boyer (2001) noted, however, that the species has a distinctive subovate outline by comparison with Recent species referred to Dentimargo, and he also recorded its affinity with Longinella tollere Laseron, 1957 from Queensland. The latter was said by Coovert (1999: 35) to be a radulate species with “ Volvarina - type ” comb-like plates, whereas the genus Dentimargo was said to lack a radula (Coovert & Coovert 1995), despite very few species having been checked from this perspective.

This situation led Boyer (2001) to a discussion about the possible occurrence of several lineages of radulate and nonradulate species descending from an early stock of radulate Volvarina-Dentimargo. Boyer (2001) suggested that until the issue could be clarified, his new species should be provisionally allocated to the genus Dentimargo, as D. stylaster.

The more recent discovery of a Volvarina -like radula (comb-like pattern with few cusps) in a typical Dentimargo species off Yucatan (Boyer 2005) proved that the genus Dentimargo cannot be characterized by the lack of a radula, and the thesis of a common Volvarina-Dentimargo clade was reinforced. However, this leads to the need to examine more closely the possible discrimination among Dentimargo, Volvarina and Serrata on the basis of shell features.

The range of morphological variation in Serrata, as recorded in this study, demonstrates that D. stylaster belongs in this genus. Very disparate shell morphologies are currently accepted in the genus Dentimargo, which contains biconical plain or weakly decorated shells with a smooth or denticulate outer lip. However, when present, the denticles in Dentimargo are more prominent on the upper part of the lip, and in most species the largest one, very sharp and produced, is uppermost, contrary to the situation in Serrata where the denticles are relatively subequal in denticulate species. In Dentimargo species with a smooth labrum, the central part of the inner lip is never distinctly reflected and the anterior part is not excavated. However, some non-denticulate or regularly denticulate species currently attributed to Dentimargo may not be distinguishable from Volvarina or Serrata on the simple basis of shell morphology.

Dentimargo stylaster is transferred to the genus Serrata on the basis of its sinuous labrum, which is much narrowed anteriorly with a sharp edge, reflected over its upper half, and distinctly excavated over its lower half, with regularly distributed, indistinct denticles, among which the most prominent penultimate upper one is not much more pronounced than its neighbours. This labrum pattern matches very well the most common pattern found in Serrata and is not found in Dentimargo.

Serrata stylaster seems to be a rare endemic species from Stylaster Bank, on the northern Norfolk Ridge, with no close relatives known to me in New Caledonian waters.