Levisulculus, Jaanusson, 1957

Schallreuter, Roger E. L. & Hinz-Schallreuter, Ingelore C. U., 2010, Sexual dimorphism and pore systems in Ordovician ostracodes, Acta Palaeontologica Polonica 55 (4), pp. 741-760 : 743-744

Levisulculus , Ampletochilina , and Swantina and the construction of their antra

General features.—All morphological features are called sculptures. Structures refer to special constructions of the material of the shell (differently constructed layers, for example prismatic layer, organic rich layer etc.).

Size and shape.—For definition of size see Table 1.

The shape (Gestalt) is defined by the length:height (L:H) ratio ( Table 2). According to the definitions given above, adult Levisulculus , Ampletochilina , and Swantina are smallto medium−sized. Their shape varies between rather high and rather long, and the domicilium is rather long to very long. The carapaces are essentially equivalved but the left valve overlaps the right valve at the contact margin.

The outline is amplete or preplete. The cardinal corners are approximately rectangular to distinctly blunt. Often, the cardinal corners of the left valve carry acroidal spines ( Figs. 1A , 3C, D 1, H, 4F 2, 5E, M 1, 6A, F), which are relatively longer in larvae (Fig. 6K, L).

Lobal and sulcal sculptures.—Lobal and sulcal features are very weakly developed in these genera. Most Baltic species are nonsulcate or nearly nonsulcate, having only a faint sulcal depression as in the type species of Levisulculus ( L. lineatus ). Often a rounded muscle spot is present (Figs. 5E, 6C), rarely exhibiting muscle scars (Fig. 6E). Anterodorsally of the muscle spot an indistinct small preadductorial node may be present, which is most recognisable from the inner surface view of the respective valves (e.g., Fig. 4A 1).

doi:10.4202/app.2009.0056

Stronger lobes and sulci are developed in some non−Baltic species referred formerly to Levisulculus . In L. michiganensis the S2 is slightly sigmoidal, consisting of a shallow sulcus that extends down from the dorsal border immediately above the small, distinct, vertically elongate preadductorial node, then curves around L2, and terminates in a distinct pit at midheight on the domicilium ( Kesling 1960: 353; pl. 1: 2). The anteroventral portion of the postsulcal area is the highest part of the domicilium, thus indicating a weak indistinct posteroventral lobe, which is very distinct in L. posteroventrolobatus . In that species the S2 consists of a sulcal depression with a muscle spot in its centre posteroventrally to a small but distinct preadductorial node (Schallreuter et al. 2001: pl. 2: 1). In contrast, the Bohemian species L. smolai is characterised by a long and deep sigmoidal sulcus ( Schallreuter and Krůta 1991: figs. 1–2, 2001: text−fig. 1.1–2).

Velar sculptures.—In the genera discussed the velum is a restricted frill, which is developed mainly in the ventral half of the valve at some distance from the free margin ( Fig. 1A ). The frill terminates posteroventrally in a more or less strong spine (terminal velar spine, Fig. 1A ). The characteristic tubules of the frill are mostly recognisable by the radial striation of the latter. They are hollow but this is extremely difficult to observe (Fig. 6I).

The velum displays dimorphic features. It is broader in females and forms an antrum, which also incorporates the marginal surface between velum and marginal sculpture (dolonal−admarginal antrum; see Schallreuter and Hinz−Schallreuter 2007: fig. 2). The dolon is generally only weakly convex (Fig. 5C, D) but may be strongly convex in certain parts. In Levisulculus it is strongly convex in the anterior part and weakly convex ventrally (Fig. 4A). In Swantina crassiconvexa it is strongly convex ventrally and weakly convex anteriorly (Fig. 4C). In strongly convex dolones the wall of the dolon itself forms the outer antral fence of the antrum, but in weakly convex dolones or parts of the antrum it is bordered by a special antral fence close to the peripheral border of the dolon ( Figs. 1B , 5C).

The antrum may be open at either end (Fig. 5C) or closed anteriorly, resulting in a strongly convex dolonal portion (Fig. 4A). Alternatively, it may be closed posteriorly (Fig. 4C), often by a terminal velar spine ( Fig. 1B , Table 3).

Surface sculptures.—In the type species of Levisulculus and in L. dactylus the lateral surface is covered with irregular, more or less vertically directed faint ridges. The adventral sculptures, especially the dolon, may show ornamentation different from that of the lateral surface (Fig. 3A; Schallreuter et al. 2001: pl. 2: 7).

Very faint ridges occur at the adventral sculptures, the outer part of the dolon ( Fig. 7B, C), and the marginal flange ( Fig. 7A 3). Special vertical pillars are developed at the outermost part of the marginal flange between the borders of the flange and inner antral fence ( Fig. 7A 2 ).

The tubules of the velar flange may appear as radial ridges on the outer surface. In some cases they form into separate spines (Fig. 4E). The spines are declined posteriorly, and the ridges may show an abrupt bend in the posterior direction (Figs. 5E, 7D).

In a few cases the lateral surface appears completely smooth (Fig. 5A 1), except for the densely set normal pores (Fig. 5B, D 1). However, in most cases the pores are developed as pustules resulting in a more or less tuberculate outer surface (Figs. 5E, H, J, K 1, 6).

Figures

Fig. 1. Morphological terminology explained in Swantina pseudobliqua.

Fig. 2. Oepikellid ostracods from Palaeozoic erratic boulders. A. Holotype of the type species of the oepikellid ostracod Levisulculus, Levisulculus lineatus Jaanusson, 1957 (UM T89), female left valve, length (L) 0.89 mm. B. Holotype of Primitia extraria Öpik, 1937 (TUG 1120−1; Kukruse Stage, Estonia), tecnomorphic right valve, L 0.88 mm. C. Holotype of Primitia troedssoni Thorslund, 1940 (UM T10), tecnomorphic right valve, L 0.79 mm. D. Holotype of Primitia granulosa Thorslund, 1940 (UM T11), tecnomorphic right valve, L 0.86 mm (Jaanusson 1957: pl. 8: 12, Öpik 1937: pl. 10: 19, Thorslund 1940: pl. 1: 16, 13). E–H. Primitia elongata obliqua Steusloff, 1895: type series, all tecnomorphic valves embedded in rock. Geschiebe (glacial erratic boulder) from Neubrandenburg. E. Lectotype GG 114−27, left valve, L 1.16 mm (without velum). F. GG 114−26, right valve, L 1.07 mm. G. GG 114−28, right valve, L 0.99 mm (without velum). H. GG 114−29, right valve, L 0.82 mm. I. Primitia canaliculata Steusloff, 1895, holotype GG 114−25, steinkern of a juvenile right valve embedded in rock, L 0.70 mm, same erratic boulder.