Rissoa turricula Eichwald, 1853

Kowalke, Thorsten & Harzhauser, Mathias, 2004, Early ontogeny and palaeoecology of the Mid-Miocene rissoid gastropods of the Central Paratethys, Acta Palaeontologica Polonica 49 (1), pp. 111-134 : 115-116

Rissoa turricula Eichwald, 1853

Fig. 4A .

Rissoa turricula sp. nov. Eichwald 1853: 267–268 (pro parte, non text ad 9*), pl. 10: 9 (non 9*).

Rissoa soceni sp. nov. Jekelius 1944: 68, pl. 14: 9–10.

Rissoa soceni Jekelius ; Papp 1954: 32, pl. 5: 36–37.

Rissoa turricula Eichwald ; Strausz 1966: 69, pl. 45: 20, pl. 46: 1–5.

Rissoasoceni Jekelius; Svagrovský 1971: 258, pl. 37: 1–9, pl. 38: 1–4.

Rissoa turricula Eichwald ; Bohn−Havas 1973: 1039, pl. 3: 3–4.

Material.— Five specimens from the Early Sarmatian of Waldhof / Styria, Austria, from the collection of A. Papp, NHMW 2002 z0030/0001.

Description.—The low conical to slender oval shell usually measures up to 6 mm in height—up to 8 mm according to Strausz (1966). The width amounts to slightly more than half of the height. The shell comprises up to six only slightly rounded whorls separated by moderately incised sutures. Sculpture of the teleoconch whorls consists of 10–12 prominent, slightly rounded axial ribs. The last whorl forms 60–70% of total shell height. The large, inclined, rounded triangular aperture is characterised by a broad, slight basal notch and by a pointed parietal edge. The thick outer lip is rounded. The thin columellar and parietal lip is bent.

The protoconch comprises 2.25 rounded whorls measuring 0.3 mm in height and 0.35 mm in maximum diameter. Sculpture is not preserved. The first whorl measures 0.03–0.04 mm in the width of the initial cap and 0.15 mm in maximum diameter. The protoconch is terminated by a thickened rim on the shell.

Remarks.—Morphology and dimensions of the protoconch indicate an indirect development with inclusion of a planktotrophic larval stage.

Slightly more slender morphs from the Early Sarmatian have been described as R. soceni by Jekelius (1944). A comparison with the material from Styria reveals no conchological differences warranting a separation from the generally variable R. turricula . Svagrovský (1971), who presented a very strict species concept for the Sarmatian representatives of Rissoa , treated the morphs from Waldhof/Styria as a separate species, based on the less slender outline. Other distinguishing features mentioned by Svagrovský (1971), such as the shallow sutures and the less convex whorls, display considerable variability and range well within the variability of Rissoa turricula from Romania and Slovakia. Rather than separating the Styrian shells from Rissoa turricula , we consider them to merely be a stout morph. Following the concept of Svagrovský (1971), the Styrian specimens with regard to shell shape might be considered as closely related to Rissoa certa Svagrovský, 1971 and Rissoa rugosa Svagrovský, 1971 , which could be distinguished by their additional spiral teleoconch sculpture.

The species has commonly been described from Badenian sediments of the Central Paratethys in the Polish foredeep, Hungary, and Austria ( Meznerics 1933; Bohn−Havas 1973). It is also documented from the Early Sarmatian of Austria, Romania, and Slovakia ( Papp 1954; Svagrovský 1971). At least in Austria, Rissoa turricula settled the same environments as Mohrensternia spp.

Modern representatives of Rissoa from the Mediterranean Sea and from the North Atlantic are characterised by a protoconch morphology very similar to R. turricula ( Warén 1996; Kowalke 1998). Closest are R. membranacea (J. Adams, 1800) from the North Atlantic coasts as figured by Warén (1996: fig. 9) and Ponder (1985: fig. 77C, D), and R. guerini Récluz, 1843 from the Mediterranean Sea ( Kowalke 1998: pl. 9: 6). They have nearly the same protoconch dimensions, but are distinguished by larger, more bulbous embryonic shells measuring about 0.17–0.18 mm in maximum diameter (versus only 0.15 mm in R. turricula ), reflecting a yolk−rich embryogenesis. The protoconch of the type species differs by its smaller dimensions, including a considerably smaller embryonic shell, and by its slender shape, which is higher than wide ( Thiriot−Quiévreux 1980; Kowalke 1998).

Figures

Fig. 4. Four species of Rissoa Desmarest, 1814. A. Rissoa turricula Eichwald, 1853. Specimen (collection A. Papp, NHMW 2002z0030/0001) from the Early Sarmatian of Waldhof/Styria (A1). Protoconch of the same specimen in lateral view (A2). The protoconch is terminated by a thickened rim, reflecting the thickened apertural margin of the veliger ready for metamorphosis. B. Rissoa parva (Da Costa, 1778). Specimen (NHMW 2002z0030/0002), Recent, from the Northern Adriatic Sea (B1). Protoconch of the same specimen in lateral view (B2). The protoconch is terminated by a prominent slightly sinuous, thickened rim. C. Rissoa acuticosta (Sacco, 1895). Specimen (NHMW 2002z0030/0003) from the Badenian of Steinebrunn, Lower Austria (C1). Apical and lateral view of the protoconch of the same specimen (C2, C3). The protoconch is terminated by sinuous, closely spaced, thickened growth lines. D. Rissoa clotho Hörnes, 1856. Specimen (NHMW 2002z0030/0004) from the Badenian of Steinebrunn, Lower Austria (D1). Protoconch of the same specimen in lateral view (D2). The protoconch is terminated by an indistinct, very fine thickened rim on the shell. Regular closely spaced growth lines indicate the onset of the teleoconch.