Rissoa acuticosta (Sacco, 1895)

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 : 116-117

Rissoa acuticosta (Sacco, 1895)

Fig. 4C .

Rissoa Lachesis Basterot ; Hörnes 1856: 572, pl. 48: 16.

Turbella [sic!] acuticosta sp. nov.; Sacco 1895: 23.

Turboella acuticosta Sacco ; Meznerics 1933: 329, pl. 13: 7a, b.

Rissoa turricula acuticosta (Sacco) ; Strausz 1966: 69–70, pl. 46: 6, 7.

Rissoa acuticosta Sacco ; Bohn−Havas 1973: 1039, pl. 3: 7.

Turboella (Turboella) acuticosta (Sacco) ; Bałuk 1975: 69, pl. 8: 9–11.

Turboella (Turboella) acuticosta (Sacco) ; Svagrovský 1981: 121, pl. 37: 8, 9.

Turboella acuticosta (Sacco) ; Iljina 1993: 41, pl. 3: 27–30.

Material.— 145 specimens from the Badenian of Steinebrunn , Lower Austria, NHMW 2002 z0030/0003.

Description.—The small egg−shaped shell with acute apex comprises 5–6 slightly rounded low whorls. It measures slightly more than 3 mm in height. Teleoconch sculpture consists of 14–16 prominent, straight axial ribs and 10–15 subordinated, flat spiral threads, which are most strongly developed on the abapical half of the whorls. The last whorl forms 60–70% of total shell height. The base is ornamented by three spiral threads. The aperture is drop−shaped. It is holostomatous and only slightly thickened. The parietal portion is slightly pointed.

The protoconch (n = 9) comprises 2.5–2.6 slightly rounded whorls measuring 0.38–0.4 mm in height and in maximum diameter. The first whorl measures 0.03–0.04 mm in the width of the initial cap and 0.13–0.14 mm in maximum diameter. The transition from the embryonic shell to the larval shell is indistinct, indicated by a slight thickening of the shell. In the course of the second whorl of the protoconch, two granulated spiral threads are visible just above the abapical suture. The protoconch is terminated by sinuous, closely spaced, thickened growth lines. The beginning of the teleoconch is indicated by the formation of fine, regular growth lines and by the onset of the adult sculpture.

Remarks.— R. acuticosta has frequently been regarded as a species of Turboella , which represents a synonym of Rissoa (see remarks to the genus Rissoa ). R. acuticosta from the Badenian of Várpalota (West Hungary) has been assigned to the genus Rissoa as a subspecies of R. turricula by Strausz (1966). The protoconch of R. acuticosta differs by its larger size, comprising about a quarter of a larval whorl more and measuring up to 0.4 mm in height and in maximum diameter. The transition to the teleoconch is indicated by closely spaced, thickened growth lines (versus straight, thickened rim in R. acuticosta ). The protoconch of R. clotho is of about the same size and slightly wider than high, whereas the protoconch of R. turricula is higher than wide.

The protoconch indicates an indirect development with inclusion of a free planktotrophic larval stage. The characteristic larval sculpture is similar to that of Recent rissoids from the North Atlantic and from the Mediterranean Sea. Warén (1996) described the protoconch−morphology of R. parva from the Swedish west coast. Ponder (1985) described a specimen from Plymouth, England, with well−preserved protoconch. This modern species ( Fig. 4B ) differs from the Badenian congener R. acuticosta with very similar teleoconch morphology by its larger protoconch comprising about 0.4 larval whorls more. The acute conical protoconch of Pusillina dolium from the Mediterranean Sea is characterised by a similar sculptural pattern (see Kowalke 1998: pl. 9: 7). P. dolium differs by its more slender and considerably smaller protoconch, which measures only 0.26 mm in height.

R. acuticosta occurred in the Badenian of Poland, Hungary, Romania, the Czech Republic, and Austria but is also known from the Miocene of SW France ( Cossmann and Peyrot 1919). During the Late Badenian it invaded also the Eastern Paratethys, where it was described from the Konkian by Ilyina (1993).

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.