Rissoa clotho Hörnes, 1856

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 : 117

Rissoa clotho Hörnes, 1856

Fig. 4D .

Rissoa clotho sp. nov.; Hörnes 1856: 574–575, pl. 48: 20 a, b. Alaba (Gibborissoa) clotho Hörnes ; Meznerics 1933: 328 Turboella (Turboella) clotho (Hörnes) ; Bałuk 1975: 70, pl. 8: 8.

Material.— Nine specimens from the Badenian of Steinebrunn , Lower Austria, NHMW 2002 z0030/0004.

Description.—The small, elongated egg−shaped shell comprises six regularly increasing, moderately rounded whorls separated by deep sutures. It measures slightly more than 3 mm in height. Sculpture of the teleoconch whorls consists of 12 regularly rounded straight ribs and 10 subordinated flat spiral threads, which are most strongly developed on the abapical half of the whorls. The last whorl forms about 50–60% of total shell height. The drop−shaped, holostomatous aperture is slightly thickened and is characterised by a pointed parietal portion. The base is sculptured by 4–5 flat spiral threads about equally as strong as the threads sculpturing the whorls.

The protoconch comprises 2.25 slightly rounded whorls measuring 0.3 mm in height and 0.33 mm in maximum diameter. The first whorl measures 0.03–0.04 mm in the width of the initial cap and 0.14–0.15 mm in maximum diameter. The transition from the embryonic to the larval shell is not preserved. In the course of the second whorl, larval sculpture consisting of two granulated fine spiral threads just above the abapical suture is evident; these threads decrease in the course of the last quarter whorl of the protoconch. The protoconch is terminated by an indistinct, very fine thickening on the shell. The onset of the teleoconch is indicated by the formation of regular, closely spaced growth lines and by the formation of the regular adult sculpture.

Remarks.— R. clotho is similar to the syntopic R. acuticosta , but differs by its more slender shell, with a sculpture of fewer but well−rounded ribs causing a more angulate shell outline. The spiral sculpture is more strongly developed and the aperture is less thickened in R. clotho . The protoconch of R. clotho differs by comprising 0.25 whorls less. It is slightly smaller in height and in maximum diameter, whereas the first whorl is slightly larger. The transition to the teleoconch is indistinct, not indicated by the formation of thickened growth lines as in R. acuticosta .

Rissoa clotho is known from Poland, Austria, the Czech Republic, and Romania (Boettger 1901; Bałuk 1975).

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.