Mohrensternia angulata ( 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 : 125

Mohrensternia angulata ( Eichwald, 1853)

Fig. 9A, B .

Rissoa angulata sp. nov.; Eichwald 1853: 268–269, pl. 10: 10.

Rissoaangulata Eichwald; Hörnes 1856: 577–578 (partim), pl. 48: 23 a.

Mohrensterniaangulata (Eichwald); Friedberg 1923: 392, pl. 23: 15, 16.

Mohrensternia angulata (Eichwald) ; Jekelius 1944: 71, pl. 15: 7, 8.

Mohrensternia angulata (Eichwald) ; Papp 1954: 35–36, pl. 5: 27.

Rissoa (Mohrensternia) angulata angulata Eichwald ; Boda 1959: 707, pl. 24: 12–16.

Mohrensternia angulata (Eichwald) ; Svagrovský 1971: 294–296, pl. 46: 1–9.

Material.— Thirteen specimens from the Early Sarmatian of

Lednicé (formerly Eisgrub) E of Mikulov, Czech Republic,

NHMW 2002z0029/0001.

Description.—The slender shell comprises up to 7 rounded whorls measuring up to 6 mm in height and up to 2.5 mm in width. Whorls slowly increase, separated by incised sutures. Sculpture consists of 10 broad orthocline to slightly opisthocyrt axial ribs whose intensity may decrease in the course of the last whorl. Slender morphs with regularly curving ribs exist alongside those with ribs increasing in thickness in the mid to abapical portion of the whorls. The last whorl forms 50–60% of total shell height. The holostomatous aperture is broad oval to rounded and is characterised by a very thin peristome.

The protoconch has 2.25–2.3 rounded whorls, which appear to be smooth. It measures 0.35–0.36 mm both in maximum diameter and in height. The first whorl measures 0.14–0.15 mm in maximum diameter and 0.03–0.04 mm in the width of the initial cap−like onset. The transition to the teleoconch is indistinct. The onset of the teleoconch is indicated by the formation of wavy axial folds which grade into axial ribs.

Remarks.—The dimension of the protoconch indicates an indirect development with inclusion of a free veliger stage. The veliger spent a brief period of probably one or two weeks within the plankton, feeding on phytoplankton.

Mohrensternia angulata is a typical and widespread species during the Early Sarmatian of the Vienna Basin and the Styrian Basin and is also known from Sarmatian deposits of the Mid Danube Basin ( Jekelius 1944), the Carpathian Foredeep ( Friedberg 1923), Slovakia ( Svagrovský 1971), and South−East Europe, e.g., Bulgaria and Romania ( Kolesnikov 1935; Iljina 1998). Svagrovský (1971) noted a certain variability of the shell, with more or less slender outline and variable strength of the axial ribs; this was confirmed by the investigation of our material. Its roots reach back to the Badenian, when it is a rare element throughout the Central Paratethys, documented from Hungary, Poland, and Romania ( Csepreghy−Meznerics 1950; Kokay 1966; Friedberg 1923).

Figures

Fig. 9. Four species of Mohrensternia Stoliczka, 1868. A, B. Mohrensternia angulata Eichwald, 1853. Specimen (NHMW 2002z0029/0001) from the Early Sarmatian of Lednicé, Czech Republic (A1). Protoconch of the same specimen as in A1 in apical view (A2). The onset of the teleoconch is indicated by the formation of wavy axial folds which grade into axial ribs. B. Apex fragment of the specimen (NHMW 2002z0029/0001) from the Early Sarmatian of Lednicé, Czech Republic. C. Mohrensternia banatica (Jekelius, 1944). Specimen (NHMW 2002z0029/0002) from the Early Sarmatian of Eichkogel/ Mödling, Vienna Basin (Austria) (C1). Apical and lateral view of the protoconch of the same specimen as in C1 (C2, C3). The protoconch is terminated by a slightly thickened rim. Detailed view of the embryonic shell of the same protoconch as in C3 (C4). The embryonic shell is well separated from the larval shell by a marked incision and a subsequent thickened rim. D. Mohrensternia hollabrunnensis sp. nov. Holotype (NHMW 2002z0029/0003) from the Early Sarmatian of Hollabrunn (Molasse Basin, Lower Austria) (D1). Lateral and apical view of the protoconch of the same specimen (D2, D3). E, F. Mohrensternia hydrobioides Hilber, 1897. Specimen (NHMW 2002z0029/0004) from the Early Sarmatian of Waldhof, Styria/Austria (E1). Apical view of the initial whorl of the same specimen (E2). F. Specimen (collection A. Papp, NHMW 2002z0029/0004) from the Early Sarmatian of Siebenhirten (F1). Protoconch of the same specimen in lateral view (F2).