Mohrensternia banatica ( Jekelius, 1944 )

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 : 126

Mohrensternia banatica ( Jekelius, 1944)

Fig. 9C .

Mohrensternia pseudoangulata banatica var. nov.; Jekelius 1944: 72, pl. 15: 16–18.

Mohrensternia banatica (Jekelius) ; Papp 1954: 36–37, pl. 5: 30–33.

Mohrensternia banatica (Jekelius) ; Svagrovský 1971: 301–303, pl. 49: 1–12.

Material.— Four specimens from the Early Sarmatian of Eichkogel / Mödling, Austria (Vienna Basin ), NHMW 2002 z 0029/0002.

Description.—The small, high−spired shell comprising up to seven regularly increasing whorls measures up to 4 mm in height and up to 1.5 mm in width. The rounded whorls are separated by moderately deep sutures. Whorls are sculptured by 14 delicate, closely spaced, slightly wavy to sickle−shaped axial ribs, which regularly curve in the course of the whorls’ flanks. Weaker spiral sculpture is more or less developed. The last whorl forms about 40% of total shell height. The holostomatous aperture is rounded to oval. It is characterised by a thin peristome.

The low conical protoconch comprises 2.25 rounded whorls measuring 0.36–0.37 mm both in height and in maximum diameter. The apparently smooth embryonic shell comprises one whorl, measuring 0.14–0.15 mm in maximum diameter and 0.03 mm in the width of the initial cap. It is well separated from the larval shell by a marked incision and a subsequent thickened rim. The larval shell is smooth and is terminated by a slightly thickened rim.

Remarks.—The protoconch indicates an indirect development with inclusion of a free planktonic larval stage. Size and dimensions of the protoconch closely resemble those of Mohrensternia angulata . Mohrensternia banatica differs, however, by a distinct demarcation of the embryonic shell from the subsequent larval shell with an incision and a thickened rim, indicating the thickened apertural margin of the hatchling. The transition from the larval shell to the teleoconch is sharp in the case of M. banatica , indicated by a rim of the aperture of the pediveliger ready for metamorphosis and the abrupt onset of the adult sculpture, whereas in M. angulata this transition is indistinct.

According to Papp (1954) this species was also found in the Early Sarmatian of the drilling St. Marx in the Vienna Basin. Svagrovský (1971) mentioned its occurrence in the Sarmatian of Slovakia, but remarked its absence from the Pannonian Basin, the Carpathian Foredeep, and from the Sarmatian of South−East Europe, whereas Iljina (1998) mentioned it from the Early Sarmatian of Hungary.

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).