Mohrensternia hydrobioides Hilber, 1897

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 : 126-127

Mohrensternia hydrobioides Hilber, 1897

Fig. 9E, F .

Mohrensternia hydrobioides sp. nov.; Hilber 1897: 199, pl. 1: 12–14.

Mohrensterniahydroboides [sic!] Hilber; Papp 1954: 35; pl. 5: 22, 23.

Rissoa (Mohrensternia) inflata hydrobioides Hilber ; Boda 1959: 706, pl. 23: 15–23.

Mohrensternia hydrobioides Hilber ; Svagrovský 1971: 289–292, pl. 45: 1–8.

Material.— Two specimens (original material in Papp 1954: pl. 5: 22, 23) and four specimens from the Early Sarmatian of Waldhof , Styria / Austria, NHMW 2002 z0029/0004.

Description.—The shell comprises six regularly increasing, slightly rounded whorls measuring up to 5.5 mm in height and up to 3.2 mm in width. Whorls are separated by distinct, moderately deep sutures. Sculpture consists of 14–16 straight axial ribs about as broad as the gaps between them. The ribs are most strongly developed on the initial two whorls of the teleoconch, and they decrease on later whorls. The last two whorls are smooth in some individuals ( Svagrovský 1971). The last whorl forms 50–60% of total shell height. The small holostomatous aperture is rounded to oval. The peristome is very thin.

The conical protoconch comprises two rounded whorls. It measures 0.36 mm in height and in maximum diameter. 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. Transitions of the embryonic to the larval shell and sculpture are not evident. The transition to the adult shell is indistinct. The onset of the teleoconch is indicated by the formation of axial folds that grade into ribs.

Remarks.—The protoconch is indicative of an indirect development including a free planktonic veliger stage. According to Papp (1954), Mohrensternia hydrobioides closely resembles M. inflata , but differs by its smaller, more slender shell. The “transitional” morphs he also mentioned could not be confirmed by our investigations. Mohrensternia hydrobioides is well distinguished from M. inflata by its smaller protoconch, comprising about 0.5 whorls less, and by its considerably broader teleoconch. Svagrovský (1971) mentioned the variability of M. hydrobioides , with the occurrence of phenotypes having a slender shell outline and of those with broad conical shape and a higher body whorl. These are most probably represented by Mohrensternia hollabrunnensis sp. nov., which is distinguished by its flattened protoconch with bulbous embryonic whorl and by its broad, conical teleoconch with a sculpture of more and weaker axial ribs.

Papp (1954) mentioned Mohrensternia hydrobioides also from the Early Sarmatian of Siebenhirten in the northern Vienna Basin. According to Svagrovský this species occurred in several South−East European localities (see also Kolesnikov 1935; Iljina 1998).

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).