Mohrensternia hollabrunnensis, Kowalke & Harzhauser, 2004

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 hollabrunnensis sp. nov.

Fig. 9D .

Derivation of the name: Named after the type locality.

Type horizon and locality: Early Sarmatian silt at Hollabrunn ( Molasse Basin / Lower Austria) .

Material.— Only the holotype ( NHMW 2002 z0029/0003) is known .

Description.—The broad−conical shell comprises six slightly rounded whorls separated by deeply incised sutures. The holotype measures 4 mm in height and 2.16 mm in width. Sculpture consists of 20–22 straight axial ribs about as broad as the gaps between them. The last whorl forms 70% of total shell height. The large oval aperture is slightly inclined to the main axis of the shell.

The low−conical protoconch comprises 2.2 slightly rounded whorls measuring 0.21–0.22 mm in height and 0.34 mm in maximum diameter. The first whorl measures 0.17 mm in maximum diameter and 0.04–0.05 mm in the width of the initial cap. Transitions from the embryonic to the larval shell and sculpture are not evident. The transition to the teleoconch is indistinct. The onset of the adult shell is indicated by the formation of weak axial folds that grade into regular axial ribs.

Remarks and differences.—The protoconch indicates a free larval stage during early ontogeny. The large initial whorl may indicate a yolk−rich embryogenesis, probably with a subsequent lecithotrophic larval stage.

Mohrensternia hollabrunnensis has a similar sculpture as M. hydrobioides , but differs by a protoconch that is wider than high, by a larger embryonic shell, and by a broader teleoconch; the latter is sculptured by more but thinner axial ribs that do not decrease in the course of the last teleoconch whorls. Mohrensternia hollabrunnensis is distinguished from M. inflata , which has a similar teleoconch shape, by its smaller larval shell but larger embryonic shell and by its teleoconch sculpture of more but thinner axial ribs.

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).