Mohrensternia pfaffstaettensis, 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 : 130

Mohrensternia pfaffstaettensis sp. nov.

Fig. 11C .

Derivation of the name: Named after the type locality. Type horizon and locality: Early Sarmatian silty marls of Pfaffstätten , Vienna Basin/ Lower Austria.

Material.— The holotype ( NHMW 2002 z0029/0014) and three paratypes ( NHMW 2002 z0029/0015) from Pfaffstätten, Vienna Basin / Lower Austria are present .

Description.—The small, slender shell comprises six regularly increasing, rounded whorls separated by deep sutures. The holotype measures 2.43 mm in height and 1.06 mm in width. Teleoconch whorls are sculptured by 14 sickle−shaped axial ribs which diminish in their abapical portion on the last whorl. Additional sculpture consists of about ten more or less distinct, granulated spiral threads, which appear to be strongest in the mid portion of the whorls. The last whorl forms about 50% of total shell height. A slit−like umbilicus is indicated. The inclined, egg−shaped aperture is characterised by a broad anterior notch and by a slightly pointed posterior portion.

The protoconch comprises 2.5 rounded whorls measuring 0.37 mm in height and 0.36 mm in maximum diameter. The embryonic shell has about one whorl. It measures 0.04 mm in the width of the initial cap and 0.13–0.14 mm in maximum diameter. Three very finely granulated spiral threads sculpture the embryonic shell. It is terminated by a very fine but distinct, slightly sinuous rim on the shell. The subsequent larval shell is smooth except for growth lines. It is terminated by a slightly sinuous rim. The onset of the teleoconch is indicated by the formation of the adult sculpture of strong axial ribs and subordinated spiral threads.

Remarks and differences.—The protoconch points to an indirect development including a free planktotrophic larval stage. Mohrensternia pfaffstaettensis sp. nov. differs from the similar M.banatica by its smaller shell with higher body whorl and by a sculpture consisting of more and sickle−shaped ribs. The protoconch of M. pfaffstaettensis has about the same size, but comprises 0.25 larval whorls more, and it lacks the prominent incision characterising the onset of the larval shell in M. banatica . The rim terminating the embryonic shell is less thickened than in M. banatica . M. pfaffstaettensis differs from M. pseudangulata politioanei by its much weaker ribs and less angular shell outline, by the higher body whorl, by its protoconch comprising about 0.25 whorls more, and by its larger embryonic shell but slightly smaller initial cap. Mohrensternia pfaffstaettensis is distinguished from M. waldhofensis sp. nov. by its smaller, less angular shell, and weaker sculpture of sickle−shaped ribs; its protoconch, however, is larger, slightly higher than wide (versus considerably wider than high in Mohrensternia waldhofensis ).

Figures

Fig. 11. Four species of Mohrensternia Stoliczka, 1868. A. Mohrensternia pseudoangulata politioanei Jekelius, 1944. Specimen (NHMW 2002z0029/ 0009) from the Early Sarmatian of Waldhof, Styria/Austria (A1). Protoconch of the same specimen in lateral view (A2). The onset of the teleoconch is indicated by the formation of weak axial folds, grading into axial ribs. B. Mohrensternia soceni Jekelius, 1944. Specimen (NHMW 2002z0029/0012) from the Early Sarmatian of Hollabrunn (B1). Protoconch of the same specimen as in B1 in lateral view (B2). The onset of the teleoconch is indicated by the formation of axial folds. Detailed apical view of the initial whorl of the same protoconch as in B2 (B3). The embryonic shell is demarcated from the subsequent larval shell by a slightly thickened rim followed by an incision of the shell. C. Mohrensternia pfaffstaettensis sp. nov. Holotype (NHMW 2002z0029/0014) from the Early Sarmatian of Pfaffstätten, Vienna Basin/Lower Austria (C1). Lateral and apical view of the protoconch of the same specimen as in C1 (C2, C3). The protoconch is terminated by a slightly sinuous rim. The onset of the teleoconch is indicated by the formation of strong axial ribs and subordinated spiral threads. Detailed view of the initial whorl of the same protoconch as in C3 (C4). The embryonic shell has a finely granulated spiral sculpture. It is terminated by a slightly sinuous rim. D. Mohrensternia waldhofensis sp. nov. Holotype (NHMW 2002z0029/0004) from the Early Sarmatian of Waldhof, Styria/Austria (D1). Lateral and apical view of the protoconch of the same specimen (D2, D3). The transition to the teleoconch is indicated by a thickened rim which is not projecting.