Early Ontogeny and Palaeoecology of the Mid−Miocene Rissoid Gastropods of the Central Paratethys

Early ontogeny and palaeoecology of the Mid−Miocene rissoid gastropods of the Central Paratethys THORSTEN KOWALKE and MATHIAS HARZHAUSER Kowalke, T. and Harzhauser, M. 2004. Early ontogeny and palaeoecology of the Mid−Miocene rissoid gastropods of the Central Paratethys. Acta Palaeontologica Polonica 49 (1): 111–134. Twenty−six species of Rissoidae (Caenogastropoda: Littorinimorpha: Rissooidea) are described from the Badenian and Early Sarmatian of 14 localities in Austria and the Czech Republic (Molasse Basin, Styrian Basin, Vienna Basin) and from the Badenian of Coştei (Romania). For the first time, the early ontogenetic skeletal characters of these gastropods are de− scribed. Based on these features an indirect larval development with a planktotrophic veliger could be reconstructed for all investigated Mid−Miocene species. The status of Mohrensterniinae as a subfamily of the Rissoidae is confirmed by the mor− phology of the low conical protoconch, consisting of a fine spirally sculptured embryonic shell and a larval shell which is smooth except for growth lines. Transitions from embryonic shells to larval shells and from larval shells to teleoconchs are slightly thickened and indistinct. Whilst representatives of the subfamily Rissoinae characterise the marine Badenian assem− blages, Mohrensterniinae predominate the Early Sarmatian faunas. We hypothesize that this take−over by the Mohren− sterniinae was triggered by changes in the water chemistry towards polyhaline conditions. Consequently, the shift towards hypersaline conditions in the Late Sarmatian is mirrored by the abrupt decline of the subfamily. Four new species Rissoa costeiensis (Rissoinae) from the Badenian and Mohrensternia hollabrunnensis, Mohrensternia pfaffstaettensis,and Mohrensternia waldhofensis (Mohrensterniinae) from the Early Sarmatian are introduced. Key words: Gastropoda, Rissoidae, Littorinimorpha, protoconch−morphology, Miocene, Badenian, Sarmatian, Paratethys. Thorsten Kowalke [[email protected]−muenchen.de], Ludwig−Maximilians−Universität, Department für Geo− und Umwelt− wissenschaften, Sektion Paläontologie, Richard−Wagner−Str. 10, 80333 München, Germany; Mathias Harzhauser [mathias.harzhauser@nhm−wien.ac.at], Naturhistorisches Museum, Geologisch−Paläontologische Abteilung, Burgring 7, 1014 Wien, Austria. Introduction their Recent Mediterranean and NE Atlantic congeners (cf. Thiriot−Quiévreux 1980; Ponder 1985; Kowalke 1998). The superfamily Rissooidea comprises minute littorinimorph The evolution of the brackish−marine rissoid genus gastropods, which settled various shallow to deeper normal Mohrensternia is connected to the Sarmatian biogeographic marine and brackish−water habitats at least since Jurassic history of the Paratethys. A conspicuous maximum diversity times (Gründel and Kowalke 2002). Rissoidae usually prefer of Mohrensternia in the Central Paratethys coincides with the shallow littoral habitats, where they either live on hard sub− maximum transgression of the Early Sarmatian Sea. This strates as micro−algal grazers or on softer substrates as detriti− phase is consequently termed the Mohrensternia Zone accord− vores (see e.g., Bandel and Kowalke 1999). Most rissoid spe− ing to the ecostratigraphic zonation by Papp (1956). During cies are characterised by protoconchs that reflect a larval de− the subsequent Mid−Sarmatian Ervilia Zone, the diversity of velopment. This includes a free−swimming larval stage with Mohrensternia drops distinctly, and in shallow coastal habi− either feeding on the remaining embryonic yolk (lecithotro− tats the genus is apparently replaced by Cerithium−dominated phic development) or on phytoplankton (planktotrophic de− assemblages in carbonate−oversaturated marine waters. velopment) (Thiriot−Quiévreux 1980; Ponder 1985; Warén Coelacanthia Andrussov, 1890 and Archaschenia Zhgenti, 1996; Kowalke 1998; Harzhauser and Kowalke 2001). Both 1981, restricted to the Eastern Paratethys, are distinguished types of larval development facilitated a wide distribution of from Mohrensternia by the formation of hollow spines on species and also enabled populations to massively settle tem− the teleoconch whorls (see e.g., Wenz 1938–1944: 597, fig. porary and extreme biotopes (”r−strategies”). This strategy has 1642). The genera co−occurred with Mohrensternia, e.g., dur− also been documented in other lower caenogastropods (cf. ing the Karaganian and Maeotian of the Eastern Paratethys, Kowalke 2001, 2003; Harzhauser and Kowalke 2002). and could well be related to Mohrensternia, however, the Littoral gastropod assemblages of the Central Paratethys protoconch morphology is unknown. were characterised by modern rissoid genera such as Rissoa The rissoid genera Rissoa, Alvania, and Manzonia typi− and Alvania at least since the Eggenburgian, Early Miocene cally have a similar teleoconch morphology in Badenian as− (Steininger 1963). The protoconch morphologies of Badenian semblages, but could be differentiated from Mohrensternia species with planktotrophic development are very similar to based on protoconch morphology. Acta Palaeontol. Pol. 49 (1): 111–134, 2004 http://app.pan.pl/acta49/app49−111.pdf 112 ACTA PALAEONTOLOGICA POLONICA 49 (1), 2004 This study describes the protoconch morphology of Bade− nian and Early Sarmatian rissoid taxa in detail. The early Mediterran Central Paratethys Eastern ontogenetic shells of the species are described and figured for ages ages Paratethys ages (Ma) the first time. The palaeoecological context of the taxa and Epochs 5.3 marked changes in ecological parameters in Paratethyan as− Messinisan Pontian Pontian semblages are discussed. Maeotian Tortonian Pannonian Material and methods 10 Late Miocene Khersonian The investigated material mainly derives from the Badenian Bessarabian 11.5 “pauperization” and Early Sarmatian of Central Paratethys Basins (Figs. 1, 2). Mactra Zone Sarmatian Ervilia Zone Volhynian The Badenian (early Middle Miocene) faunas were collected Serravallian Mohrensternia Zone Konkian in the Vienna Basin (Steinebrunn, Baden/Sooß, Vöslau, Aus− 13.6 L. tria; Sedlec/Nový Rybník, Lednicé, Czech Republic). Addi− Wielicien Karaganian E. Chokrakian tional material derives from the Badenian of Coştei (Roma− 15 Langhian Badenian nia). The Early Sarmatian (late Middle Miocene) material was Middle Miocene Tarkhanian found at Hollabrunn (Molasse Basin), Pfaffstätten, Sieben− 16.4 Karpatian hirten (Vienna Basin) and Waldhof/Wetzelsdorf close to Graz (Western Styrian Basin). Recent material for comparison de− Ottnangian Kotsakhurian rives from the Adriatic Sea (Mediterranean Sea). Burdigalian Special emphasis was placed on early ontogenetic shells. 20 Eggenburgian Sakaraulian The investigated small to medium−sized gastropods were ex− tracted from marly and silty sediment samples after processing Early Miocene with H O , drying and fractionating with sieves. Shells with 2 2 Aquitanian Egerian Karadzhalgian preserved protoconchs were mounted on stubs, sputtered with gold and documented using scanning electron microscopy 23.8 (digital scan “Leo 1525”). Protoconch dimensions were measured during the SEM in− Fig. 1. Stratigraphic correlation chart of the standard scale with the Central Paratethys and the Eastern Paratethys; modified after Rögl (1998) and vestigations. Maximum height and diameter of the proto− Studencka et al. (1998). The investigated faunas derive exclusively from conchs are given. The dimensions (i.e. the width of the initial the Badenian and Early Sarmatian of the Central Paratethys. Serravallian cap−like onset and the maximum diameter of the embryonic and Tortonian ages after Foresi et al. (2002) and Lirer et al. (2002). shell) and the ornament of the embryonic shell are described along with the morphology of the transition to the larval shell, if preserved. Larval sculptural patterns and the morphology of et al. 2001: 11)—no distinct preference for siliciclastic or cal− the transition to the teleoconch are also described. For termi− careous substrates can be stated. Diverse shallow marine habi− nology of early ontogenetic shells in gastropods see also tats have been settled, like by their modern Mediterranean Kowalke (1998). counterparts. Mohrensterniinae, in contrast, are usually accompanied Additional original material derives from the collection of by few euryhaline taxa, indicating the ”extreme” character of A. Papp at the Natural History Museum, Vienna (NHMW) and their habitats. Nonetheless, the protoconch morphology in all from the Institut Royal des Sciences Naturelles de Belgique, investigated species reflected a planktotrophic larval devel− Brussels (IRSNB IG 23693). All figured specimens are depos− opment. This indicates a connection of the habitats to the ited in the collection of the NHMW (NHMW 2002z0029/0001– open sea. The Sarmatian Sea displayed its maximum exten− 2002z0029/0017; NHMW 2002z0030/0001–2002z0030/0019). sion during the Mohrensternia Zone of the Central Para− tethys. The palaeogeographic position of the investigated sections was along the western margin of the Sarmatian Sea. Palaeoecology The north−western−most section Hollabrunn in Lower Aus− tria represents a flat shore of a small and shallow embay− Badenian assemblages of the Vienna Basin, with large num− ment; it reached about 40 km from the Vienna Basin to the bers of Rissoa and Alvania, also yield abundant pectinids, west into the otherwise already dry Molasse Basin. This situ− echinoderms, calcareous algae and various sharks, indicating ation established a tidelands environment settled by large

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