Decapod crustaceans from the Ottnangian (middle Burdigalian, Lower Miocene) of the Western and Central Paratethys remain poorly known. In this study, we review and re-describe mud shrimps (Jaxea kuemeli), ghost shrimps (Gourretia sp., Calliax michelottii) and brachyuran crabs of the families Leucosiidae, Polybiidae and Portunidae. A dorsal carapace of the genus Calliax is reported for the first time in the fossil record. Re-examination of the type material of Randallia strouhali (Leucosiidae) and Geryon ottnangensis (Geryonidae) resulted in a transfer of these species into Palaeomyra (Leucosiidae) and Liocarcinus (Polybiidae), respectively. Achelous vindobonensis, originally described as a chela of a portunid crab, probably belongs to a member of Polybiidae and is provisionally treated as Liocarcinus sp. Only two species, J. kuemeli and C. michelottii, are also known from the Karpatian, the succeeding Paratethyan stage. In most cases, the decapod assemblages of the Ottnangian consist of rather shallow-water taxa whereas the assemblages of the Karpatian consist of deep-water taxa from the middle and outer shelf. The Central Paratethyan assemblages show similarities in genus composition to the Proto-Mediterranean and recent Indo-Pacific regions. Gourretia sp. represents the earliest occurrence of the respective genus in the fossil record. The Oligocene-Early Miocene appearance of Palaeomyra and Liocarcinus in the circum-Mediterranean implies that sources of present-day diversity hotspots in the Indo-Pacific trace to the Western Tethys (as for other decapod genera), although coeval decapod assemblages in the Indo-Pacific remain poorly known.

During the Ottnangian (Burdigalian, early Miocene), the North Alpine Foreland Basin operated as a sea-way connecting the Central Paratethys Sea with the Rhône Basin in the Western Mediterranean. Within this short time window, an intensive faunal exchange between the two paleo-biogeographic units occurred, which is reflected in macrofossil assemblages. The extraordinarily rich fossil record of the study site, Allerding, located in the westernmost Central Paratethys, provides valuable insights into the composition and origins of the bivalve fauna colonizing the marine gateway. The site documents the early Ottnangian marine transgression over deeply weathered crystalline basement, grading from fossil bearing shallow water clay and sand into the open marine "Schlier" facies of the Ottnang Formation. Despite considerable taphonomic overprint including aragonite leaching and mechanical abrasion of bivalve shells under turbulent shallow-water conditions, a total of 46 species are recorded, including two new species (Lima allerdingensis n. sp. and Astarte danningeri n. sp.). The dominance of suspension feeders, and the presence of several deposit feeders and chemosymbiotic taxa point to well diversified inshore settings, including low intertidal mudflats, as well as seagrass meadows. An abundance of primary and secondary hardgrounds is reflected in the high number of cementing and byssate species, as well as in the occurrence of species drilling actively into hard substrate. Finally, the dominance of active burrowers suggests a patchwork of habitats, where sandy and muddy soft bottoms occur interspersed. Biostratigraphic analysis constrains the deposits to the early to middle Ottnangian, based on the presence of the index species Flexopecten davidi and the concurrence of several taxa, which have their last or first occurrences within this interval. These are predominantly taxa persisting into the Badenian, therefore allowing for a straightforward differentiation between late Eggenburgian and early Ottnangian assemblages. While a few Central Paratethys endemics reflect a continued semi-isolated position of the area, the majority of the newly arriving species are shared with the Mediterranean and NE Atlantic, documenting the establishment of a faunal migration route via the North Alpine Foreland Basin. In the present study the lectotypes of Nucula mayeri Hörnes, 1865, Saccella subfragilis (Hörnes, 1875) and Lucinoma wolfi (Hoernes, 1875) are designated.