Mississippian Ostracoda from the Bonaparte and Canning basins, NW Australia: Platycopina and Podocopida, their biostratigraphy, palaeoecology and palaeozoogeographic links

Platycopines and podocopids are represented in the Mississippian benthic ostracod fauna of the Bonaparte Basin by at least sixty species referable to twenty-five genera distributed among the ostracod superfamilies Kloedenelloidea (3), Cytherelloidea (5), Barychilinoidea (2), Healdioidea (5), Thlipsuroidea (1), Bairdiocypridoidea (3), Sigillioidea (3), Bairdioidea (27), Cypridoidea (4), Cytheroidea (3), and Youngielloidea (1), and the family Geisinidae (4). The genera are cosmopolitan, being found in the major Mississippian provinces (e.g. Acratia, Tschigovana, Bairdia, Bairdiacypris, Bairdiocypris, Beyrichiopsis, Cavellina, Carboprimitia, Healdia, Healdianella, Microcheilinella, Praepilatina, Sulcella). Nine new species are described from the Bonaparte Basin (Evlanella? conversa, Glyptolichvinella acrista, Cavellina septima, Paracavellina olempskae, Healdia veeversi, Healdianella ogmoconchelloides, Healdiacypris drucei, Reversocypris australis, Tschigovana thomasi) of which Cavellina septima, Glyptolichvinella acrista and Tschigovana thomasi are also present in the Canning Basin. Thirty-five species are closely related to, if not conspecific with, established taxa (Glyptolichvinella ovicella, Cavellina cf. taidonensis, C. glandella, Healdia cornigera, Healdianella cf. cuneola, Orthocypris zhaksiconica, Microcheilinella cf. intumescens, M. cf. spinosa, Healdiacypris cf. egorovae, Bairdiocypris proxima, Tschigovana sinica, T. cf. angulata, Blessites feluyensis) while 13 are placed in open nomenclature, most of which are comparable with previously described taxa.

The current taxonomic study of the platycopine and podocopid elements of the benthic ostracod fauna refines an interim biostratigraphic scheme previously proposed for the Mississippian succession of the Bonaparte Basin. The revised biostratigraphic scheme now consists of a succession of eight ostracod assemblage zones based on the first appearance (in ascending order) of Welleriella atypha, Shishaella porrecta, Shivaella armstrongiana, Coryellina cesarensis, Malnina spinosa, Healdia cornigera, Scrobicula inaequalis and Amphissites sp. B. This scale of local assemblage zones is controlled by those based on conodonts and foraminiferids, which in turn are calibrated against the global time-scale. The atypha, porrecta, armstrongiana and spinosa assemblage zones are also recognised in the early Mississippian (Tournaisian) sequence of the Canning Basin.

In terms of the Mississippian environment of the Bonaparte Basin the benthic ostracods of the Tournaisian Langfield Group and the Visean Utting Calcarenite are representative of a shallow marine offshore realm where Bairdioidea dominate, followed by Paraparchitioidea, suggesting warm-water conditions. This taxonomic association is common in the Mississippian carbonate platform deposits in western Europe, cordilleran Canada, and several basins in Siberia and Kazakhstan. The ostracod fauna of the dominantly clastic part of the Visean Weaber Group (Milligans Formation, Yow Creek Formation and Kingfisher Shale) represents the marine nearshore realm, with fewer species of Bairdioidea, and with the addition of species of Healdioidea and Youngielloidea, which are absent in the shallow marine offshore realm.

The major affinities of the Mississippian platycopines, and podocopids from the Bonaparte Basin, like those of previously described Beyrichicopida (including Kirkbyoidea and Paraparchitoidea) are with species from western Europe (Belgium, northern England), the Russian Platform, Kazakhstan and south China. The Podocopida (especially Bairdioidea) described here, show that species with North American affinities are of greater significance than previously thought. The palaeozoogeographic distribution of benthic ostracods during the Late Devonian and Mississippian has been explained in terms of migration by palaeocurrents and genetic exchange along juxtaposed shallow shelves of the adjacent blocks of Gondwana and Laurentia-Baltica.