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Our investigation of the taxonomy, biostratigraphy, and phylogeny of the microperforate Oligocene species generally included in the planktonic genus Streptochilus documents that biserial planktonic foraminifera occur in the upper Oligocene, despite previous descriptions of a global gap in occurrence of such taxa over that interval. We describe a total of 4 distinct morphological species, namely Streptochilus martini (Pijpers), Streptochilus pristinum Brönnimann and Resig, Streptochilus rockallkiddensis Smart and Thomas, and Streptochilus tasmanensis Smart and Thomas n. sp. Some Recent biserial foraminifera (Streptochilus globigerus) live tychopelagically, i.e., the species lives in the plankton but is genetically identical to the neritic benthic species Bolivina variabilis. Fossil species could have had a similar lifestyle, from which they could have evolved into true planktonic species, implying polyphyletic, multiple evolution of planktonic from benthic biserial groups. It is likely, therefore, that the stratigraphic distribution of the genus Streptochilus represents one or more expatriation events from the coastal benthos to the pelagic zone, and that not all or no species traditionally placed in the genus Streptochilus are descended from the genus Chiloguembelina. The name Streptochilus has been used for species which morphologically cannot be distinguished from species in the benthic genus Bolivina but live planktonically, and the genus thus is polyphyletic. We do not have sufficient information on evolutionary patterns to define clades of biserial planktonic species, and here propose to keep the name Streptochilus provisionally for biserial planktonic species until evolutionary relations have been clarified. We assign the genus to the Superfamily Bolivinoidea.

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