Abstract

Three new species of Streptochilus, a biserial planktic foraminiferal genus, were recognized in the lower Miocene of the eastern Atlantic and western Indian Oceans. These species had been formerly thought to be benthic species of the genus Bolivina, but evidence on their apertural morphology and stable isotopic composition indicates that they lived as plankton and should be assigned to the genus Streptochilus. The observation that three morphological species occurred in different regions of the oceans during the same short period of time (18.9-17.2 Ma) suggests that these biserial planktic species may have evolved polyphyletically, either from biserial planktic or from benthic ancestors, possibly in response to the occurrence of relatively eutrophic environmental conditions caused by intermittent upwelling, leading to high algal growth rates but low transport efficiency of organic matter to the sea floor. The new species of Streptochilus are described, illustrated and named: S. rockallkiddensis sp. nov. (from the northeastern Atlantic), S. cetacensis sp. nov. (from the equatorial and southeastern Atlantic) and S. mascarenensis sp. nov. (from the western equatorial Indian Ocean) and the description of the genus is emended.

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