Switches in coiling direction of shells of the planktonic foraminifer Neogloboquadrina pachyderma have been extensively used as a paleotemperature proxy from the late Miocene through the Quaternary, exploiting the specific modern polar affinity of the sinistrally coiled form. Here we have reexamined the origin of the modern sinistral form in Pliocene and Pleistocene sediments from Ocean Drilling Program Site 1014 in the eastern North Pacific. By using morphometric and stable isotope data, we show that Pliocene and Pleistocene sinistral forms of N. pachyderma are morphologically and ecologically different and that modern-type sinistral form first appeared ca. 1 Ma. These findings suggest that N. pachyderma sinistral should not be used for calibrated paleoceanographic reconstructions prior to the middle Pleistocene and reinforce the need to assess the origin of specific ecological affinities of other modern planktonic foraminifer species. The first appearance of the modern form and the subsequent evolution of its mean shell size appear to have been synchronous in the North Pacific and the North Atlantic. Such hemispheric evolutionary synchronicity implies existence of trans-Arctic gene flow. The modern-type shell morphology and the specific cold-water affinity of N. pachyderma sinistral may have evolved in response to the onset of the 100 k.y. climate regime in the middle Pleistocene.

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