Radiations are commonly believed to be linked to the evolutionary appearance of a novel morphology or ecology. Previous studies have demonstrated a close relationship between the evolutionary appearance of algal photosymbiosis in planktonic foraminifera and evolutionary diversification of Paleogene photosymbiotic clades. For example, the evolution of photosymbiosis was synchronous with the abrupt evolution of four major groups of Paleogene planktonic foraminifera including two clades within the genus Morozovella, as well as the genera Acarinina and Igorina. Our new isotopic and biogeographic data suggest that the acarininids evolved from a photosymbiotic ancestor (which we identify as Praemurica inconstans or early representatives of Praemurica uncinata), but also demonstrate that photosymbiosis did not trigger an immediate species-level radiation in this group. Instead, the acarininids remained a low-diversity taxon restricted to high latitudes for nearly 1.8 million years before radiating ecologically and taxonomically. The eventual radiation of the acarininids is tied to an expansion of their geographic range into the mid and low latitudes. Biogeographic analyses of modern plankton suggest that high-latitude environments may be less conducive to establishing radiations simply because there are fewer niches available to be filled than there are in the tropics. Accordingly, the acarininids may have initially failed to diversify because they started off in environments that presented few opportunities to sustain a large radiation. The high-latitude origin of the acarininids continued to retard their overall diversification until they were able to develop strategies that allowed them to expand into tropical environments and fully exploit their photosymbiotic ecology.

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