Role of photosymbiosis and biogeography in the diversification of early Paleogene acarininids (planktonic foraminifera)

Radiations are commonly believed to be linked to the evolutionary appearanc e of a novel morphology or ecology. Previous studies have demonstrated a do se relationship between the evolutionary appearance of algal photosymbiosis in planktonic foraminifera and evolutionary diversification of Paleogene p hotosymbiotic clades. For example, the evolution of photosymbiosis was sync hronous with the abrupt evolution of four major groups of Paleogene plankto nic foraminifera including two clades within the genus Morozovella, as well as the genera Acarinina and Igorina. Our new isotopic and biogeographic da ta suggest that the acarininids evolved from a photosymbiotic ancestor (whi ch we identify as Praemurica inconstans or early representatives of Praemur ica uncinata), but also demonstrate that photosymbiosis did not trigger an immediate species-level radiation in this group. Instead, the acarininids r emained a low-diversity taxon restricted to high latitudes for nearly 1.8 m illion years before radiating ecologically and taxonomically. The eventual radiation of the acarininids is lied to an expansion of their geographic ra nge into the mid and low latitudes. Biogeographic analyses of modern plankt on suggest that high-latitude environments may be less conducive to establi shing radiations simply because there are fewer niches available to be fill ed than there are in the tropics. Accordingly, the acarininids may have ini tially failed to diversify because they started off in environments that pr esented few opportunities to sustain a large radiation. The high-latitude o rigin 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.