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Previous studies have established a close relationship between the evolutionary origin of new clades of planktonic foraminifera and heterochrony. Studies of the Paleogene radiation of the genusMorozovellarevealed, for example, a temporal pattern of variation consistent with paedomorphosis. Our study focused on the late Paleocene species ofAcarinina, sister group ofMorozovella.Shape variations related to evolution and ontogeny are appraised through a morphometric method based on outline analysis using the elliptic Fourier transform. Patterns of developmental and evolutionary changes are studied and compared within each species (Acarinina nitida, A. subsphaerica, andA. mckannai). As no congruence is found, we suggest that the evolutionary change observed within these species is not related to a heterochronic process. We also test for similarity of both evolutionary and ontogenetic changes among species. Although we observe no significant correlation between temporal patterns of shape change among species, the tight congruence of ontogenetic trajectories suggests that the developmental constraints affecting these trajectories have been preserved in spite of the evolutionary diversification of acarininids. Heterochrony is not clearly involved in the early Paleogene diversification of acarininids and therefore may not be as common as previously claimed. The role of developmental constraints in monitoring morphological evolution therefore needs to be reassessed.

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.