EARLY STRATIGRAPHIC RECORD AND MAJOR EVOLUTIONARY CHANGES OF THE PLANKTONIC FORAMINIFERA AND DINOFLAGELLATES IN THE MESOZOIC: HINTS OF COEVOLUTION

Communities of planktonic organisms evolve passively in a continuum, following major changes in the structure of the water masses. Planktonic foraminifera (heterotrophic protozoans) and dinoflagellates (autotrophic unicellular algae) are microscopic organisms distributed throughout the world’s oceans, and play a major role in the trophic structure and nutrient/carbon recycling of the marine epipelagic realm. The first true dinoflagellates are found in the mid- to late Triassic, and have a subsequent rich and variable stratigraphic record throughout most of the Meso-Cenozoic. The earliest protoglobigerinids, Conoglobigerina, a meroplanktonic foraminifera, and Globuligerina, are first reported from the mid-Jurassic (Bajocian), and restricted to most of NW, E and central-southern Europe and the North Atlantic until the Valanginian. The transition to a planktonic mode of life from benthic ancestors (e.g., Praegubkinella racemosa in the early Toarcian), was probably induced by the Toarcian dysoxic-anoxic event, which was an obvious restriction to benthic life. The main pulses of radiation and speciation, extinction events, and biogeographic differentiation of the planktonic foraminifera and dinoflagellate populations, show remarkably coeval trends in the mid- to late Jurassic and near-coeval trends throughout the Cretaceous. These evolutionary changes appear coincident with changes in sea-level and related oceanographic conditions, and may also hint at a coevolutionary drive. The establishment of algal (dinoflagellate) symbiosis, perhaps as early as in the late Bajocian, allowed the planktonic foraminifera populations to diversify and thrive in low-nutrient (oligotrophic) oceanic settings and achieve near-global distribution by the mid-Cretaceous (Albian).