A major Pliocene coccolithophore turnover: Change in morphological strategy in the photic zone
Published:January 01, 2007
Marie-Pierre Aubry, 2007. "A major Pliocene coccolithophore turnover: Change in morphological strategy in the photic zone", Large Ecosystem Perturbations: Causes and Consequences, Simonetta Monechi, Rodolfo Coccioni, Michael Rampino
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The coccolithophores (or calcareous nannoplankton) have proven to be remarkably sensitive to changes in the earth system. However, their history is often expressed in terms of changes in species richness, a methodology that became suspect with the discovery of cryptic species through molecular techniques. To avoid this problem, I describe the extant coccolithophores in terms of morphostructural characters, tracing their changes through the Neogene. I conclude that these are regulated by a morphological strategy that favors small size of cells and coccoliths. I show that this strategy developed as a result of morphologic convergences in different lineages and in taxa inhabiting different strata of the photic zone. A long-term trend through the Neogene resulted in similar innovations in different lineages, and, also, in the loss of large and complex coccoliths. Superimposed on this trend, a short (∼2 m.y.) Pliocene turnover involved both the loss of morphostructural groups that were successful through the Paleogene and Miocene, and a critical, permanent shift to smaller size in the dominant Family Noelaerhabdaceae. The life strategy exhibited by the extant calcareous nannoplankton is rooted in this turnover, so that the same morphological strategy (Pleistocene Morphological Strategy; PLMS) has regulated the coccolithophores since the latest Pliocene–earliest Pleistocene. It is possible that biologic pressure from the microplankton induced the shift to smaller cells; it is equally possible that the progressive change in size and fine structure of coccoliths through the Neogene is linked to an increase in the Mg/Ca ratio and decrease in Ca2+ concentration in what Stanley and Hardie have called Aragonite III sea. The Pliocene turnover was likely induced by glacial intensification in the middle Pliocene, and sustained by the progressive cooling from the warm early Pliocene to the cold Pleistocene. The PLMS thus results from the combined forcing of ocean chemistry and climatic change.
The physiognomy of the extant coccolithophores, far from indicating their failure as a result of unfavorable seawater chemistry, demonstrates the remarkable adaptability of a group that evolved and first radiated in Aragonite II sea, thrived in Calcite II sea, and has “reinvented” itself in Aragonite III sea by adopting a unique morphological strategy.