FUNCTIONAL MORPHOLOGY OF COCCOLITHS: PHYLOGENETIC EVIDENCE IN THE ORDER DISCOASTERALES

As both primary producers and calcifiers the (unicellular) coccolithophores play a fundamental role in the dynamics of the Earth System. Although well documented in the modern ocean, the past diversity of these protists has remained poorly known for a number of reasons, primarily because phylogenetic relationships among ‘nannofossils’ are poorly established. Except for selective biomarkers restricted to members of the Order Isochrysidales, the fossil record of the coccolithophores consists of tiny skeletal pieces, or coccoliths, that surround the cell, forming an external skeleton, or coccosphere. The role of the coccoliths has eluded us until now, most being seen as protective, some as floating devices, others as ballast, still others as means to concentrate nutrients. Only recently has the suggestion been made that coccoliths may serve as food traps in species living in oligotrophic environments. The discoasters, a prominent group that have constituted >50 % of equatorial nannofossil assemblages from late Paleocene to late Pliocene (~58 Ma to 2.0 Ma) have long been classified as incertae sedis. Morphostructural analysis shows that discoasters are phylogenetically related to a large group of other Paleocene nannofossils which can be traced back to Biantholithus, the single coccosphere-bearing taxon that marks earliest Paleocene marine strata. This shows that discoasters are highly specialized coccoliths, and justifies the introduction of the new Order Discoasterales. Examination of the sequential changes in coccolith morphostructure and their timing indicates that evolutionary novelties in this order were largely driven by oceanic oligotrophy, coccoliths becoming increasingly specialized to passively capture, concentrate, and possibly farm organisms of the picoplankton capable of providing the cells that secrete them with nitrogen in N-deficient environments.