COCCOLITHS, the calcite plates formed by unicellular phytoplanktonic algae (coccolithophores, phylum Prymnesiophyta) 1, are produced in enormous quantities and probably constitute the largest single carbonate sink in oceanic biogeochemical cycles. They are major sediment formers, are of great value to geologists as biostratigraphic indicators and have been extensively studied as models for biomineralization 2,3. We have applied the understanding of coccolith biomineralization to the fossil record 4 and present evidence from electron and optical microscopy that there has been a conserved mechanism of crystal nucleation throughout the 230 million year history of coccolithophores. This fundamental feature of coccolith growth, which we term the V/R model, involves the assembly of a ring of single crystals with alternating orientations, radial (R) and vertical (V), and provides a powerful tool for tracing phylogenies, identifying homologous structures and rationalizing the higher taxonomy of the group. The living coccolithophorid, Emiliania huxleyi, seemed anomalous because only R crystals had been observed; transmission electron microscopy of proto-coccolith rings, however, revealed relict V crystals which are overgrown by preferential development of R units in complete coccoliths. A speculative model based on a plicated macromolecular template is presented as a possible explanation for the conserved nucleation process.
