MORPHOLOGIC VARIABILITY OF THE COCCOLITHOPHORID CALCIDISCUS-LEPTOPORUS IN THE PLANKTON, SURFACE SEDIMENTS AND FROM THE EARLY PLEISTOCENE

On a global scale, morphological variability of the extant coccolithop horid Calcidiscus leptoporus (Murray and Blackman, 1898) Loeblich and Tappan was investigated in surface sediments and plankton samples and from an Early Pleistocene time-slice (1.8 Ma to 1.6 Ma). In the bivari ate space coccolith diameter versus number of rays in the distal shiel d, Holocene samples follow a single, unimodal morphocline. Sample mean s of coccolith size and number of elements group in three clusters, I, II and III, which are of biogeographic significance. Clusters II and III coccoliths (mean coccolith size of 5.0 mu m and 20.9 elements, and 6.6 mu m and 25.6 elements, respectively) are found in a tropical bel t extending from 11 degrees N to 17 degrees S with an annual minimum s ea-surface temperature above 23.5 degrees C. Cluster I coccoliths (5.8 mu m, 20.7 elements) are found in samples outside that belt. The dist ribution of coccoliths in the surface sediments is tentatively interpr eted to be a result of mixing to a varying degree of at least three di fferent morphotypes ('small', 'intermediate' and 'large'), which were identified in the living plankton, and which are separated from each o ther at 5 mu m and 8 mu m mean coccolith diameter, respectively. A com parison of the surface sediments with the Early Pleistocene assemblage s revealed that between 1.6 Ma and 1.8 Ma two morphoclines A and B exi sted, the first of which persisted until the Holocene in the form of C . leptoporus, while the second comprises only extinct morphotypes incl uding Calcidiscus macintyrei as one end-member. During the Early Pleis tocene morphocline A was more homogeneous and no clusters were evident . Morphocline B shows a clear bimodality with a separation of morphoty pes at 9.5 mu m. Our observations suggest that morphoclines are subset s within the total stratigraphical range of a single species, and repr esent the global variability of that species in a particular time inte rval. Morphotypes, which belong to a morphocline, represent the infra- specific variability of that species within the biogeographic and stra tigraphic limits of that species.