Miocene variability of Calcidiscus gr. leptoporus and possible evolutionary relationship with another Coccolithaceae: Umbilicosphaera gr. sibogae.

Bibliographic data concerning the variability of coccosphere and coccoliths during the life-cycle of two extant Coccolithophorid species (Calcidiscus gr. leptoporus and Umbilicosphaera gr. sibogae) are summarized and completed by new observations on the structure of the proximal shield by fossil coccoliths. Thorough investigation of the proximal shield of C. gr. leptoporus in the latest Miocene (Messinian) assemblages from three oceanic localities (central Pacific, eastern equatorial Atlantic and southwestern Indian oceans) indicates the co-occurrence of two types of coccoliths: (1) typical C. gr. leptoporus and (2) specimens constituted by a distal shield similar to that of C. leptoporus, but with a proximal shield comparable to that of U. sibogae, although no typical U. sibogae occurs on the basis of the distal views. Such intermediate coccoliths have been previously described from the Badenian (middle Miocene) of central Europe. Whereas the authors considered them as a new species (Cycloperfolithus carlae), they are interpreted here as transitional morphotypes revealing an evolutionary link between C. gr. leptoporus and U. gr. sibogae, during the late Miocene. Consequently, the palaeontological 'species' carlae is hereby transferred to the genus Calcidiscus. These observations also point out the possibility of a diachronous evolution of the two shields of the same coccolith type, underlining the necessity for a better knowledge of the proximal shield structure, usually somewhat neglected.