Speciation of extant Umbilicosphaera (Prymnesiophyceae) during the Pliocene

Abstract This paper examines the morphological variations in fossil Umbilicosphaera spp. from Pliocene sediments recovered at ODP Hole 709C (equatorial western Indian Ocean). An undescribed extinct species was found in calcareous nannofossil Subzones CN10b to CN12b (Zones NN12–16) and named Umbilicosphaera patera n. sp. Comparison of the central opening size and kink pattern of the distal shield showed that U. patera and two extant species, U. sibogae and U. foliosa, were similar to each other but different from extinct species U. jafari and U. rotula. U. patera, U. sibogae, and U. foliosa can be clearly distinguished from each other based on the widths of the slope and collar of the distal shield. Gradual morphometric differentiation of U. patera from both U. sibogae and U. foliosa across the studied horizons suggests that the two extant species were derived from their common ancestor, U. patera. U. patera has also been identified in sediments dated to between 4.4 and 3.7 Ma in the northwestern Pacific and ca. 3.8 Ma in the northwestern Atlantic. The speciation of U. sibogae and U. foliosa from U. patera is estimated to have occurred during the warm Pliocene intervals around 4.3 and 4.2 Ma, respectively.