Following the mass extinction event at the end of the Cretaceous, the marine molluscan faunas of the high southern latitudes underwent a marked period of diversification during the early Paleocene. The appearance of four new species belonging to the new genus Seymourosphaera, tentatively placed in the subfamily Pseudolivinae, from the lower Paleocene strata of Seymour Island, Antarctic Peninsula, clearly illustrates the post-Cretaceous extinction diversification. The abrupt radiation of the buccinids during the early Paleocene, was also apparently related to geographic isolation of Antarctica during final breakup of Gondwana. Comparative analysis of shell morphology of Seymourosphaera, new genus reveals close morphologic similarities, not only with taxa within Pseudolivinae, but also with several genera and subgenera belonging to the families Buccinidae and Nassariidae. However, incompleteness of the fossil record and a “generalized” shell morphology make difficult establishment of unequivocal phylogenetic relationships for Seymourosphaera. A taxonomic review of most closely related, and possibly ancestral genus Austrosphaera Camacho, 1949, is provided. The following new species of genus Seymourosphaera new genus are described: Seymourosphaera bulloides new species, S. subglobosa new species, S. depressa new species, and S. elevata new species.