Sphaerospyris? sp. is a unique megalocephalic nassellarian that has been studied in two piston cores from the central equatorial Pacific Ocean. This large morph first appeared about 9.5 Ma (near the top of the Diartus petterssoni Biozone) and had its last occurrence about 6.5 Ma (near the base of the Dimymocyrtis penultimus Biozone). There is no descendant lineage. The species was not found in time-equivalent recovery from Deep Sea Drilling Project Sites 65, 77, 83, and 289. The rare and somewhat sporadic occurrence of Sphaerospyris? sp. is attributed to hybridization between Acanthosphaera? sp. (Spumellarida) and Nephrospyris leibnitziana (Nassellarida). Skeletal characters of the parent species appear to have been exchanged through the process of introgression, which was the most important evolutionary agent in these lineages. The introduction of new morphological variants by introgression may have general significance in pelagic plankton communities.