Fossil dinoflagellates, when asymmetrical, almost always have features such as antapical horns on the right side reduced relative to features on the left side. A new species here described, Wilsonidium pechoricum, is therefore unusual in having a reduced left antapical horn. W. pechoricum seems to have originated in the northern Tethys in the latest Paleocene. It subsequently spread northwards and became widely distributed in the Peri-Tethys and parts of the Arctic region during the short interval known as the Initial Eocene Thermal Maximum (IETM). The new species was probably favored by extraordinary paleoecological conditions (high sea-surface temperatures and probably also high nutrient levels) prevailing in neritic waters of the IETM; a time during which aberrant morphotypes were also recorded among other planktonic protists. The apparent absence of W. pechoricum from the North Atlantic region suggests that the Turgay Strait may have functioned as a waterway between the Arctic and Peri-Tethys during the IETM. W. pechoricum is the oldest species of the genus Wilsonidium and possibly descended from the genus Apectodinium. Its early appearance points to a Late Paleocene radiation of the Wetzelielloideae before the well-known Early Eocene radiation in the subfamily, and its morphology is in accordance with a monophyletic origin of the group.