DIOONOPSIS HORIUCHI ET KIMURA LEAVES FROM THE EOCENE OF WESTERN NORTH AMERICA: A CYCAD SHARED WITH THE PALEOGENE OF JAPAN

摘要

Reinvestigation of cycadalean leaf and leaflet remains from the Eocene of Hamilton Bay, Alaska, and La Porte, California, using features of epidermal anatomy as well as macromorphology, indicates that these remains, which were formerly assigned to modern cycad genera, conform to the extinct genus Dioonopsis Horiuchi et Kimura first described from the Paleocene of Japan. Recognition of Dioonopsis praespinulosa (Hollick) Erdei, Manchester et Kvacek comb. nov. from Alaska and Dioonopsis macrophylla (Potbury) Erdei, Manchester et Kvacek comb, et stat. nov. from California supports the existence of the Dioonopsis lineage in North America during the early Cenozoic. The occurrence of this genus in the Eocene of western North America as well as Japan, combined with its apparent absence in the European Tertiary, indicates that it probably spread via the Beringial phytogeographic pathway and was thus adapted to occupy high-latitude environments. Among modern cycad genera, Dioon fits best to the macro- and micromorphology of Dioonopsis and may be a remnant of a lineage closely related to that of Dioonopsis.