Fruits of Koelreuteria (Sapindaceae) from the Cenozoic throughout the northern hemisphere: their ecological, evolutionary, and biogeographic implications.

PREMISE OF THE STUDY

Koelreuteria (Sapindaceae) has four extant deciduous tree species, disjunctly distributed in eastern Asia and the Fiji Islands. While K. paniculata is widely cultivated, the biogeographic origin and evolutionary history of Koelreuteria remain unclear.

METHODS

Fruits, pollen, wood, and leaves of closely related extant taxa were examined in comparison with fossil remains to evaluate the fossil record and biogeographic history of Koelreuteria.

KEY RESULTS

Overall, characters of capsular fruits are more diagnostic than other organs for this genus. We describe two new species of fruit remains from the Eocene, K. taoana sp. nov. (northeastern China and far eastern Russia) and K. dilcheri sp. nov. (western United States), and give emended descriptions of three species: K. allenii (Lesq.) W. N. Edwards (early-late Eocene of the United States), K. macroptera (Kováts) W. N. Edwards (late Oligocene-early Pliocene of Europe), and K. miointegrifoliola Hu et R. W. Chaney (Miocene of eastern Asia).

CONCLUSIONS

Reliable fossil records of capsules and ring-porous wood indicate that Koelreuteria may have originated in North Pacific-Rim area of the northern hemisphere by the early Eocene, representing an early temperate lineage in Sapindaceae adapted for wind dispersal. The fossils herein place a minimum age (ca. 52 Ma) for the divergence of Koelreuteria from tropical genera that appear more basal in the molecular phylogeny of Sapindaceae. Regional extinctions after the Eocene in North America and the Pliocene in Europe, reduced the range of Koelreuteria to eastern Asia, where three species occur today. The present distribution of another species in the Southern Pacific may be explained by long-distance dispersal.