Earliest fossil record of the genus Tetradium (Rutaceae) in Asia: implications for its evolution and palaeoecology

A new species, Tetradium nanningense sp. nov. (Rutaceae), is described on the basis of well-preserved mummified wood from the upper Oligocene Yongning Formation of Nanning Basin, Guangxi Province, South China. This species represents the most ancient fossil evidence of the genus Tetradium in Asia, the region of its modern distribution. Its occurrence in the late Oligocene is consistent with the diversification age of the modern Asian species within this genus as estimated by molecular dating: T. nanningense could be closely related to an ancestor of extant Tetradium species. The fossil record of Tetradium suggests that this genus migrated from North America to eastern Asia in the Oligocene. The presence of (semi-)ring-porous wood and helical thickenings on vessel walls in T. nanningense provides new evidence for the independent gains of these traits in the course of evolution among different plant groups from eastern and south-eastern Asia in the Oligocene. These wood features might have arisen in response to the increase in climate seasonality following the abrupt climate cooling across the Eocene–Oligocene boundary.