Triplochitioxylon (Sterculiaceae): A New Genus of Wood From the Eocene of Oregon and Its Bearing on Xylem Evolution in the Extant Genus Triplochiton

A new sterculiaceous wood, Triplochitioxylon oregonensis gen. et sp. n., was collected from a Middle Eocene locality in the Clarno Formation of Oregon. Anatomical data indicate a close natural relationship between T. oregonensis and the living species of Triplochiton, a genus endemic to tropical Africa. The fossil is believed to represent a population of the group or complex from which Triplochiton evolved. The basic differences in the xylem organizations of the two genera are explained by a significant reduction of fusiform initial length and by a complete suppression of post‐cambial parenchyma strand elongation in the extant genus. Paleobotanical and biogeographical evidence suggest that the xylem evolution has been strongly influenced by the increasing aridity of the African continent.