Trochodendron and Nordenskioldia (Trochodendraceae) from the middle Eoceneof Washington State, USA

Fossil remains of the family Trochodendraceae are found in the early middle Eocene (49-50 Ma) Republic flora of northeastern Washington, a flora that contains a highly diverse and extensive montane warm-temperate assemblage. In this study, we document the earliest known fossil record of Trochodendro n Sieb. & Zucc. (Trochodendraceae) based on the distinctive leaves of Troch odendron nastae Pigg, Wehr, & Ickert-Bond sp. nov., and two infructescences , and an isolated fruit assigned to Trochodendron sp. The Republic Trochode ndron fruits are smaller but otherwise closely resemble those of extant Tro chodendron aralioides Sieb. & Zucc. and those of Neogene fossil fruits. Tro chodendron nastae leaves have the features of extant and other fossil Troch odendron leaves, except for their palmate rather than pinnate primary venat ion. This feature is more typical of the trochodendralean sister genus Tetr acentron Oliver. Trochodendron nastae leaves have venation that thus appear s to be intermediate between these two genera, suggesting that the palmate condition may be basal within this group. The Republic flora also contains one of the first known Eocene occurrences of the trochodendralean infructes cence Nordenskioldia Heer in western North America, along with leaves simil ar to Zizyphoides Seward & Conway, its apparently congeneric foliar genus. The presence of two distinctive trochodendralean plants, Trochodendron and the Nordenskioldia/Zizyphoides plant at Republic, demonstrates that the Tro chodendraceae were a diverse group of plants during the middle Eocene in we stern North America. This finding further documents the greater diversity d uring the Tertiary and wider distribution of a group known today exclusivel y from Asian endemics.