Trochodendron and Nordenskioldia (Trochodendraceae) from the middle eocene of Washington State, U.S.A.

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 Trochodendron Sieb. & Zucc. (Trochodendraceae) based on the distinctive leaves of Trochodendron nastae Pigg, Wehr, & Ickert-Bond sp. nov., and two infructescences, and an isolated fruit assigned to Trochodendron sp. The Republic Trochodendron fruits are smaller but otherwise closely resemble those of extant Trochodendron aralioides Sieb. & Zucc. and those of Neogene fossil fruits. Trochodendron nastae leaves have the features of extant and other fossil Trochodendron leaves, except for their palmate rather than pinnate primary venation. This feature is more typical of the trochodendralean sister genus Tetracentron Oliver. Trochodendron nastae leaves have venation that thus appears 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 infructescence Nordenskioldia Heer in western North America, along with leaves similar 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 Trochodendraceae were a diverse group of plants during the middle Eocene in western North America. This finding further documents the greater diversity during the Tertiary and wider distribution of a group known today exclusively from Asian endemics.