Trochodendron (Trochodendraceae) is documented from the early middle Eocene (49-50 Ma) Republic flora of Washington state, USA, based on distinctive leaves, an inflorescence and an isolated fruit. Leaves are morphologically similar to Miocene and extant Trochodendron, but demonstrate basally actinodromous venation like that of the extant sister genus Tetracentron rather than pinnate venation characteristic of the single extant species of Trochodendron. This new Trochodendron thus has venation that appears evolutionarily intermediate between these two genera as previously known and retains the palmate condition thought to be primitive within the Hamamelideae as a whole. Leaves are simple, symmetric, 3-8 cm long x 1.5-5 cm wide with an acute apex and acute to cuneate base, with exstipulate, striate, deeply caniculate petioles. Venation includes 2-3 pairs of lateral primaries, a marginal vein and up to three secondaries that diverge at angles of 40-45 degrees. Weak intersecondary veins anastomose at right angles into widely and irregularly spaced orthogonal reticulate tertiary veins. Leaves typically have an unlobed crenate margin with numerous small appressed chloranthoid (papillate) teeth in the apical portion of the lamina, while more basal portions are entire. Reproductive structures are quite similar to those of Miocene and extant forms. An infructescence specimen consists of a single woody axis 3.2 cm long and 16-23 mm wide that bears fruits on striated pedicels. Fruits have an acute base and truncate apex and sometimes show evidence of persistent recurved styles. Occurring in the same beds, is the extinct infructescence Nordenskioldia (Trochodendraceae) and its associated leaf Zizyphoides that show similiarities to those of Paleocene and Neogene relatives in North America, Europe and Asia. The presence at Republic of both a distinctive Trochodendron plant and Nordenskioldia and Zizyphoides remains demonstrates that the Trochodendraceae, today known exclusively from Asian endemics, was undergoing rapid diversification in the Eocene of western North America.

Key words: middle Eocene, Nordenskioldia, Trochodendraceae, Trochodendron, Zizyphoides