PIGG, KATHLEEN B.* AND WESLEY C. WEHR. Department of Plant Biology, Arizona State University, Box 871601, Tempe, AZ 85287-1601; Burke Museum of Natural History and Culture, University of Washington, Seattle, WA 98195. - Trochodendron (Trochodendraceae) from the early middle Eocene Republic Flora, Washington, USA.
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