Vegetative and Reproductive Morphology of an Extinct Plane Tree (Platanaceae) from the Eocene of Western North America

An extinct type of plane tree, common in the Middle to Late Eocene of Oregon, is reconstructed as a "whole plant" on the basis of repeated co-occurrence of isolated platanaceous organs at numerous localities. It is described in detail on the basis of leaf architecture, petiole and stem anatomy, pistillate inflorescence morphology, fruit anatomy, staminate inflorescence morphology, and in situ pollen morphology, rendering it the most completely documented fossil angiosperm species. The leaves, Macginitiea angustiloba (Lesq.) comb. nov., are relatively large, with five to seven palmately arranged lobes and palinactinodromous venation. They resemble those of some modern Platanus species except in features associated with a narrower angle between adjacent primary veins. The petioles are basally expanded and hollow and appear to have enclosed the axillary buds, as in most extant Platanus species. Inflorescences are borne as globose heads along separate staminate and pistillate axes. The pistillate inflorescences, Macginicarpa glabra gen. et sp. nov., and staminate inflorescences, Platananthus synandrus gen. et sp. nov., both have florets with well-developed perianths and whorls of five carpels or stamens. The fruits are achenes with persistent styles and lack dispersal hairs. The stamens have short filaments, elongate anthers with small (12-16 μm) tricolpate, reticulate pollen and elongate-peltate connectives. The reconstructed plant, informally termed the "Clarno Plane," combines features of both extant subgenera of Platanus with a suite of unique characters that distinguish it as an extinct genus in the Platanaceae. Differences in perianth development, fruit indumentum, and pollen size between the Clarno Plane and Platanus indicate that the modern genus may be more specialized for wind dispersal of fruits and pollen than was the Eocene plant.