A permineralized tree fern trunk, 23.2 cm long X 11.7 cm wide, has been recovered from Shoup Ranch Gulch near Cottonwood, California. The specimen comes from the Upper Chickabally Member of the Budden Canyon Formation (Aptian), Upper Cretaceous, and was studied by the cellulose acetate peel technique. The outer surface of the stem shows helically arranged persistent petiole bases and large numbers of adventitious roots surrounded by a dense indument of multicellular trichomes. The broad, parenchymatous pith with mucilaginous cells and sclerotic nests is surrounded by an amphiphloic dictyostele, parenchymatous inner cortex and outer sclerenchymatous cortex. Sclerenchyma also surrounds the cauline vasculature and leaf traces. Medullary or cortical bundles are lacking. In transverse sections, meristeles of the cauline system form an arc with the convex surface toward the stem periphery. Phloem contains both axially elongated and tangential sieve elements. Petiole bases are roughly oval in outline with three vascular bundles, including an undulating abaxial arc and an adaxial pair. The protoxylem in the petiole is associated with cavity parenchyma. Primary xylem maturation in the petiole is endarch. Radially aligned cells surrounding a dark area of amorphous material suggest tissue damage and wound response in the cortex. Roots of an epiphytic seed plant are found among the trichomes and roots of the stem. Stem size, xylem configuration in the petiole, presence of tangential cells in the phloem, persistent petiole bases and extensive indument of trichomes and roots are indicative of cyatheaceous tree ferns. This fossil fern was included in a phylogenetic analysis with living and fossil tree ferns. The combination of characters, including the distinct leaf trace configuration, suggests that the California fossil represents a new genus.

Key words: Cretaceous, Cyatheaceae, Dicksoniaceae, phylogeny, tree ferns