The North American records of Campanian (Upper Cretaceous) megafossil plant localities are known primarily from the Atlantic Coastal Plain, the central and northern Rocky Mountains and California. A significant gap exists concerning plant megafossil localities that border the southwestern margin of the former Western Interior Seaway. A rare collection of leaf impressions, from the Upper Shale Member of the Aguja Formation at Big Bend National Park, helps to reduce this gap. These megafossils are preserved in tan to dark orange-brown, oxidized shales and consist of extinct forms of Hamamelidaceae, Lauraceae (Persea-tribe), Palmaceae, Zingiberales, Cheirolepidiaceae, and Taxodiaceae. Specimens of juvenile and mature leaves belonging to the Hamamelidaceae represent an undescribed genus. This extinct genus has affinities to several extant genera of the Hamamelidaceae including Corylopsis, Hamamelis, Fothergilla, and Parrotiopsis. For example, major and minor marginal venation patterns resemble those of Parrotiopsis jacquemontiana Rehder; tooth apical projections are similar to those of Corylopsis spicata Siebold and Zucc.; and the pattern of secondaries, tertiaries, quaternaries and finer veins throughout the middle portion of the lamina are similar to cleared leaf specimens of Hamamelis virginiana L. The plant megafossils are characteristic of plants that thrived on well-drained, channel levee, or floodplain environments that experienced a pronounced dry season.

Key words: Aguja Formation, Campanian, Hamamelidaceae, Hamamelis, Lauraceae