The ~38-35 Ma Cedarville Flora, NE California, is a unique leaf-dominated impression flora intermediate between "Boreotropical" Paleogene vegetation and Metasequoia-dominated "Arctotertiary" vegetation of the early Oligocene Northwest. The assemblage grew at ~1000 m paleoelevation on the seaward slope of the Paleogene Cordillera, in biogeographic connection with both upland interior and lowland coastal vegetational sources. Cedarville evergreen broadleaved taxa (eg. Annonaceae, Magnolia, Meliosma, Quercus, Schizandra) while not diverse, closely resemble species from Eocene Oak-Laurel vegetation of the Northwest lowlands. Deciduous broadleaved taxa (eg. Alnus, Cedrelospermum, Cercidiphyllum, Decodon, Parrotia, Plafkeria, Platanus exaspera, Pterocarya, Ulmus) are conspecific with or similar to taxa from the latest Eocene Florissant Flora, Colorado, and Oligocene Bridge Creek Flora, central Oregon. With the exception of Rosaceae (seven species) however, diverse "Arctotertiary" lineages that distinguish Bridge Creek-type assemblages (eg. Aceraceae, Betulaceae, Tiliaceae, etc.) are absent or represented by one species in the Cedarville Flora. At Cedarville, the combination of moderate elevation and maritime climate (cool summers, moderate winters, and copious, non-seasonal precipitation) permitted the intergradation of "Boreotropical" lineages from lowland coastal vegetation and "Arctotertiary" lineages from the interior Cordillera. The Cedarville Flora documents both the early westward migration of seasonally-adapted microthermal lineages, and the adaptation of megathermal "Boreotropical" lineages to increasingly cool and seasonal climate.

Key words: Cedarville, Cordillera, Eocene, paleobotany, paleoclimate