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Extant baleen whales (Mysticeti) uniquely use keratinous baleen for filter-feeding and lack dentition, but the fossil record clearly shows that "toothed" baleen whales first appeared in the Late Eocene. 1 Globally, only two Eocene mysticetes have been found, and both are from the Southern Hemisphere: Mystacodon selenensis from Peru, 36.4 mega-annum (Ma) ago 1,2 and Llanocetus denticrenatus from Antarctica, 34.2 Ma ago. 3,4 Based on a partial skull from the lower part of the Lincoln Creek Formation in Washington State, USA, we describe the Northern Hemisphere's geochronologically earliest mysticete, Fucaia humilis sp. nov. Geology, biostratigraphy, and magnetostratigraphy places Fucaia humilis sp. nov. in the latest Eocene (ca. 34.5 Ma ago, near the Eocene/Oligocene transition at 33.9 Ma ago), approximately coeval with the oldest record of fossil kelps, also in the northeastern Pacific. 5 This observation leads to our hypothesis that the origin and development of a relatively stable, nutrient-rich kelp ecosystem 5,6 in the latest Eocene may have fostered the radiation of small-sized toothed mysticetes (Family Aetiocetidae) in the North Pacific basin, a stark contrast to the larger Llanocetidae (whether Mystacodon belongs to llanocetids or another independent clade remains unresolved) with the latest Eocene onset of the Antarctic Circumpolar Current in the Southern Hemisphere. 7,8,9 Our discovery suggests that disparate mechanisms and ecological scenarios may have nurtured contrasting early mysticete evolutionary histories in the Northern and Southern hemispheres. • A new fossil mysticete is described from Washington State, USA • This fossil represents the oldest known mysticete from the Northern Hemisphere • The small body size contrasts with larger coeval whales in the Southern Hemisphere • The coastal kelp ecosystem likely fostered the diversification of early whales Tsai et al. describe a new mysticete from the latest Eocene (dated at approximately 34.5 Ma) in Washington State, USA. This is the oldest known mysticete from the Northern Hemisphere, revealing a previously unknown early evolutionary history of whales and suggesting contrasting ecological settings for baleen whale evolution between two hemispheres. [ABSTRACT FROM AUTHOR]