A number of fossil species have been discovered across the Northern Hemisphere dating from the Early Eocene,[3] although they became confined to eastern Asia during the Pleistocene ice ages.[4][5]
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^Wing, S. L.; Hickey, L. J. (1984). "The Platycarya perplex and the evolution of the Juglandaceae". American Journal of Botany. 71 (3): 388–411. doi:10.1002/j.1537-2197.1984.tb12525.x.