Temperate deciduous forests have been the focus of northern hemisphere biogeographical studies, yet the origins of the floristic elements in these forests stem in part from tropical deciduous forests that pervaded northern latitudes during the early Tertiary. The legume family Fabaceae may provide some insight into the fate of the early elements in North American deciduous forests. Most did not radiate into the temperate deciduous forests, but rather remained in the seasonally deciduous tropical forests that migrated southward with the global cooling of the late Tertiary. Most of the temperate largely herbaceous diversity in legumes seems to have migrated in from the Old World during the late Tertiary rather than have evolved in situ in North America from tropical, woody ancestors. We illustrate the early Tertiary North American pattern with tropical legume groups including Robinia and close relatives, as well as the dalbergioid legumes. The late Tertiary to Quaternary North American pattern is illustrated with temperate legumes including Hologalegina and Gleditsia. For tropical lineages, a molecular clock based on nuclear rDNA ITS/5.8S sequences is calibrated with an early Tertiary Caribbean vicariance event for the endemic Greater Antillenan radiations Poitea and Pictetia.

Key words: Fabaceae, molecular biogeography, North America, Tertiary