Fossil history of the leguminosae from the Eocene of southeastern North America

A diverse assemblage of fossil legumes from the Eocene Claiborne Formation in southeastern North America has been the focus of this study of the early fossil record of the Leguminosae. These fossils are significant because they constitute one of the oldest known assemblages of fossil legumes. Both extinct and extant genera in all three subfamilies have been recognized and provide unequivocal evidence for the occurrence of the tribes: Caesalpinieae, Cassieae, Detarieae, Mimoseae, Swartzieae and Sophoreae. Additional fossil material not yet identified may provide evidence for other tribes. The Caesalpinieae and Sophoreae are particularly diverse; each is represented by at least four genera. This may indicate a tropical American center of evolution for these tribes. The extant genera so far documented as fossils from North America have modern distributions in (i) South America, (ii) North America-Asia disjunct, (iii) Old World tropics, and (iv) tropical Africa. Some of these fossils are best explained by migration, whereas others are best interpreted as representing once widespread taxa that underwent regional extinction, resulting in more restricted, sometimes disjunct distributions. The first fossil evidence of the legume genera commonly thought to be most archaic postdates these Eocene fossils. The paleobotanical data from this study are providing an understanding of the early history of the Leguminosae and a means to interpret extant biogeographical distribution patterns.