The fossil record of the Leguminosae is abundant and diverse in Eocene and younger sediments. Caesalpinioideae, Mimosoideae, and Papilionoideae were all diverse by the middle Eocene, and many fossils are referable to extant genera. By contrast, the Paleocene record is not diverse and the several taxa that have been reported are all referable to the Caesalpinioideae. The fossils we describe here are significant because they are Paleocene in age and are clearly referable to the subfamily Papilionoideae. The fossil locality is a fine-grained, abandoned channel fill deposit in the lower part of the Willwood Formation in the northwestern Bighorn Basin of Wyoming. Mammalian fossils from above and below the plant site indicate a latest Paleocene age, probably not more than 100 ky prior to the carbon isotope excursion that marks the Paleocene/Eocene boundary. Other fossil plants from the site are consistent with a late Paleocene age, including, Acer silberlingii, Deviacer, Metasequoia, Corylites, Platanus raynoldsii, and Macginitiea gracilis. The legume fossils at this site consist of a single type of fruit and a single type of leaf, which are inferred to represent the same plant species. The fossil fruits are stipitate, membranous, and narrowly winged on the placental suture. The fruits are ca. 10 cm long and contain numerous ovules. The seeds are transversely oriented and have a prominent radicular lobe, below which is the funiculus attachment. The co-occurring leaves are imparipinnate and leaflet position varies from opposite to alternate on a single leaf. This fossil taxon is not referable to an extant genus, but it is most comparable to several genera in the tribe Sophoreae, including Acosmium, Bowdichia, Diplotropis, Sakoanala, and Maackia. A precise understanding of relationships of this extinct taxon will require a phylogenetic analysis that includes these and other basal woody papilionoid legumes.

Key words: Leguminosae, Paleobotany, Paleocene, Tertiary