LAUSR

Premise of research. Fossil leaves from the early Eocene Green River Formation of Wyoming and late Eocene Florissant Formation of Colorado have been studied and described here as two species in the monospecific extant genus Arcoa (Leguminosae, subfamily Caesalpinioideae). The single living species of Arcoa is endemic to the Caribbean island of Hispaniola. The species from Florissant has been known since the late 1800s but has been incorrectly treated as several different legume genera. Methodology. The compression fossils were studied using standard methods of specimen preparation and microscopy. Fossils were compared with extant taxa using herbarium collections at the Field Museum and Smithsonian Institution. Pivotal results. The fossil bipinnate leaves exhibit an unusual morphological feature of the primary rachis, which terminates in a triad of pinnae, one terminal flanked by two lateral pinnae, all of which arise from the same point at the apex of the rachis. This feature, combined with other features that are diagnostic of the family Leguminosae or subgroups within it, allows the taxonomic affinities of the fossil leaves to be definitively determined as representing the extant genus Arcoa, which is restricted to the Caribbean island of Hispaniola today. Conclusions. The fossil leaves described in this article demonstrate that the monospecific genus Arcoa was more diverse and had a much more widespread distribution in the past than it has today. Although the two fossil species are clearly referable to the same genus, differences between them in leaf size are consistent with differences in climate that are inferred for the more tropical Fossil Lake flora of the Early Eocene Green River Formation as compared with the warm temperate Late Eocene Florissant flora.