Fruit of Staphylea (Staphyleaceae) from the Oligocene of Montana, USA

Staphylea has abundant isolated seeds in the fossil record, but no fossil fruits of this genus have been confirmed before. In this paper, we report a capsular fruit of Staphylea from the Oligocene Renova Formation of Woodworth, western-central Montana, USA. Detailed investigation of this fruit gives new insights on the phylogenetic and biogeographic history of the genus. Morphological comparison with modern Staphylea fruits shows its closest resemblance to the extant Asian species, S. bumalda DC., which is similar in fruit shape, size, and pericarp venation, but differs by the angle of interlobal sinus and the stipe development. We establish a new fossil species, S. woodworthensis Zhu & Manchester sp. nov. for this fossil fruit. This discovery indicates that Staphylea was already established in western North America at least by the Oligocene, which is inconsistent with the recent interpretation of a late Oligocene origin of the genus in Europe. The distribution of fossils indicates an earlier diversification of Staphylea, along with other staphyleaceous genera, back to early Paleogene in North America. The genus subsequently retracted its range to the east and south in North America as evidenced by younger fossils and its modern geographic range.