Reappraisal of Eocypselus—a stem group apodiform from the early Eocene of Northern Europe

Abstract

Eocypselus vincenti is a small swift-like bird from the early Eocene of England and Denmark, whose phylogenetic affinities are controversial. Here, a new skeleton from the Danish Fur Formation is reported, in which for the first time the skull and pelvis are preserved. Further described are previously overlooked features of the other Fur Formation specimens, which bear on the systematic position of the species. It is detailed that E. vincenti falls outside crown group Apodiformes and represents the earliest diverging apodiform bird, with its osteology providing a link between that of apodiform birds and their morphologically disparate sister taxon, the Aegothelidae (owlet-nightjars). The poorly developed processus internus indicis on the proximal phalanx of the major wing digit indicates that Eocypselus had less elongated outer primaries than extant Apodiformes. Together with differences in the pectoral girdle, such as the lack of a coracoid/furcula articulation, this suggests that the fossil taxon was less well adapted than its extant relatives to feeding on the wing.