Notes on the osteology and phylogenetic affinities of the Oligocene Diomedeoididae (Aves, Procellariiformes)

New specimens of the procellariiform taxon Diomedeoididae are reported from the early Oligocene (Rupelian) deposits of Wiesloch-Frauenweiler in southern Germany. Two skeletons belong to Diomedeoides brodkorbi, whereas isolated legs of larger individuals are tentatively assigned to D. lipsiensis, a species which has not yet been reported from the locality. The fossils allow the recognition of some previously unknown osteological features of the Diomedeoididae, including the presence of a vestige of the hallux. Diomedeoidids are characterized by extremely wide phalanges of the third and fourth toes, which also occur in some species of the extant procellariiform Oceanitinae (southern storm-petrels). The poorly developed processus supracondylaris dorsalis of the humerus supports a position of these Oligocene tubenoses outside a clade including the Diomedeidae (albatrosses), Procellariidae (shearwaters and allies), and Pelecanoididae (diving-petrels). It is hypothesized that like modern Oceanitinae, which have an equally short supracondylar process, diomedeoidids probably employed flap-gliding and used their immersed feet to remain stationary.