The Palaeogene Diomedeoididae are amongst the earliest representatives of procellariiform birds (albatrosses, tubenoses, and allies). Although several fossils of these birds have been reported in the past, many details of their osteology remained unknown. Here we describe a comprehensive collection of diomedeoidid fossils from the Rupelian stratotype in Belgium, which was found more than 100 years ago. The material includes all major limb elements as well as other cranial and postcranial bones, and allows the recognition of previously unknown features of phylogenetic significance. Based on these new osteological data, diomedeoidids were for the first time subjected to a phylogenetic analysis, which resulted in a position outside a clade including Hydrobatidae (northern storm-petrels), Pelecanoididae (diving-petrels), and Procellariidae (fulmars, petrels, shearwaters, and allies), either as the sister taxon of Diomedeidae (albatrosses) or as that of all crown group Procellariiformes. The latter placement is better supported by the osteological evidence, and diomedeoidids lack several apomorphies of crown group Procellariiformes. Previously unrecognized derived features are reported that support a monophyletic Hydrobatidae, thus contradicting recent proposals that Oceanitinae (southern storm-petrels) are the earliest diverging crown group Procellariiformes. The new fossils also have a bearing on the convoluted taxonomy of diomedeoidids, and Diomedeoides Fischer, 1985 is synonymized with Rupelornis van Beneden, 1871. Diomedeoides lipsiensis (Fischer, 1983) is synonymous with Rupelornis definitus (van Beneden, 1871), a species that exhibits a large size range.

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Paleontology