Phylogenetic affinities of the enigmatic avian taxon Zygodactylus based on new material from the early Oligocene of France

Until now, the avian taxon Zygodoctylus has been known only from distal tibiotarsi and tarsometatarsi from the early Miocene of Europe. Although the tarsometatarsus exhibits derived similarities to that of extant Pici (barbets, toucans, woodpeckers and their allies), it differs in many aspects and the phylogenetic affinities of Zygodactylus were uncertain. Here, a well-preserved skeleton with a tentatively referred skull is described from the early Oligocene of the Luberon, southern France, as a new species, Z. luberonensis. It is about ict million years older than previously assigned species and exhibits derived characters which support a clade comprising Zygodoctylus and Eocene zygodactyl birds that were assigned to the taxon 'Primoscenidae'. Primoscenidae Harrison & Walker, 1977 is synonymised with Zygodactylidae Brodkorb, 1971. Zygodactylus lacks derived features of crown group Piciformes and the derived similarities in the distal tarsometatarsi of Zygodactylus and the Pici are thus a striking example of convergence. Instead, the analysis results in a sister group relationship between Zygodactylidae and Passeriformes (passerines), and morphological characters are listed which support this hypothesis. Recognition of Zygodactylidae as the possible sister taxon of the Passeriformes implies that stem group representatives of passerines already existed by the early Eocene.