Further Hassiavis specimens from the Eocene of Messel (Germany) and the phylogenetic affinities of the Archaeotrogonidae (Aves, Strisores)

We report new specimens of the archaeotrogon Hassiavis laticauda from the latest early/earliest middle Eocene of Messel (Germany), including the first complete and well-preserved skeleton of the species. Evaluating the new and previously published fossils, we find that the proposed sister group relationship between Hassiavis and the Aegotheliformes cannot be supported. Previously unknown features are reported, which also conflict with a sister group relationship between the Archaeotrogonidae and the Caprimulgiformes (absence of a large lacrimal and presence of 18 presacral vertebrae). A derived morphology of the quadrate conforms to the hypothesis that archaeotrogons are the sister taxon of the Daedalornithes, the clade formed by the Aegotheliformes and Apodiformes, but further osteological data are required for a firmly supported phylogenetic placement of archaeotrogons. We note that the wide interorbital section of the frontal bones of Hassiavis contrasts with the very narrow interorbital bar of the Aegotheliformes, although it is currently not possible to infer the activity pattern of archaeotrogons from this difference.