The Postcranial Osteology and Phylogenetic Position of the Middle Eocene Messelastur Gratulator Peters, 1994—a Morphological Link between Owls (strigiformes) and Falconiform Birds?

An articulated skeleton of Messelastur gratulator Peters, 1994, from the middle Eocene of Messel in Germany is described. This species was hitherto known from two skulls with associated vertebrae only, and has been tentatively classified in the Accipitridae (hawks). The new specimen shows that M. gratulator closely resembles the Lower Eocene Tynskya eocaena Mayr, 2000a, which is known from an articulated skeleton from the North American Green River Formation and isolated bones from the London Clay in England. Both species, M. gratulator and T. eocaena, are assigned to the new taxon Messelasturidae. The new specimen exhibits previously unknown osteological details bearing on the phylogenetic position of the Messelasturidae, most notably the absence of an ossified supratendineal bridge on the distal tibiotarsus. Cladistic analysis of 110 characters supports sister-group relationship between Messelastur and Tynskya, and shows the Messelasturidae to be the sister taxon of owls (Strigiformes). The clade (Messelasturidae + Strigiformes) is shown to be the sister group of the clade (Falconidae [falcons] + Accipitridae). Among the known fossil raptorial birds, the Messelasturidae most substantially differ from the extant taxa. They provide a morphological link between Strigiformes and Falconiformes (diurnal birds of prey), and support the highly disputed falconiform affinities of owls in combining derived tibiotarsus and tarsometatarsus characters of owls with a more plesiomorphic, ‘falcon-’ or ‘hawk-like’,