New fossils from the London Clay show that the Eocene Masillaraptoridae are stem group representatives of falcons (Aves, Falconiformes)

The Eocene taxon Masillaraptoridae includes long-legged, raptorial birds, the phylogenetic affinities of which are poorly resolved. Here, fossils from the London Clay of Walton-on-the-Naze (Essex, U.K.) are described, which corroborate the hypothesis that masillaraptorids are stem group representatives of the Falconiformes (falcons). Two partial skeletons are assigned to a new genus and species, <i>Danielsraptor phorusrhacoides</i>, gen. et sp. nov., whereas a smaller species closely resembles <i>Masillaraptor parvunguis</i> from the German fossil site Messel. The new fossils show previously unrecognized derived characters shared by masillaraptorids and falconiform birds, and a phylogenetic analysis also supported a sister group relationship between the Masillaraptoridae and the Falconiformes. By contrast, the long and deep beak of masillaraptorids is remarkably similar to that of the cariamiform Phorusrhacidae and the tarsometatarsus also shows a resemblance to that of some Cariamiformes. The similarities between masillaraptorids and the Cariamiformes are notable, because most sequence-based analyses show the Falconiformes and Cariamiformes to be closely related, and a few studies even suggest a sister group relationship between both taxa. These similarities may be plesiomorphic for a more inclusive clade containing falconiforms and cariamiforms, but it is also possible that masillaraptorids represent a direct evolutionary link between the Falconiformes and Cariamiformes.