ENANTIORNITHINE (AVES) TARSOMETATARSI AND THE AVIAN AFFINITIES OF THE LATE CRETACEOUS AVISAURIDAE

Several characters of the tarsometatarsus of the Cretaceous enantiornithine birds are discussed, with emphasis on the genus Avisaurus, which was considered a non-avian theropod taxon by its authors. Two synapomorphies (metatarsal IV reduced with respect to metatarsals II and III; well-developed knob on the anterior face of metatarsal II) relate Avisaurus to the remaining enantiornithine tarsometatarsi types and support reference of this taxon within the Enantiornithes. Three other synapomorphies (laterally compressed, J-shaped metatarsal I; anterior surface of the mid-shaft of metatarsal III strongly convex transversely; strong posterior projection of the internal rim of trochlea on metatarsal III) shared by Avisaurus and a Late Cretaceous enantiornithine from northwestern Patagonia further support its avian affinities. Avisaurus and the family Avisauridae are assigned to the avian subclass Enantiornithes.