Eozygodactylus, Weidig, 2010

Weidig, Ilka, 2010, New Birds from the Lower Eocene Green River Formation, North America, Records of the Australian Museum 62 (1), pp. 29-44 : 36

Zygodactylidae Brodkorb, 1971

Included genera: Zygodactylus Ballmann, 1969 , Primoscens Harrison & Walker, 1977 , Primozygodactylus Mayr, 1998 , Eozygodactylus n.gen.

Taxonomic remarks. The Primoscenidae Harrison & Walker, 1977 were described based on a single carpometacarpus. Mayr (1998) included additional species which were represented by complete skeletons and emended the diagnosis, mentioning the resemblance of Primoscenidae to the Zygodactylidae Brodkorb, 1971 . Due to a new species of Zygodactylidae, Mayr (2008b) synonymized the Primoscenidae Harrison & Walker, 1977 with the Zygodactylidae Brodkorb, 1971 .

The two species from the Green River Formation fit well with Mayr’s (1998) emended diagnosis of Primoscenidae , being small zygodactyl birds with a long, slender tarsometatarsus and a large processus intermetacarpalis of the carpometacarpus. Furthermore, the new species exhibit the apomorphies of the clade ( Primozygodactylus + Zygodactylus ) listed by Mayr (2008b): (1) trochlea metatarsi II of tarsometatarsus much shorter than trochlea metatarsi III and (2) with well-developed, plantarly projecting process, and (3) trochlea metatarsi IV with large trochlea accessoria, which is separated by a furrow from the trochlea metatarsi IV. According to Mayr (1998), the very long and slender tarsometatarsus that distinctly exceeds the humerus in length is another possible apomorphy of this clade, a character that is also found in the species described below.