On the taxonomy and osteology of the Early Eocene North American Geranoididae (Aves, Gruoidea)
Whereas our knowledge of small arboreal Paleogene birds has greatly increased in the past years, that of the larger birds coexisting with them is far less detailed. Particularly poorly known and, hence, widely ignored are the North American Geranoididae, of which six species were described from the Early Eocene Willwood Formation. The published material of all of these consists only of incomplete leg elements, and previous descriptions and comparisons mainly focused on selected bones. Here, a revision of some geranoidids from the Willwood Formation is undertaken, and it is concluded that the taxon Geranoides Wetmore, 1933, the type genus of the Geranoididae, is a junior synonym of Palaeophasianus Shufeldt, 1913. Eogeranoides campivagus Cracraft, 1969 is poorly differentiated from Paragrus prentici (Loomis, 1906), and a synonymy of both species also seems likely. Complete leg bones of a large Eogeranoides/Paragrus-like species are reported, which constitute the most substantial record of a geranoidid from the Willwood Formation. It is detailed that geranoidids are likely to be stem group representatives of the Gruoidea, the clade including trumpeters, cranes, and allies, and the potential occurrence of geranoidids in the Eocene of Europe is discussed.