While initially considered a member of the family Opisthodactylidae, further examination of the fossil remains showed that it was more similar to the modern rhea. According to Gerald Mayr, Diogenornis is best considered a stem-group member of the Rheidae.[3] However, recent phylogenetic studies have shown a closer affiliation to Australian ratites, the cassowaries and emus. This may reevaluate the origins and distribution of this clade, expanding their range to the South American Paleocene, well before the appearance of Emuarius.[4] Recent findings nonetheless show that it co-existed with early rheas, meaning the ratite diversity of South America was very high during the Paleogene.[5]
Diogenornis possesses a rather narrow beak, similar to that of tinamous, lithornithids and cassowaries, as well as rather large wings.[2] These traits, both rather unspecialised, seem to suggest a then recent development from a flying ancestor.[4]
^ abAlvarenga, H.M.F. (1983) Uma ave ratitae do Paleoceno Brasileiro: bacia calcária de Itaboraí, Estado do Rio de Janeiro, Brasil. Boletim do Museu Nacional, Nova Série, Geologia 41: 1–8
^ abMayr, G. (2009). Paleogene fossil birds. Springer.
^ abH. Alvarenga, Diogenornis fragilis Alvarenga, 1985, restudied: a South American ratite closely related to Casuariidae, 2010
^Agnolin et al,『Unexpected diversity of ratites (Aves, Palaeognathae) in the early Cenozoic of South America: palaeobiogeographical implications Article in Alcheringa』An Australasian Journal of Palaeontology July 2016 DOI: 10.1080/03115518.2016.1184898
