Diogenornis is an extinct genus of ratites, that lived during the Early Eocene (Itaboraian to Casamayoran in the SALMA classification). It was described in 1983 by Brazilian scientist Herculano Marcos Ferraz de Alvarenga based on fossils found in the Itaboraí Formation in southeastern Brazil. The type species is D. fragilis. While initially considered a member of the family , 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. It grew to about two thirds the size of the modern greater rhea, at about 90 centimetres (3.0 ft) of height.