The order Gruiformes, for which even familial composition remains controversial, is perhaps the least well understood avian order from a phylogenetic perspective. The history of the systematics of the order is presented, and the ecological and biogeographic characteristics of its members are summarized. Using cladistic techniques, phylogenetic relationships among fossil and modern genera of the Gruiformes were estimated based on 381 primarily osteological characters; relationships among modern species of Grues (Psophiidae, Aramidae, Gruidae, Heliornithidae and Rallidae) were assessed based on these characters augmented by 189 characters of the definitive integument. A strict consensus tree for 20,000 shortest trees compiled for the matrix of gruiform genera (length = 967, CI = 0.517) revealed a number of nodes common to the solution set, many of which were robust to bootstrapping and had substantial support (Bremer) indices. Robust nodes included those supporting: a sister relationship between the Pedionomidae and Turnicidae; monophyly of the Gruiformes exclusive of the Pedionomidae and Turnicidae; a sister relationship between the Cariamidae and Phorusrhacoidea; a sister relationship between a clade comprising Eurypyga and Messelornis and one comprising Rhynochetos and Aptornis; monophyly of the Grues (Psophiidae, Aramidae, Gruidae, Heliornithidae and Rallidae); monophyly of a clade (Gruoidea) comprising (in order of increasingly close relationship) Psophia, Aramus, Balearica and other Gruidae, with monophyly of each member in this series confirmed; a sister relationship between the Heliornithidae and Rallidae; and monophyly of the Rallidae exclusive of Himantornis. Autapomorphic divergence was comparatively high for Pedionomus, Eurypyga, Psophia, Himantornis and Fulica; extreme autapomorphy, much of which is unique for the order, characterized the extinct, flightless Aptornis.
In the species–level analysis of modern Grues, special efforts were made to limit the analytical impacts of homoplasy related to flightlessness in a number of rallid lineages. A strict consensus tree of 20,000 shortest trees compiled (length = 1232, CI = 0.463) confirmed the interfamilial relationships resolved in the ordinal analysis and established a number of other, variably supported groups within the Rallidae. Groupings within the Rallidae included: monophyly of Rallidae exclusive of Himantornis and a clade comprising Porphyrio (including Notornis) and Porphyrula; a poorly resolved, basal group of genera including Gymnocrex, Habroptila, Eulabeornis, Aramides, Canirallus and Mentocrex; an intermediate grade comprising Anurolimnas, Amaurolimnas, and Rougetius; monophyly of two major subdivisions of remaining rallids, one comprising Rallina (paraphyletic), Rallicula, and Sarothrura, and the other comprising the apparently paraphyletic ‘long–billed’ rails (e.g. Pardirallus, Cyanolimnas, Rallus, Gallirallus and Cabalus and a variably resolved clade comprising ‘crakes’ (e.g. Atlantisia, Laterallus and Porzana, waterhens (Amaurornis), moorhens (Gallinula and allied genera) and coots (Fulica). Relationships among ‘crakes’ remain poorly resolved; Laterallus may be paraphyletic, and Porzana is evidently polyphyletic and poses substantial challenges for reconciliation with current taxonomy. Relationships among the species of waterhens, moorhens and coots, however, were comparatively well resolved, and exhaustive, fine–scale analyses of several genera (Grus, Porphyrio, Aramides, Rallus, Laterallus and Fulica) and species complexes (Porphyrio porphyrio –group,Gallirallus philippensis –group and Fulica americana –group) revealed additional topological likelihoods. Many nodes shared by a majority of the shortest trees under equal weighting were common to all shortest trees found following one or two iterations of successive weighting of characters. Provisional placements of selected subfossil rallids (e.g. Diaphorapteryx, Aphanapteryx and Capellirallus ) were based on separate heuristic searches using the strict consensus tree for modern rallids as a backbone constraint.
These analyses were considered with respect to assessments of robustness, homoplasy related to flightlessness, challenges and importance of fossils in cladistic analysis, previously published studies and biogeography, and an annotated phylogenetic classification of the Gruiformes is proposed.