structural:expansion
Systematics

There are only two suprafamilial clades (natural groups) among the birds traditionally classified as Gruiformes. Rails (Rallidae), flufftails (Sarothruridae), finfoots and sungrebe (Heliornithidae), adzebills (Aptornithidae), trumpeters (Psophiidae), limpkin (Aramidae), and cranes (Gruidae) compose the suborder Grues and are termed "core-Gruiformes". These are the only true Gruiformes. The suborder Eurypygae includes the kagu (Rhynochetidae) and sunbittern (Eurypygidae). These are not even remotely related to Grues. The families of mesites or roatelos (Mesitornithidae), button-quails (Turnicidae), Australian plains-wanderer (Pedionomidae), seriemas (Cariamidae), and bustards (Otididae) each represent distinct and unrelated lineages. Many families known only from fossils have been assigned to the Gruiformes, e.g., Ergilornithidae, Phorusrhacidae, Messelornithidae, Eogruidae, Idiornithidae, Bathornithidae, to name just a few (see below). Though some of these are superficially 'crane-like' and the possibility exists that some may even be related to extant families traditionally included in the Gruiformes, there are no completely extinct families that can be confidently assigned to core-Gruiformes.

The traditional order Gruiformes was established by the influential German avian comparative anatomist Max Fürbringer (1888). Over the decades, many ornithologists suggested that members of the order were in fact more closely related to other groups (reviewed by Olson 1985, Sibley and Ahlquist 1990). For example, it was thought that sunbittern might be related to herons and that seriemas might be related to cuckoos. Olson and Steadman (1981) were first to correctly disband any of the traditional Gruiformes. They recognized that the Australian plains-wanderer (family Pedionomidae) was actually a member of the shorebirds (order Charadriiformes) based on skeletal characters. This was confirmed by Sibley and Ahlquist (1990) based on DNA–DNA hybridization and subsequently by Paton et al. (2003), Paton and Baker (2006) and Fain and Houde (2004, 2006). Sibley and Ahlquist furthermore removed button-quails (Turnicidae) from the Gruiformes based on large DNA–DNA hybridization distances to other supposed Gruiformes. However, it was not until the work of Paton et al. (2004) and Fain and Houde (2004, 2006) that the correct placement of buttonquails within the shorebirds (order Charadriiformes) was documented on the basis of phylogenetic analysis of multiple genetic loci. Using 12S ribosomal DNA sequences, Houde et al. (1997) were the first to present molecular genetic evidence of gruiform polyphyly, although apparently they were not convinced by it. However, on the basis of numerous additional sequence data, it has been shown decisively that the traditionally recognized Gruiformes consist of five to seven unrelated clades (Fain and Houde 2004, Ericson et al. 2006, Hackett et al. 2008).

Fain and Houde (2004) proposed that Neoaves are divisible into two clades, Metaves and Coronaves, although it has been suggested from the start that Metaves may be paraphyletic (Fain and Houde 2004, Ericson et al. 2006, Hackett et al. 2008). Sunbittern, kagu, and mesites all group within Metaves but all the other lineages of "Gruiformes" group either with a collection of waterbirds or landbirds within Coronaves. This division has been upheld by the combined analysis of as many as 30 independent loci (Ericson et al. 2006, Hackett et al. 2008), but is dependent on the inclusion of one or two specific loci in the analyses. One locus, i.e., mitochondrial DNA, contradicts the strict monophyly of Coronaves (Morgan-Richards et al. 2008), but phylogeny reconstruction based on mitochondrial DNA is complicated by the fact that few families have been studied, the sequences are heavily saturated (with back mutations) at deep levels of divergence, and they are plagued by strong base composition bias.

The kagu and sunbittern are one another's closest relatives. It had been proposed (Cracraft 2001) that they and the recently extinct adzebills (family Aptornithidae) from New Zealand constitute a distinct Gondwanan lineage. However, sunbittern and kagu are believed to have diverged from one another long after the break-up of Gondwanaland and the adzebills are in fact members of the Grues (Houde et al. 1997, Houde 2009). The seriemas and bustards represent distinct lineages within neoavian waterbirds.