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The diverse Gruiformes contains a considerable number of living and extinct bird families with, on first sight, little in common. Gruiform means "crane-like."

Traditionally, a number of wading and terrestrial bird families that did not seem to belong to any other order were classified together as Gruiformes. These include, the very large cranes, or the relatively small and secretive water-loving crakes and rails (families with a wide distribution and a dozen or more member species), as well as a variety of very small families, some of them containing very few species, such as the Heliornithidae, the limpkin, or the trumpeters.

The gruiformes are morphologically diverse birds, with little apparent similarity. However, anatomical evidence indicates that several groups within the traditional Gruiformes do appear to be evolutionarily related, and this is further supported by molecular analyses. Other birds have been placed in this order more out of necessity to place them somewhere; this has caused the expanded Gruiformes to lack distinctive apomorphies. Luckily for phylogeneticists, more recent studies indicate that these "odd Gruiformes" are if at all only loosely related to the cranes, rails, and relatives ("core Gruiformes"). A notable feature in several gruiform lineages is that flighlessness evolves far more easily than in most other birds. About one-third of the extinct families were at least partially flightless, and numerous living and extinct flightless rails are known.

Systematics

The Gruiformes have been further divided into suborders. The Rallidae and the Heliornithidae are united in the Ralli; although the Rallidae are sometimes separated as order Ralliformes, this is not supported by the current data.^[1] Aramidae, Gruidae and the Psophiidae form the Grui. This arrangement is quite well-supported by the available molecular and morphological data.

A number of potential candidates for Gruiformes status have been rejected by Sibley & Ahlquist (1990) based on DNA-DNA hybridization studies. Their research led them to remove the Otididae, Eurypygidae, Rhynochetidae, Cariamidae, Mesitornithidae and Turnicidae from the Gruiformes. The Eurypygidae and Rhynochetidae were similar to the Ardeidae, and were therefore associated with certain birds that are traditionally interpreted as Ciconiiformes and others such as Charadriiformes in a vastly expanded Ciconiiformes. The Mesitornithidae and Cariamidae were indicated to be close to the Cuculiformes, but the Otididae did not seem to be close to other extant birds and on the placement of the Turnicidae they could reach no firm conclusion.

However, DNA-DNA hybridization is often insufficient to determine more than the most basic phylogenies; Sibley & Ahlquist's "Ciconiiformes" are not a natural group for example. While the families that they did not believe to belong to this group probably indeed do not do so, their proposed affinities according to Sibley and Monroe are generally seen as erroneous nowadays. Removing these taxa from the Gruiformes has the additional benefit of enabling the order to be defined by morphological apomorphies, as noted above.

DNA sequence analyses has led to the Plains Wanderer - which was formerly believed to be an aberrant buttonquail in the Gruiformes - being reassigned as a charadriiform wader related to jacanas (Paton et al. 2003, Thomas et al. 2004, van Tuinen et al. 2004). The buttonquails have in the meantime also turned out to be Charadriiformes, but form a lineage rather distinct from other members of that order (van Tuinen et al., 2005; Paton & Baker, 2006).

Fain & Houde (2004) found that the families split out of the Gruiformesby Sibley & Ahlquist do indeed seem to be distinct. Their data indicated that a motley clade of families might exist which they termed Metaves, but the suggested internal structure of these is hardly likely to be correct and the group as a whole is not robustly supported. Thus, the "Metaves" might be a pseudo-clade based on molecular homoplasies. However, the case for reducing the Gruiformes to a core of families is nonetheless strengthened as despite it not being known where the "odd" families should be placed, maintaining them in the Gruiformes is less and less supported either.

The kagu and sunbittern seem to constitute a distinct Gondwanan lineage, which might also include the extinct adzebills of New Zealand. Its relationship to the more distinct but possibly still somewhat related mesites and the core Gruiformes is unresolved. The seriemas and bustards are possibly lineages fairly basal in a clade uniting the "higher waterbirds", aquatic Neoaves such as Procellariiformes and Ciconiiformes but not the waterfowl which are Galloanserae.

The enigmatic Kagu may not be a gruiform

ORDER GRUIFORMES

• Suborder Ralli
  • Family Rallidae (crakes and rails)
  • Family Heliornithidae (finfoots and Sungrebe)
• Suborder Grui
  • Family †Eogruidae (fossil)
  • Family †Ergilornithidae (fossil) - may belong in Eogruidae
  • Family Gruidae (cranes)
  • Family Aramidae (Limpkin)
  • Family Psophiidae (trumpeters)
• Families incertae sedis and doubtful Gruiformes
  • Family †Parvigruidae (fossil)
  • Family †Songziidae (fossil)
  • Family †Gastornithidae (diatrymas) (fossil)
  • Family †Messelornithidae (Messel-birds)
  • Family †Salmilidae (fossil)
  • Family †Geranoididae (fossil)
  • Family †Bathornithidae (fossil)
  • Family †Idiornithidae (fossil)
  • Family †Phorusrhacidae (terror birds) (fossil)
  • Family Cariamidae (seriemas) - basal "higher waterbirds"?
  • Family Otididae (bustards) - basal "higher waterbirds"?
  • Family Eurypygidae (Sunbittern) - distinct order?
  • Family Rhynochetidae (Kagu) - distinct order?
  • Family †Aptornithidae (adzebills) (prehistoric) - distinct order? Grui?
  • Family Mesitornithidae (mesites) - distinct order?

The gruiforms are one of the older lineages of modern birds. Ancestral gruiforms were in all probability among the survivors of the Cretaceous–Tertiary extinction event c. 65 mya already. No unequivocal basal gruiforms are known from the fossil record. However, there are several genera which are not unequivocally assignable to the known families and which may occupy a more basal position:

• Propelargus (Late Eocene/Early Oligocene of Quercy, France) - cariamid or idornithid
• Rupelrallus (Early Oligocene of Germany) - rallid? parvigruid?
• Badistornis (Brule Middle Oligocene of Shannon County, USA) - aramid?
• Probalearica (Late Oligocene? - Middle Pliocene of Florida, USA, France?, Moldavia and Mongolia) - gruid? A nomen dubium?
• "Gruiformes" gen. et sp. indet. MNZ S42623 (Bathans Early/Middle Miocene of Otago, New Zealand) - Aptornithidae?
• Aramornis (Sheep Creek Middle Miocene of Snake Creek Quarries, USA) - gruid? aramid?
• Euryonotus (Pleistocene of Argentina) - rallid?
• Occitaniavis - cariamid or idiornithid, includes Geranopsis elatus

Other even more enigmatic fossil birdsa re occasionally suggested to belong into this order, such as the proposed Late Cretaceous family Laornithidae and the following taxa:

• Horezmavis (Bissekty Late Cretaceous of Kyzyl Kum, Uzbekistan)
• Telmatornis (Navesink Late Cretaceous?)
• Eobalearica (Ferghana Late? Eocene of Ferghana, Uzbekistan) - gruid?
• "Phasianus" alfhildae (Washakie B Late Eocene of Haystack Butte, USA)
• Talantatos (Late Eocene of Paris Bain, France)
• Telecrex (Irdin Manha Late Eocene of Chimney Butte, China) - rallid?
• Aminornis (Deseado Early Oligocene of Rio Deseado, Argentina) - aramid?
• Loncornis (Deseado Early Oligocene of Rio Deseado, Argentina) - aramid?
• Riacama (Deseado Early Oligocene of Argentina)
• Smiliornis (Deseado Early Oligocene of Argentina)
• Pseudolarus (Deseado Early Oligocene - Miocene of Argentina) - gruiform?
• Gnotornis (Brule Late Oligocene of Shannon County, USA) - aramid?
• Anisolornis (Santa Cruz Middle Miocene of Karaihen, Argentina) - aramid?