Hackett et al., 2008
Eurypygiformes is a clade formed by the kagus, comprising two species in the Rhynochetidae family endemic to New Caledonia, and the Sunbittern (Eurypyga helias) from the tropical regions of the Americas.
The Eurypygiformes's affinities are not too well resolved. They are two families from a Gondwanan lineage of birds. Suggested by some morphological characteristics they were initially classed as members of the family Ardeidae and later the Gruiformes.
When seen as a gruiform, the Kagu is generally considered related to the extinct adzebills from New Zealand and the Sunbittern from Central and South America. Recent studies do indicate that the Sunbittern is the closest living relative of the Kagu. For example, Fain & Houde found these to be certainly sister taxa. They and the mesites did not group with traditional Gruiformes in their study, but instead with their proposed clade Metaves, which also includes the Hoatzin, pigeons, Caprimulgiformes, flamingos, tropicbirds, Apodiformes, sandgrouse and grebes. The internal structure of this group was not well resolvable by their data, and contains numerous groupings which are almost certainly incorrect (such as Caprimulgidae and flamingos). Thus, the entire "Metaves" may be nothing more than a collection of lineages united by molecular homoplasies. Notwithstanding, the Kagu and Sunbittern – and possibly the adzebills – seem to form a distinct Gondwanan lineage of birds, possibly one order, possibly more, even though the relationships between them, the mesites, and the "core Gruiformes" are not yet resolved. It is notable, however, that the Sunbittern and the mesites possess powder down too, whereas the "core Gruiformes" do not.
While the Kagu is the only living species in the family Rhynochetidae, a larger species, the Lowland Kagu (Rhynochetos orarius), has been described from Holocene subfossil remains. The measurements of this species were 15% bigger than Rhynochetos jubatus, with no overlap in measurements except those of the forelimbs. Given that the sites where R. orarius were all lowland sites, and that no fossils of R. jubatus were found in these sites, the scientists that described the fossils suggested they represented highland and lowland species respectively. R. orarius is one of many species to have become extinct in New Caledonia after the arrival of humans. The validity of the species has been questioned by some authors, but accepted by others.
- Hackett, Shannon J.; et al. (2008-06-27). "A Phylogenomic Study of Birds Reveals Their Evolutionary History". Science 320 (5884): 1763–1768. doi:10.1126/science.1157704. PMID 18583609. Retrieved 2008-10-18.
- Fain, Matthew G. & Houde, Peter (2004)"Parallel radiations in the primary clades of birds." Evolution 58(11): 2558-2573. doi:10.1554/04-235 PMID 15612298
- Balouet, Jean C.; Storrs L. Olson (1989). "Fossil birds from Late Quaternary deposits in New Caledonia". Smithsonian Contributions to Zoology 469: 28–32.
- del Hoyo, J. Elliott, A. & Sargatal, J. (editors). (1996) Handbook of the Birds of the World. Volume 3: Hoatzin to Auks. Lynx Edicions. ISBN 84-87334-20-2
- Steadman, David (2006). Extinction and Biogeography in Tropical Pacific Birds. Chicago: University of Chicago Press. p. 158. ISBN 978-0-226-77142-7.