Eurypygiformes is an order formed by the kagus, comprising two species in the

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

Rhynochetidae endemic to

New Caledonia New Caledonia ( ; ) is a group of islands in the southwest Pacific Ocean, southwest of Vanuatu and east of Australia. Located from Metropolitan France, it forms a Overseas France#Sui generis collectivity, ''sui generis'' collectivity of t ...

, and the

sunbittern The sunbittern (''Eurypyga helias'') is a bittern-like bird of tropical regions of the Americas, and the sole member of the family Eurypygidae (sometimes spelled Eurypigidae) and genus ''Eurypyga''. It is found in Central and South America, an ...

(''Eurypyga helias'') from the tropical regions of the

Americas The Americas, sometimes collectively called America, are a landmass comprising the totality of North America and South America.''Webster's New World College Dictionary'', 2010 by Wiley Publishing, Inc., Cleveland, Ohio. When viewed as a sing ...

. Its closest relatives appear to be the tropicbirds of the tropical Atlantic, Indian, and Pacific oceans.

Classification

The affinities of Eurypygiformes are not very well resolved. The group consists of two families from a

Gondwana Gondwana ( ; ) was a large landmass, sometimes referred to as a supercontinent. The remnants of Gondwana make up around two-thirds of today's continental area, including South America, Africa, Antarctica, Australia (continent), Australia, Zea ...

n lineage of birds. Based on some morphological characteristics, they were initially classed as members of the family Ardeidae, and later the Gruiformes. According to Jarvis, et al.'s 2014 "Whole-genome analyses resolve early branches in the tree of life of modern birds", the group is distantly related to the Phaethontiformes. The oldest known fossil eurypygiform is an indeterminate fossil eurypygid from the

Early Eocene In the geologic timescale the Ypresian is the oldest age (geology), age or lowest stage (stratigraphy), stratigraphic stage of the Eocene. It spans the time between , is preceded by the Thanetian Age (part of the Paleocene) and is followed by th ...

-aged

Green River Formation The Green River Formation is an Eocene geologic formation that records the sedimentation in a group of intermountain lakes in three basins along the present-day Green River (Colorado River), Green River in Colorado, Wyoming, and Utah. The sedimen ...

of the United States. When seen as a gruiform, the

kagu The kagu or cagou (''Rhynochetos jubatus'') is a crested, long-legged, and bluish-grey bird endemism, endemic to the dense mountain forests of New Caledonia. It is the only surviving member of the genus ''Rhynochetos'' and the family Rhynoche ...

is generally considered related to the extinct adzebills from

New Zealand New Zealand () is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and List of islands of New Zealand, over 600 smaller islands. It is the List of isla ...

and the

from Central and

South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a considerably smaller portion in the Northern Hemisphere. It can also be described as the southern Subregion#Americas, subregion o ...

. Recent studies do indicate that the sunbittern is the closest living relative of the

. For example, Fain & Houde found these to be certainly

sister taxa In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and ...

and Furo ''et al.'' indicated a close phylogenetic relationship between them by

cytotaxonomy Cytotaxonomy is the classification of organisms using comparative studies of Chromosome, chromosomes during meosis. Description Cytotaxonomy is a branch of taxonomy that uses the characteristics of cellular structures to classify organisms. In cyt ...

; they suggest that their common ancestor was separated by the Gondwana vicariance in South America and New Caledonia, respectively. They and the mesites did not group with traditional Gruiformes in their study, but instead with their proposed

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

Metaves Metaves ("other birds") is a controversial group proposed by Fain & Houde (2004) and later rescued on the studies of Ericson ''et al.'' (2006)Ericson, P.G.P. ''et al.'' (2006Diversification of Neoaves: integration of molecular sequence data and f ...

, which also includes the

hoatzin The hoatzin ( ) or hoactzin ( ) (''Opisthocomus hoazin'') is a species of tropical bird found in swamps, riparian forests, and mangroves of the Amazon and the Orinoco basins in South America. It is the only extant species in the genus ''Opisthoco ...

, pigeons,

Caprimulgiformes Nightjars are medium-sized nocturnal or crepuscular birds in the family Caprimulgidae and order Caprimulgiformes, characterised by long wings, short legs, and very short bills. They are sometimes called bugeaters, their primary source of food ...

, flamingos, tropicbirds,

Apodiformes The Apodiformes is an Order (biology), order, or Taxonomy, taxonomic grouping, of Bird, birds which traditionally contained three living Family (biology), families—the Swift (bird), Apodidae (swifts), the Treeswift, Hemiprocnidae (treeswifts), ...

sandgrouse Sandgrouse is the common name for Pteroclidae (), a family (biology), family of sixteen species of bird, members of the order Pterocliformes (). They are traditionally placed in two Genus, genera. The two central Asian species are classified as ...

and grebes. The internal structure of this group was not well resolvable by their data, and contains numerous groupings not otherwise corroborated (such as Caprimulgidae and flamingos). The usefulness and monophyly of "Metaves" is therefore unclear. Notwithstanding, the

and sunbittern – and possibly the adzebills – seem to form a distinct

n 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 The down of birds is a layer of fine feathers found under the tougher exterior feathers. Very young birds are clad only in down. Powder down is a specialized type of down found only in a few groups of birds. Down is a fine thermal insulator and p ...

, whereas the "core Gruiformes" do not. While the

is the only living species in the family Rhynochetidae, a larger species, the lowland kagu (''Rhynochetos orarius''), has been described from late

Holocene The Holocene () is the current geologic time scale, geological epoch, beginning approximately 11,700 years ago. It follows the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene to ...

subfossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

remains. The body measurements of this species were 15% greater than '' Rhynochetos jubatus'', with no overlap in measurements except those of the wings. Given that the sites where ''R. orarius'' has been found are all lowland sites, and that no fossils of ''R. jubatus'' were found in these sites, the scientists who 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,del Hoyo, J. Elliott, A. & Sargatal, J. (editors). (1996) ''Handbook of the Birds of the World. Volume 3: Hoatzin to Auks''. Lynx Edicions. but accepted by others. Fossil Messelornithidae once included in Eurypygiformes are now considered the oldest known members of Ralloidea ( Gruiformes).

Species by global population

References

{{Taxonbar, from=Q147907 Neognathae Bird orders