The Phaethontiformes are an order of birds. They contain one extant family, the

tropicbird Tropicbirds are a family, Phaethontidae, of tropical pelagic seabirds. They are the sole living representatives of the order Phaethontiformes. For many years they were considered part of the Pelecaniformes, but genetics indicates they are most cl ...

s (Phaethontidae), and one extinct family

Prophaethontidae Prophaethontidae is an extinct family of Early Paleogene phaethontiforms that include the genera ''Lithoptila'' and ''Prophaethon'', as well as possibly '' Phaethusavis''. The anatomy of the prophaethontids were more similar to those of membe ...

from the early Cenozoic. Several fossil genera have been described. The tropicbirds were traditionally grouped in the order

Pelecaniformes The Pelecaniformes are an order of medium-sized and large waterbirds found worldwide. As traditionally—but erroneously—defined, they encompass all birds that have feet with all four toes webbed. Hence, they were formerly also known by such n ...

, which contained the pelicans, cormorants and shags,

darters The darters, anhingas, or snakebirds are mainly tropical waterbirds in the family Anhingidae, which contains a single genus, ''Anhinga''. There are four living species, three of which are very common and widespread while the fourth is rarer and ...

, gannets and boobies and

frigatebirds Frigatebirds are a family of seabirds called Fregatidae which are found across all tropical and subtropical oceans. The five extant species are classified in a single genus, ''Fregata''. All have predominantly black plumage, long, deeply forked ...

; in the Sibley-Ahlquist taxonomy, the Pelecaniformes were united with other groups into a large "Ciconiiformes". More recently this grouping has been found to be massively paraphyletic (missing closer relatives of its distantly related groups) and split again. Microscopic analysis of eggshell structure by Konstantin Mikhailov in 1995 found that the eggshells of tropicbirds lacked the covering of thick microglobular material of other Pelecaniformes. Some early studies in the last decade suggested Phaethontiformes were distantly related to

Procellariiformes Procellariiformes is an order (biology), order of seabirds that comprises four family (biology), families: the albatrosses, the Procellariidae, petrels and shearwaters, and two families of storm petrels. Formerly called Tubinares and still call ...

, but since 2004 they have been placed in

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 ...

, or in a lineage with no affinities with Procellariiformes, by the results of most recent molecular studies.Naish, D. (2012). "Birds." Pp. 379-423 in Brett-Surman, M.K., Holtz, T.R., and Farlow, J. O. (eds.), ''The Complete Dinosaur (Second Edition)''. Indiana University Press (Bloomington & Indianapolis). Jarvis, ''et al''.'s 2014 paper "Whole-genome analyses resolve early branches in the tree of life of modern birds" aligns the Phaethontiformes most closely with 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, and ...

and the

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

of the

Eurypygiformes Eurypygiformes is an order formed by the kagus, comprising two species in the family Rhynochetidae endemic to New Caledonia, and the sunbittern (''Eurypyga helias'') from the tropical regions of the Americas. Its closest relatives appear to b ...

, with these two clades forming the sister group of the "core water birds", the Aequornithes, and the Metaves hypothesis abandoned.

References

{{Taxonbar, from=Q12765692 Bird orders Seabirds