''Zhylgaia'' is a

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

fossil 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 preserve ...

bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...

. Its remains, consisting of two partial humeri, were recovered from an upper

Paleocene The Paleocene ( ), or Palaeocene, is a geological epoch (geology), epoch that lasted from about 66 to 56 mya (unit), million years ago (mya). It is the first epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), ...

deposit in

Kazakhstan Kazakhstan, officially the Republic of Kazakhstan, is a landlocked country primarily in Central Asia, with a European Kazakhstan, small portion in Eastern Europe. It borders Russia to the Kazakhstan–Russia border, north and west, China to th ...

. The relationships of this genus are unknown; it was initially placed in the Presbyornithidae, which at that time were believed to be some sort of "transitional shorebird". Upon recognition that the presbyornithids were more likely a prehistoric lineage of fairly advanced

waterfowl Anseriformes is an order of birds also known as waterfowl that comprises about 180 living species of birds in three families: Anhimidae (three species of screamers), Anseranatidae (the magpie goose), and Anatidae, the largest family, which i ...

, ''Zhylgaia'' was assigned to the

form taxon Form classification is the classification of organisms based on their morphology (biology), morphology, which does not necessarily reflect their biological relationships. Form classification, generally restricted to palaeontology, reflects uncer ...

" Graculavidae", an assemblage of Late Cretaceous and

shorebirds which are not a natural

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 ...

but merely an assemblage of superficially similar birds. All that can be said about this taxon is that it was a modern bird, most likely a neognath. In 2008, ''Zhylgaia'' was assigned to

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

—an extinct family related to modern

tropicbirds 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 ...

—on the basis of its anatomy and size closely matching the prophaethontid ''

Lithoptila ''Lithoptila abdounensis'' is an extinct species of prophaethontid seabird that lived during the Palaeogene period. It is a distant relative of the tropicbirds. Distribution ''L. abdounensis'' lived in Morocco, its fossils having been foun ...

''.

References

* Nessov, L. A. (1988): ew Birds from the Cretaceous and Paleogene of Central Asia and Kazakhstan and Natural Conditions in Their Habitats ''Trudy Zoologicheskogo Instituta Akademii Nauk SSSR'' 182: 116–123. rticle in Russian Phaethontiformes Paleocene birds Paleogene birds of Asia Fossils of Kazakhstan Fossil taxa described in 1988 {{Phaethontiformes-stub