Belongs within: Palaeognathae.
The Casuariiformes are an Australasian clade of large flightless birds including the emus (Dromaius) and cassowaries (Casuarius). The emu Dromaius novaehollandiae is an inhabitant of open habitats in mainland Australia, wheras the less cursorial cassowaries are found in thick forest in New Guinea and northern Queensland. Cassowaries have mostly black plumage with bare, bright blue head and neck, often with one or two bright, contrastingly coloured wattles hanging from the neck, and a horny casque on the top of the head. The emu has shaggy grey plumage with visible blue skin on much of the head and neck (Arlott & van Perlo 2021).
Characters (from here): Large graviportal flightless birds with 3 toes (hind toe lost). Sexes alike in coloration; plumage sombre brown or brownish black, naked blue skin on the head and neck. Plumage loose and hairlike due to the lack of barbules; no differentiated tail feathers. Wings and keel on sternum reduced, legs enlarged. Vestigial clavicles present in shoulder girdle; humerus much reduced, shorter than combined forewing and manus. Manus with a single digit (the third), bearing a long claw. Palate palaeognathous or dromaeognathous, with large vomers extending back to separate short palatines.
Casuariidae [Casuariiformes, Casuariimorphae]
|--Casuarius Brisson 1760B94 (see below for synonymy)
| |--C. australis Wall in Gould 1865 [incl. C. johnsoni Krefft 1867]N13
| |--C. bennettii Gould 1857APB99
| |--C. bicarunculatusS66
| |--C. casuariusML14
| |--C. galeatusN13
| |--C. lydekkeriF71 [=C. bennetti lydekkeriFP64]
| `--C. uniappendiculatus [incl. C. kaupi Rosenberg 1861]S66
`--Dromaiinae [Dromaiidae, Dromiceiidae]M09
|--Metapteryx bifronsWH02
|--EmuariusM09
| |--E. gidju (Patterson & Rich 1987) [=Dromaius gidju]M09
| `--E. guljaruba Boles 2001M09
`--Dromaius Vieillot 1816 [incl. Dromiceius Vieillot 1816]B94
|--D. aterFS01
|--D. baudinianusFS01
|--‘Dromaeus’ minorA14
|--D. novaehollandiae (Latham 1790)CC10 [=Casuarius novaehollandiaeCC10, Dromiceius novaehollandiaeFP64]
| |--D. n. novaehollandiaeWS48
| |--D. n. ater [incl. D. minor]F02
| |--D. n. baudinianus [incl. D. n. diemenianus]F02
| |--D. n. diemenensisD81
| |--D. n. rothschildi (Mathews 1912) [=Dromiceius novaehollandiae rothschildi]WS48
| `--D. n. woodwardi (Mathews 1912) [=Dromiceius novaehollandiae woodwardi]WS48
`--D. patriciusF71
Casuarius Brisson 1760B94 [incl. Cela Möhring 1752 (pre-Linnean) non Cela Reichenbach 1852B94, Hippalectryo Gloger 1842B94; CasuariinaeM09, Celidae, Hippalectryonidae]
*Type species of generic name indicated
References
[A14] Anderson, W. 1914. Notes on the occurrence of the sand-rock containing bones of extinct species of marsupials (emu, kangaroo, wombat, etc.) on King Island, Bass Strait, Tasmania. Records of the Australian Museum 10 (9): 275–283.
[APB99] Aplin, K. P., J. M. Pasveer & W. E. Boles. 1999. Late Quaternary vertebrates from the Bird’s Head Peninsula, Irian Jaya, Indonesia, including descriptions of two previously unknown marsupial species. Records of the Western Australian Museum Supplement 57: 351–387.
[B94] Bock, W. J. 1994. History and nomenclature of avian family-group names. Bulletin of the American Museum of Natural History 222: 1–281.
[D81] Day, D. 1981. The Doomsday Book of Animals: A unique natural history of three hundred vanished species. Ebury Press: London.
[FP64] Fisher, J., & R. T. Peterson. 1964. The World of Birds: A comprehensive guide to general ornithology. Macdonald: London.
[FS01] Flannery, T., & P. Schouten. 2001. A Gap in Nature: Discovering the World’s Extinct Animals. Text Publishing: Melbourne.
[F71] Fletcher, H. O. 1971. Catalogue of type specimens of fossils in the Australian Museum, Sydney. Australian Museum Memoir 13: 1–167.
[F02] Fuller, E. 2002. Foreword. Extinct birds. In: Hoyo, J. del, A. Elliott & J. Sargatal (eds) Handbook of the Birds of the World vol. 7. Jacamars to Woodpeckers pp. 11–68. Lynx Edicions: Barcelona.
[M09] Mayr, G. 2009. Paleogene Fossil Birds. Springer.
[ML14] Mitchell, K. J., B. Llamas, J. Soubrier, N. J. Rawlence, T. H. Worthy, J. Wood, M. S. Y. Lee & A. Cooper. 2014. Ancient DNA reveals elephant birds and kiwi are sister taxa and clarifies ratite bird evolution. Science 344 (6186): 898–900.
[N13] North, A. J. 1913. On the early history of the Australian cassowary (Casuarius australis, Wall). Records of the Australian Museum 10 (4): 39–48, pls 8–9.
[S66] Schlegel, H. 1866. Extract from a letter addressed to Mr. P. L. Sclater respecting Casuarius uniappendiculatus. Proceedings of the Zoological Society of London 1866: 168.
[WS48] Whittell, H. M., & D. L. Serventy. 1948. A systematic list of the birds of Western Australia. Public Library, Museum and Art Gallery of Western Australia, Special Publication 1: 1–126.
[WH02] Worthy, T. H., & R. N. Holdaway. 2002. The Lost World of the Moa: Prehistoric life of New Zealand. Indiana University Press: Bloomington (Indiana).