The bird order Casuariiformes has four surviving members: the three species of cassowary, and the only remaining species of emu. The emus are classified in the family Dromaiidae, while the cassowaries are all located within the Casuariidae family.
All four living members of the order are very large flightless birds native to Australia-New Guinea, but some possible extinct taxa occurred in other landmasses.
Systematics and evolution
The emus form a distinct family, characterized by legs adapted for running. As with all ratites, there are several contested theories concerning their evolution and relationships. As regards this order, it is especially interesting whether emus or cassowaries are the more primitive form: the latter are generally assumed to retain more plesiomorphic features, but this does not need to be true at all; the fossil record is also ambiguous, and the present state of genomics does not allow for sufficiently comprehensive analyses. A combination of all these approaches with considerations of plate tectonics at least is necessary for resolving this issue.
The total number of cassowary species described, based on minor differences in casque shape and color variations, formerly reached nine. Now, however, only 3 species are recognized, and most authorities only acknowledge few subspecies or none at all.
The fossil record of casuariforms is interesting, but not very extensive. Regarding fossil species of Dromaius and Casuarius, see their genus pages.
Some Australian fossils initially believed to be from emus were recognized to represent a distinct genus, Emuarius, which had a cassowary-like skull and femur and an emu-like lower leg and foot. In addition, the first fossils of mihirungs were initially believed to be from giant emus, but these birds were completely unrelated.
It has been suggested that the South American genus Diogenornis was a casuraiiform bird, instead of a member of the current South American ratite lineage, the rheas. If this was the case, not only it expanded the fossil range of this lineage spatially, but temporally as well, since Diogenornis occurs in the late Paleocene and is among the earliest known ratites. The Pliocene indian Hypselornis has also been linked to emus and cassowaries, but no recent (post-30's) studies have been performed in this extremely obscure taxon.
†D. ocypus Miller 1963 [Casuarius ocypus (Miller 1963)]
D. novaehollandiae (Latham 1790) Vieillot 1816 [Casuarius novae-hollandiae Latham 1790; †Dromaius gracilipes De Vis 1892; †Dromaius patricius De Vris 1888; Metapteryx bifrons De Vis 1892] (emu)
†D. n. minor Spencer 1906 [Dromaeus minor (sic) Spencer 1906; Dromaius novaehollandiae ater Vieillot 1817; Dromiceius spenceri (sic) Mathews 1912; Dromaeus bassi Legge 1907; Dromaius ater Vieillot 1817 nomen novum; Peronista spenceri (Mathews 1912)] (King Island/black emu)
