Bullockornis Rich, 1979 Dromornis is a genus of large to enormous prehistoric birds native to Australia during the Oligocene to Pliocene epochs.

The species were flightless, possessing greatly reduced wing structures but with large legs, similar to the modern ostrich or emu.

The male of the largest species, Dromornis stirtoni, is a contender for the tallest and heaviest bird, and possibly exhibited aggressive territorial behaviour.

[1] Owen's new taxon was published in a series on prehistoric birds, read before the Zoological Society of London then appearing in its Transactions.

Like those ratites who also evolved alongside mammals, the diversity of species was very low, apparently monotypes that emerged in succession and increased in size.

[13] The height of D. stirtoni would probably have met or exceeded the females of the tallest species of the genus Dinornis, the giant moa of New Zealand.

The fossil remains of a large femur were discovered at Peak Downs in Queensland, at a depth of around 55 metres (180 ft) in a well shaft.

A femur that he had noted in the appendix of Thomas Mitchell's explorations, found in a cave, did not allow him to confirm an alliance with any previously described species of large flightless birds.

[9] The new material had been found while digging a well at Peak Downs and forwarded to Owen via W. B. Clarke, a geologist employed by the state of New South Wales, with a remark by Gerard Krefft that placed it with the New Zealand moas of Dinornis.

Richard Owen found affinities and distinctions in an osteological comparison to species of the extinct Dinornis and the extant Dromaius (the emu) and proposed that it represented a new genus.

rested a short, thick femur, so filled with mineral matter (calc spar and iron pyrites) as to give the internal structure more the appearance of a reptilian than an ornithic bone.

[12] A comparative analysis that included this femur indicated morphological characters assignable to either Dromornis or a continuation of a Ilbandornis woodburnei lineage, allied to more gracile species of the family, but these results were not considered to be necessarily characteristic to any dromornithid genera.

The fossil specimens used to describe Dromornis murrayi have been dated to 26 million years ago, being discovered at a 'shelf', a rich layer of fossilised bones, that included leg and cranial remains of the unknown species.

[6] Dromornis planei, formerly placed in a separate genus Bullockornis, lived in the Middle Miocene, approximately 15 million years ago.

[citation needed] In 2010, Nguyen and Boles first suggested that Bullockornis represents another species of Dromornis on the basis of many common traits observed in the cranial and postcranial skeleton of both taxa and their close relationship strongly supported by their phylogenetic analyses.

[citation needed] This, in addition to the bird's tremendous size and earlier misclassification as a carnivore, gave rise to its colourful nickname.

Features of skull, including a very large beak suited to shearing, have made some researchers consider that the bird may have been carnivorous, but most currently agree that it was a herbivore.

[10] A species known from the Bullock Creek fossil fauna in the Northern Territory, the habitat during the time of deposition was a seasonally wet floodplain and river.

The area was occupied by herbivores favoring shrubland, horned turtles, marsupial tapirs and diprotodontid species, but the fauna associated with this site were rarely the forest dwelling paleospecies of the period.

[10] Dromornis planei remains are found with other large contemporaries, such as the diprotodont Neohelos, and the crocodiles Baru that preyed upon them as they came to the water's edge.

Large amounts of fragmentary material found at the Alcoota fossil site in Central Australia, the type location, are the only certain occurrence of the bird.

Rich proposed the specific epithet for fellow palaeontologist Ruben A. Stirton,[23][4] an American who undertook extensive research on Australian taxa.

[11] This height is thought to have exceeded the tallest species of the genus Dinornis, which were the giant moas of New Zealand, and the Elephant Birds of Madagascar.

Peter F. Murray and Patricia Vickers-Rich, in their work "Magnificent Mihirungs" (2004), utilised three varying scientific methods to derive the approximate weight and size of the D.

[24] The disparity in robustness was interpreted by the researchers as evidence of the biology of the species, behaviours such as incubation by the female, pair bonding, parental care and aggression while nesting, and courtship or display habits exhibited by extant waterfowl, the anseriforms.

The size and proportions of the head and its bill are comparable to that of mammals such as camels or horses The large bird had "stubby", reduced wings, which ultimately deemed it flightless.

Two forms of unearthed specimens are considered to be due to strong sexual dimorphism, concluded in a 2016 morphometric analysis using landmark based and actual measurements which also supported earlier conclusions regarding the species enormous size.

[26] This histological technique has been applied to other large and extinct avian species, including investigation into the paleobiology of the elephant birds Aepyornithidae.

[25] Osteohistological analysis of its femora, tibiotarsi, and tarsometatarsi has also revealed that D. stirtoni was extremely K-selected, likely requiring over a decade to reach its adult body size, after which skeletal maturity occurred and its growth rate retarded.

In recognition of the varying opinions, it is widely accepted that whilst the large bird may have occasionally scavenged or eaten smaller prey, they were mostly herbivorous.