Dromornis


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. They were likely to have been predominantly, if not exclusively, herbivorous browsers. The male of the largest species, Dromornis stirtoni, is a contender for the tallest and heaviest bird, and possibly exhibited aggressive territorial behaviour. They belong to the family Dromornithidae, extinct flightless birds known as mihirungs.

Taxonomy

The genus was erected to separate a new species, Dromornis australis, from the previously described Dinornis, another lineage of ancient large and flightless birds found in New Zealand that was earlier described by Richard Owen in 1843. A femur that was forwarded to England, probably a dromornithid and since lost, suggested an Australian genus, but Owen withheld publication for many years. The type specimen, another femur, was found in a well at Peak Downs, Queensland, and subsequently described by Owen in 1872. Owen's new taxon was published in a series on prehistoric birds, read before the Zoological Society of London then appearing in its Transactions.
The name of the genus is derived from Ancient Greek, dromos meaning running, a race, and ornitho, a bird.
The genus and family are referred to as mihirung, distinguishing these birds from the giant emus. 'Mihirung paringmal' is an Aboriginal word from the Tjapwuring people of Western Victoria and it means 'giant bird'.
The placement of these dromornithid species may be summarised as
Dromornithidae
The Dromornis lineage is proposed to represent a monotypic succession, from earliest to latest these are D. murrayi, D. planei, D. stirtoni, and this species, D. australis.
The dromornithid family are sometimes known by appellations such as Stirton's mihirung to refer to each species. Nicknames describing the species as 'thunderbirds' etc. have appeared in reports of their discovery, later terms such as "demon ducks" refer to their relationship to the extant waterfowl of the galloanseres.

Description

Dromornis is a genus of large to gigantic flightless birds of the Dromornithidae family. Members of this family lived from 8 million years ago until less than 30,000 years ago. Although they looked like giant emus, Dromornis and its relatives are more closely related to the earliest waterfowl of the Anseriformes order or a basal galliform. Comparative studies using endocranial reconstructions of dromornithids, Ilbandornis and three Dromornis species, suggest that the head and bill of the Dromornis lineage became foreshortened.
The species resemble large birds of the Northern hemisphere, the Paleognathaes, of whom some descendants are known as ostriches and their allies. 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.
Dromornis stirtoni is amongst the largest known birds, although Aepyornis maximus, a species of elephant bird from Madagascar, were likely just as heavy, if not heavier. 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.

Species

''D. australis''

Dromornis australis fossils are found in Pliocene deposits of Australia. They were once considered the smallest species of the genus Dromornis, around three quarters the size of Dromornis stirtoni, until the discovery of Dromornis planei specimens were described in 2016.

Discovery

The fossil remains of a large femur were discovered at Peak Downs in Queensland, at a depth of around in a well shaft. This type of locality was described as an assemblage of boulders and pebbles beneath around thirty feet of alluvial soil; the femur was located over a boulder in the rock beds. The description of Dromornis australis by Richard Owen, best known for extensive work on the paleontology of Australian mammals, was the first of an extinct Australian avian species.
Owen had previously sought evidence of Dinornis in the palaeontological collections of early Australian excavations. 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. Owen withheld describing that specimen, now thought lost, until the type for this species emerged many years later. 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 and proposed that it represented a new genus.

Description

The species is known by the right femur, around twelve inches long, obtained at the Peak Downs site. The details of its deposition accompanied Owen's description, "The well was sunk through 30 feet of the black trappean alluvial soil common in that part of Australia, and then through 150 feet of drift pebbles and boulders, on one of which boulders rested a short, thick femur, so filled with mineral matter as to give the internal structure more the appearance of a reptilian than an ornithic bone."
Owen notes the specimen was reported by W. B. Clarke, attributing it to Dinornis, in the Geological Magazine several years before.
The femur is similar in size to Ilbandornis woodburnei, another dromornithid species. Other osteological features of the specimens have been compared to Dromornis stirtoni, the gigantic "Stirton's thunderbird".
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. A fragment of synsacrum found at the Canadian deep lead mine near Gulgong has been tentatively assigned to Dromornis, the slight possibility that it is referable to this species might represent the continuation of the lineage as a smaller species into the Pliocene.

''D. murrayi''

Dromornis murrayi was described in 2016 using specimens discovered amongst the Riversleigh fauna in Queensland, Australia. The period during which it existed was the Oligocene to early Miocene, making this the earliest known species of the genus Dromornis. The size of these mihirungs was also determined to be the smallest of its genus. Dromornis murrayi was described from specimens of cranial and post cranial material.
The type material is the partial remains of a cranium, which was obtained at a locality named Hiatus A Site in the Carl Creek Limestone Formation; this location is one of the numerous study sites at the Riversleigh World Heritage Area. The specimens were discovered by two of the collaborating authors, Michael Archer and Suzanne J. Hand, the head researchers of taxa at the celebrated Riversleigh site and its associated fauna.

Description

This species stood around high and weighed up to, a considerable size but smaller than its congeners; the later species Dromornis stirtoni is determined to have been up to. 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.
Wings were greatly reduced, approximately, and would not have been evident beneath the bird's plumage. The skull cavity held an exceptionally small brain, the description's leading author Trevor Worthy suggesting the comparison, "I mean, if a chicken was silly, these things were very much more silly."

Distribution

The fossil deposits of Dromornis murrayi at the Hiatus site of Riversleigh have been dated as early Miocene and another as late Oligocene to early Miocene. This was established using correlation with the evolutionary stage of vertebrate species known from other sites at Riversleigh. Hiatus site is limestone deposited in an aquatic setting, lacking indicators for methods such as radiometric dating. Another site where the species occurs is Cadbury's Kingdom, designated as Faunal Zone B which is also dated as early Miocene.
The temporal range of these finds is approximately 25 to 16 mega-annum.
The only known occurrence of this species is amongst the Riversleigh fauna, the site is located in the northeastern region of the Australian continent.

''D. planei''

Dromornis planei, formerly placed in a separate genus Bullockornis, lived in the Middle Miocene, approximately 15 million years ago. It is known from specimens of the Bullock Creek fauna, fossils found in the Northern Territory of Australia. As large as an ostrich or emu, the species possessed a stocky build. A proposed common name, referring to its discoverer and locality, is Plane's bull bird. The site of its discovery was once semi-arid, containing low vegetation around seasonal wetlands and rivers.
The species was first described by Patricia Vickers-Rich in 1979, assigning it to a new genus Bullockornis. The description's first generic epithet was derived by a partial reference to the Bullock Creek Site and the greek word for bird ornis, and the common name bull bird proposed by the author for genus. The type is a fossilised section of the right femur, with other material, vertebrae and a rib, also referred to the same species. The specific epithet honours the discoverer of the vertebrae fossils, Michael Plane, thus the proposed trivial name of "Plane's Bull Bird".
Plane had been the first to investigate the Bullock Creek site, details of which were published in a 1968 paper.
It was one of several species of mihirungs, the dromornithids, that share ancestry with ducks and geese. The nickname "Demon Duck of Doom" is a reference to the large bill and body of the species. Fossil specimens of this species and other mihirungs are common, but the example of a near complete skull discovered in the 1980s was an unusual find. The direct evidence of the beak structure was evaluated in debate over the diet and habits of dromornithids.
The bird's generic name is improperly translated as "ox-bird", but was named instead for the type locality for the genus at Bullock Creek, Australia. 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. Subsequent studies also agreed upon placing this species within the genus Dromornis.
Some paleontologists, including Peter Murray of the Central Australian Museum, believe that Bullockornis was related to geese and ducks. This, in addition to the bird's tremendous size and earlier misclassification as a carnivore, gave rise to its colourful nickname. It may be somewhat inaccurate, however, as other studies have recovered dromornithids as more closely related to Galliformes.
The existence of only this species at the Bullock Creek Site, as with the late Miocene Alcoota local fauna, correlates to the lack of diversity in large ratites, such as the evolution of the ostriches in the presence of a diversity of mammals.