Dromornithids Temporal range: Oligocene - Pleistocene, Possible Eocene record
Dromornis stirtoni
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Aves
Clade:	Anserimorphae
Order:	†Gastornithiformes
Family:	†Dromornithidae Fürbringer, 1888
Genera
†Dromornis (type) †Barawertornis †Ilbandornis †Genyornis

Dromornithidae, known as mihirungs and informally as thunder birds or demon ducks, were a clade of large, flightless Australian birds of the Oligocene through Pleistocene Epochs. All are now extinct. They were long classified in Struthioniformes (the ratites), but are now usually classified as galloanseres.^[1][2][3] Dromornithids were part of the Australian megafauna. One species, Dromornis stirtoni, was 3 m (9 ft 10 in) tall. Only a single species, Genyornis newtoni survived into the Late Pleistocene.^[4][5] They are thought to have been herbivorous.^[6]

Classification

The scientific name Dromornithidae derives from the Greek words δρομαίος, dromaios ("swift-running") and ὀρνις, ornis ("bird").^[7] The family was named by Max Fürbringer in 1888, citing W. B. Clarke and Gerard Krefft, Owen's separation from "Dromaeus" and Dinornis, and a note by von Haast allying Dromornis with Dromaeus.^[8][9] What the nearest relatives of this group are is a controversial issue. For many years it was thought that dromornithids were related to ratites, such as emus, cassowaries, rheas and ostriches. It is now believed that the similarities between these groups are the result of similar responses to the loss of flight. The latest idea on dromornithid relationships, based on details of the skull, is that they evolved early in the lineage that includes waterfowl (Anseriformes). However, some phylogenetic studies have recovered them as closer to gamefowl (Galliformes).^[2]

Below is the general consensus of the phylogeny under the hypothesis that they are members of Anserimorphae.^{[10][11][12][13][14]}

A 2017 paper concerning the evolution and phylogeny of the giant fowl by Worthy and colleagues have found phylogenetic support in finding the mihirungs to be the sister taxon to Gastornithidae.^[9] Worthy et al. (2017) incorporated several new taxa and character traits into existing matrices of Galloanserae resulted in several of their phylogenies to support this grouping.^[9] The authors did note the bootstrap support is weakly supported and one of their phylogenies even found gastornithiforms to be stem galliforms instead.^[9] These were also weakly supported.^[9] Below is a simplified phylogeny showing their one phylogeny supporting gastornithiforms as anserimorphs.^[9]

A 2021 study conversely found their internal cranial anatomy more similar to that of galliforms than to anseriforms.^[15]

A 2022 study concerning their proteins found them to be sister group to the common group of galliforms and anseriforms.^[16]

Discovery

The most recent species, Genyornis newtoni, was certainly known to Aboriginal Australians during the Late Pleistocene. Cave paintings thought to depict this bird are known, as are carved footprints larger than those considered to represent emus. At Cuddie Springs, Genyornis bones have been excavated in association with human artifacts. The issue of how much of an impact humans had on dromornithids and other large animals of the time is unresolved and much debated. Many scientists believe that human settlement and hunting were largely responsible for the extinction of dromornithids, as well as many other species of the Australian megafauna.

The first Europeans to encounter the bones of dromornithids may have been Thomas Mitchell and his team. While exploring the Wellington Caves, one of his men tied his rope to a projecting object which broke when he tried to descend the rope. After the man had climbed back up, it was found that the projecting object was the fossilised long bone of a large bird. The first species to be described was Dromornis australis. The specimen was found in a 55-metre deep well at Peak Downs, Queensland, and subsequently described by Richard Owen in 1872.

Extensive collections of any dromornithid fossils were first made at Lake Callabonna, South Australia.

In 1892, E. C. Stirling and A. H. C. Zietz of the South Australian Museum received reports of large bones in a dry lake bed in the northwest of the state. Over the several next years, they made several trips to the site, collecting nearly complete skeletons of several individuals. They named the newly found species Genyornis newtoni in 1896. Additional remains of Genyornis have been found in other parts of South Australia and in New South Wales and Victoria.

Other sites of importance were Bullock Creek and Alcoota, both in the Northern Territory. The specimen recovered there remained unstudied and unnamed until 1979, when Patricia Rich described five new species and four new genera.^[17]

The best represented bones of dromornithids are vertebrae, long bones of the hindlimb and toe bones. Ribs and wing bones are uncommonly preserved. The rarest part of the skeleton is the skull. For many years, the only skull known was a damaged specimen of Genyornis. Early reconstructions of dromornithids made them appear like oversized emus. Peter Murray and Dirk Megirian of Australia's Northern Territory Museum recovered enough skull material of Bullockornis to give a good idea of what that bird's head looked like. It is now known that the Bullockornis skull was very large, with the enormous bill making up about two-thirds of it. The bill was deep, but rather narrow. The jaws had cutting edges at the front, as well as crushing surfaces at the back. There were attachments for large muscles, indicating that Bullockornis had a powerful bite. More fragmentary remains of the skull of Dromornis suggest that it, too, had an oversized skull.

Bones are not the only remains of dromornithids that have been found:

• The polished stones that the birds kept in their gizzards (muscular stomachs) occur at a number of sites. These stones, called gastroliths, played an important role in their digestion by breaking up coarse food or matter that was swallowed in large chunks.
• Series of footprints, called trackways, have been found at several sites.^[19][20][21]
• Impressions of the inside of the skull cavity (endocranial casts or endocasts) have been found. Endocasts are formed when sediments fill the empty skull, after which the skull is destroyed. These fossils give a fairly accurate picture of dromornithid brains.

Description

Artist's reconstruction of Genyornis newtoni.

Artist's reconstruction of Dromornis stirtoni.

Dromornithids looked superficially like very large emus or moa. Most were heavy-bodied, with powerfully developed legs and greatly reduced wings. The last bones of the toes resembled small hooves, rather than claws as in most birds. Like emus and other flightless birds, dromornithids lost the keel on the breastbone (or sternum) that serves as the attachment for the large flight muscles in most bird skeletons. Their skull also was quite different from that of emus. These birds ranged from about the size of a modern cassowary 1.5 to 1.8 m (4 ft 11 in to 5 ft 11 in) up to 3 m (9 ft 10 in) in the case of Dromornis stirtoni, possibly the largest bird that ever lived after the elephant bird (Vorombe).

Diet

It has been generally thought that the dromornithids were plant eaters, a belief currently considered common scientific consensus.^[24] This belief is based on:

• the lack of a hook at the end of the bill
• the lack of talons on the toes
• the association of gizzard stones (caveat: gastroliths are also found in the stomachs of some carnivores, such as modern crocodiles)
• the large number of individuals occurring together, suggesting flocking behaviour

Functional interpretations by researchers Warren Handley and Trevor Worthy suggests that dromornithids were specialised herbivores that likely possessed well-developed stereoscopic depth perception, were diurnal and fed on soft browse such as new growth, soft leaves, and fruit. Musculature for operation of the bill is “surprisingly limited", suggesting that these birds were not capable of a particularly forceful bite. There is no temporal fossa on the side of the cranium for insertion of mandibular musculature. The culmen in Dromornis, while large, has a lightly constructed osseous core that was only partially covered in rhamphotheca, was highly vascularised and likely highly innervated, a combination of features conferring relatively weak biting ability. This suggests that dromornithids were likely not consuming coarse browse requiring strong bite forces.^[15]

It has been suggested that, despite the indications of herbivory in some dromornithids, Bullockornis may have been a carnivore or possibly a scavenger. However, most authorities now consider it to be a herbivore.^[24] The recently-extinct thylacine may have been a potential predator.

Distribution

Records of these birds are known only from Australia. Most of the records of dromornithids come from the eastern half of the continent, although fossil evidence has also been discovered in Tasmania and Western Australia. At some Northern Territory sites they are very common, sometimes comprising 60-70% of the fossil material. A fragment of a dromornithid-sized foot bone has been found in Antarctica, but whether it represents these birds is uncertain.

The earliest bones identified were found in Late Oligocene deposits at Riversleigh in northwestern Queensland. There are foot impressions from the Early Eocene in southeastern Queensland (Redbank Plains Formation) that may be referable to dromornithids.^[25] The most recent evidence, of Genyornis newtoni, has been found at Cuddie Springs in north-central New South Wales and dated at 31,000 years old.^[26]

Extinction

The reasons for the extinction of this entire family along with the rest of the Australian megafauna by the end of the Pleistocene are still debated. It is hypothesized that the arrival of the first humans in Australia (around 48-60 thousand years ago) and their hunting and landscape-changing use of fire may have contributed to the disappearance of the megafauna. However, drought conditions during peak glaciation (about 18,000 years ago) are a significantly confounding factor. Some studies^[27] appear to rule this out as the primary cause of extinction, but there has also been some dispute about these studies.^[28] It is likely that a combination of all of these factors contributed to the megafauna's demise. However, there is significant disagreement about the relative importance of each.

See also

• Fossil birds
• Later Quaternary Prehistoric Birds

Notes

References

• Field, J. H.; Boles, W. E. (1998). "Genyornis newtoni and Dromaius novaehollandiae at 30,000 b.p. in central northern New South Wales". Alcheringa. 22 (#2): 177–188. Bibcode:1998Alch...22..177F. doi:10.1080/03115519808619199.
• Miller, G. H.; Magee, J. W.; Johnson, B. J.; Fogel, M. L.; Spooner, N. A.; McCulloch, M. T.; Ayliffe, L. K. (1999). "Pleistocene extinction of Genyornis newtoni: human impact on Australian megafauna". Science. 283 (#5399): 205–208. doi:10.1126/science.283.5399.205. PMID 9880249.
• Murray, P. F.; Megirian, D. (1998). "The skull of dromornithid birds: anatomical evidence for their relationship to Anseriformes (Dromornithidae, Anseriformes)". Records of the South Australian Museum. 31: 51–97.
• Murray, Peter; Vickers-Rich, Patricia (2004). Magnificent mihirungs : the colossal flightless birds of the Australian dreamtime. Bloomington: Indiana University Press. ISBN 978-0-253-34282-9.
• Rich, P (1979). "The Dromornithidae, an extinct family of large ground birds endemic to Australia" (PDF). Bulletin of the Bureau of Mineral Resources, Geology and Geophysics. 184: 1–190.
• Rich, P.; Gill, E. (1976). "Possible dromornithid footprints from Pleistocene dune sands of southern Victoria, Australia". Emu. 76 (#4): 221–223. doi:10.1071/mu9760221.
• Rich, P.; Green, R. H. (1974). "Footprints of birds at South Mt Cameron, Tasmania". Emu. 74 (#4): 245–248. doi:10.1071/mu974245.
• Roberts, R. G.; Flannery, T. F.; Ayliffe, L. A.; Yoshida, H.; Olley, J. M.; Prideaux, G. J.; Laslett, G. M.; Baynes, A.; Smith, M. A.; Jones, R.; Smith, B. L. (2001). "New ages for the last Australian megafauna: continent-wide extinction about 46,000 years ago". Science. 292 (#5523): 1888–1892. Bibcode:2001Sci...292.1888R. doi:10.1126/science.1060264. PMID 11397939. S2CID 45643228.
• Vickers-Rich, P.; Molnar, R. E. (1996). "The foot of a bird from the Eocene Redbank Plains Formation of Queensland, Australia". Alcheringa. 20: 21–29. Bibcode:1996Alch...20...21V. doi:10.1080/03115519608619220.
• Williams, D. L. G. (1981). "Genyornis eggshell (Dromornithidae; Aves) from the Late Pleistocene of South Australia". Alcheringa. 5 (#2): 133–140. Bibcode:1981Alch....5..133W. doi:10.1080/03115518108565426.
• Williams, D. L. G.; Vickers-Rich, P. (1992). "Giant fossil egg fragment from the Tertiary of Australia". Contributions in Science, Natural History Museum of Los Angeles County. 36: 375–378.

Further reading

• Chinsamy, Anusuya; Handley, Warren D.; Worthy, Trevor H. (17 August 2022). "Osteohistology of Dromornis stirtoni (Aves: Dromornithidae) and the biological implications of the bone histology of the Australian mihirung birds". The Anatomical Record. Wiley. 306 (7): 1842–1863. doi:10.1002/ar.25047. ISSN 1932-8486. PMID 37314297.