Dromornis planei (Bullockornis planei)

Dromornis planei was a massive bird with a formidable bill. It belonged to a uniquely Australian family of extinct flightless birds, the dromornithids (mihirungs). Because of the close relationship between mihirungs and ducks, Dromornis planei has been nicknamed the 'Demon Duck of Doom'.

Identification

Dromornis planei was a heavily built bird with a long neck and enormous legs. It was as tall as an ostrich but far more massive. The skull of Dromornis planei was similar in size to a horse’s head and had a deep, curved bill.

Most of the wing bones are unknown for Dromornis planei, and what we know about its sternum (breastbone) comes from small fragments. Like other mihirung species, Dromornis planei probably also had stubby little wings and lacked a keel on its sternum. These features are associated with flightlessness.

Habitat

Dromornis planei lived alongside other mihirungs and Emuarius (a relative of emus and cassowaries) at Bullock Creek. Crocodiles, fish, frogs, and large snakes were also present at this site. Based on the geology and biology of the fossil fauna, the Bullock Creek palaeohabitat is thought to have been a large meandering river and surrounding floodplain with seasonal fluctuations in water supply. The climate was seasonally dry, perhaps semi-arid. The vegetation at Bullock Creek was a mix of shrubs and sedges, as indicated by fossils of herbivorous browsers, such as diprotodontids, marsupial tapirs, and horned turtles.

The Bullock Creek fossil fauna reflects a waterhole assemblage, where large animals such as Dromornis planei and the diprotodontid Neohelos gathered. Crocodile predation is thought to be the significant cause of mortality of these large animals. Many fossil bones at Bullock Creek, including those of Dromornis planei, have puncture marks that match the shapes of the teeth of crocodiles (Baru). These fossils also have fractures that appear to have been made while the bone was fresh.

Small terrestrial animals and arboreal marsupials, such as dasyurids, kangaroos, and possums, are rare in the Bullock Creek fossil fauna. Carnivores such as marsupial lions and thylacinids are also rare at this site. These animals are thought to have come from a gallery forest further upstream.

Distribution

Dromornis planei is known only from Bullock Creek in the Northern Territory. Two other mihirung species belonging to the genus Ilbandornis have also been found at this fossil site.

Feeding and diet

The diet of mihirungs has been much debated. The skull and massive bill of Dromornis planei is at the centre of the mihirung diet debate. It was first suggested that the bill of Dromornis planei was specialised for cropping and breaking into tough plant material.

Gastroliths (gizzard stones) have been discovered alongside the bones of mihirungs Genyornis, Dromornis stirtoni, and Ilbandornis at some fossil sites. In living birds, gastroliths are swallowed to help grind plant material, as an alternative to chewing. Fossils of mihirungs such as Dromornis planei are very common at some sites, whereas carnivores are generally rare.

But some researchers have argued that the bill of Dromornis planei was more suited to tearing meat and crushing bones, similar to the unrelated terror birds (Diatryma) of South America.

Dromornis planei had a deep yet thin bill. It had limited attachment areas for muscles to operate the bill, which suggests that it had a weak biting ability. More fossil discoveries can help to solve the mystery of the diet of Dromornis planei.

Other behaviours and adaptations

Biomechanical studies suggest that dromornithids were relatively fast runners. Their massive legs were well muscled, providing the necessary power in spite of their bulk.

Life history cycle

Little is known about the life cycle of Dromornis planei. A recent study of a related species, Dromornis stirtoni, found medullary tissue in the bones of some individuals. This specialised tissue serves as a calcium reservoir for building hard eggshell. It not only distinguishes the fossil bones of female and male mihirungs, but also showed females that were laying or about to lay eggs.

Sexual size dimorphism was also present in Dromornis stirtoni, with males being much larger than females. In living anseriforms (ducks and geese), sexual size dimorphism is associated with complex mating behaviours such as lifelong monogamy, ritual displays, shared parental care, and aggressive defence of nests. It is possible that the related Dromornis planei was also sexually dimorphic and had similar mating behaviour.

Fossil eggshells that were thought to belong to Genyornis have been found in sand dune deposits across Australia. A recent study looked at the structure of the eggshell and found that these shells were not of Genyornis but of extinct giant megapodes (landfowl).

Fossils description

Mihirungs have one of the longest fossil records of any Australian vertebrate group. Their fossil record spans from the late Pleistocene (about 50,000 years ago) to possibly as early as the Eocene (about 55 million years ago). Mihirungs are also the best known fossil birds in Australia, with over 2,500 bones discovered so far.

Well-preserved fossils of Dromornis planei have been found at Bullock Creek, Northern Territory, including skulls, bills, vertebrae, and leg bones. One of the most significant discoveries at Bullock Creek is the skull of Dromornis planei, which was the first well-preserved skull known for any mihirung species. This near-complete skull provided clues to the evolutionary relationships between mihirungs and anseriforms (ducks and geese).

Evolutionary relationships

Because they were large and flightless, mihirungs were long thought to be ratites, a group that includes emus, cassowaries, ostriches, and the extinct moa and elephant birds. With the discovery of the skull and bill of Dromornis planei, researchers suggested that mihirungs are closely related to Anseriformes, the waterfowl group that includes ducks and geese. Most recently, a close relationship has been suggested between mihirungs and the Galloanseres group, which includes anseriforms and galliforms (chickens and turkeys).

Dromornis planei was initially placed in the genus Bullockornis. But recent studies have shown that this bird is more closely related to species of the genus Dromornis, and has since been renamed Dromornis planei.

References

• Grellet-Tinner G, Spooner NA, and Worthy TH. 2016. Is the “Genyornis” egg of a mihirung or another extinct bird from the Australian dreamtime? Quaternary Science Reviews 133, 147-164.
• Handley WD, Chinsamy A, Yates AM, and Worthy TH. 2016. Sexual dimorphism in the late Miocene mihirung Dromornis stirtoni (Aves: Dromornithidae) from the Alcoota Local Fauna of central Australia. Journal of Vertebrate Paleontology 36 (5), e1180298. (DOI: 10.1080/02724634.2016.1180298)
• Worthy TH, Handley WD, Archer M, and Hand SJ. 2016. The extinct flightless mihirungs (Aves: Dromornithidae): cranial anatomy, a new species and assessment of Oligo-Miocene lineage diversity. Journal of Vertebrate Paleontology 36 (3), e1031345. (DOI: 10.1080/02724634.2015.1031345)
• Worthy TH, and Yates A. 2015. Connecting the thigh and foot: resolving the association of post-cranial elements in the species of Ilbandornis (Aves: Dromornithidae). Alcheringa 9, 407–427.
• Nguyen JMT, Boles WE, and Hand SJ. 2010. New material of Barawertornis tedfordi, a dromornithid bird from the Oligo-Miocene of Australia, and its phylogenetic implications. Records of the Australian Museum 62, 45–60.
• Pain S. 2000. The demon duck of doom. New Scientist 166, 36–39.
• Wroe S. 1999. The bird from hell? Nature Australia 26, 58–64.
• Murray PF, and Megirian D. 1998. The skull of dromornithid birds: anatomical evidence for their relationship to Anseriformes. Records of the South Australian Museum 31, 51–97.
• Olson SL. 1985. The fossil record of birds. In Avian Biology, eds DS Farner, JR King, and KC Parkes, pp. 79–252. New York, Academic Press.
• Rich PV. 1979. The Dromornithidae, an extinct family of large ground birds endemic to Australia. Bureau of Mineral Resources, Geology and Geophysics Bulletin 184, 1–196.

Further reading

Murray PF, and Vickers-Rich P. 2004. Magnificent Mihirungs: The Colossal Flightless Birds of the Australian Dreamtime. Indiana University Press, Bloomington.