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Dromornis Stirtoni is a genus of Dromornis endemic to Australia, and is colloquially known as Stirton’s Thunderbird. Dromornis Stirtoni was a large feathered bird that grew up to heights of 2.5m and weights in excess of 450 kg. Patricia Vickers-Rich first discovered the bird in the 1960’s in the Alcoota Fossil Beds (Northern Territory). The bird was characterised by a deep lower jaw and quadrate bone, large, muscular hind legs with hoof-like toes, and small wings that deemed it flightless. The bird is largely thought to have been herbivorous, although they may have occasionally scavenged or eaten small prey. Dromornis Stirtoni is thought to have lived in a semi-arid climate, characterised by seasonal rainfall and open woodlands.

Taxonomy
Patricia Vickers-Rich discovered the large bird in the 1960’s in the Alcoota Fossil beds (Northern Territory), naming it after palaeontologist Ruben A. Stirton. The description of this species, along with several other new dromornithidae was published in 1979 by Patricia Vickers-Rich. This included a major revision of the family, described a century earlier in Richard Owen's 1874 publication as a new species and genus. Large amounts of fragmentary material were found at the Alcoota Fossil Beds in Central Australia, which to this day are the only offical discoveries of the bird.

Informal names for the species include Stirton's thunderbird and Stirton's mihirung.

Description


Dromornis Stirtoni was a large feathered bird which grew up to 2.5m in height. This height is thought to have exceeded the tallest species of the genus Dinornis, which were the giant moas of New Zealand. The weight of the animal is also thought to have been exceedingly large. Peter F. Murray and Patricia Vickers-Rich, in their work Magnificent Mihirungs (2004), utilised three scientific methods to derive the approximate weight and size of Dromonis Stirtoni. This thorough analysis of the bones of Dromornis Stirtoni revealed that there was considerable sexual dysmorphism, and that a fully grown male could weigh between 528–584 kg, whilst a female would likely weigh between 441–451 kg. In comparison to other known ratite elephant birds of the Aepyornithidae family, this made Dromornis Stirtoni the heaviest of all known birds. Dromornis stirtoni was compared to Aepyornis maximus by the authors, the largest of that family. Due to the poor fossil record of Dromornis australis and the large time gap between the two Dromornis species, D. stirtoni may eventually be reassigned to the genus Bullockornis.

Dromornis Stirtoni was characterised by a deep lower jaw and a quadrate bone (which connects the upper and lower jaws) that was distinctly shaped. This narrow, deep bill, made up approximately two thirds of the skull. The front of this powerful jaw was used to cut, whilst the back of the jaw was used for crushing. Comparison of two partial crania with the near complete cranium of Dromornis planei (Bullockornis) shows the head of this species to be about 25% larger. Reconstruction of overlapping remains of the rostrum have revealed its form and size, and that the lower mandible would have been around 0.5 metres. 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. However, whilst the bird was flightless, a strong development between the bony crests and tuberosities, where the wings were attached, allowed them to flap their wings. The bird was also characterised by its large hind legs, which after the completion of biomechanical studies are confirmed to have been muscular, rather than slender, due to the size of the muscle attachments along the leg (Vicker-Rich, 1979). Due to the muscularity of these legs, the Dromornis Stirtoni is believed to have been capable of running at great speeds, whereas birds such as the emu depend on the slenderness of their legs to reach higher speeds.

The Dromornis Stirtoni was also characterised by its large, hoof-like toes, which had convex nails, rather than claws. Further typical of flightless birds, Dromornis Stirtoni did not have a breastbone. Thorough analysis of the bones of Dromornis Stirtoni has revealed that there was considerable sexual dysmorphism, and that a fully grown male would weigh between 528–584 kg, whilst a female would weigh between 441–451 kg.

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. This histological technique has been applied to other large and extinct avian species, including investigation into the paleobiology of the elephant birds Aepyornithidae. Males were comparatively more robust and heavier, although not necessarily taller than females. The disparity in robustness was interpreted by 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.

Distribution and Habitat
At present the only recorded fossil discoveries of Dromornis Stirtoni have been from the Alcoota Fossil Beds.

The vegetation type of the region in that period was open woodland favouring its semi-arid climate, within which seasonal rainfall occurs.

Dromornis stirtoni probably existed in an assemblage of fauna that included other dromornithids and browsing marsupials as the apex herbivore. The Alcoota Local Fauna were deposited at the only known upper Miocene fossil beds of Central Australia. The early conceptions of a fearsome birds receives some support from the proposed behaviour of the larger males aggressively defending a preferred range against competitors, other males or herbivores, and predators.

Extinction
It is proposed that various factors may have contributed to the extinction of Dromornis Stirtoni. Palaeontologists Murray and Vickers-Rich suggested that the diet of Dromornis Stirtoni may have overlapped considerably with the diets of other large birds and animals, and that the subsequent converging trophic morphology could have contributed to the large birds extinction as it was ‘out-competed’ of its food source.

Alternate arguments have proposed that the large birds’ breeding patterns may have contributed. This suggested that Dromornis Stirtoni lived for a relatively long period of time in a group of older birds, however, for the few young that were produced, time to maturity was considerable. Subsequently, breeding adults were replaced slowly, which left the species highly vulnerable if breeding adults were lost.

It is thought that these factors may have worked in conjunction with the introduction of humans, and the predation and environmental modification that was associated. Whilst direct evidence of human predation of Dromornis Stirtoni is limited, various experts propose that the large bird may well have been hunted, recognising that more than 85% of Australian mega fauna became extinct during the same period that humans were introduced. The use of ‘fire-stick burning’ as an ecosystem management technique is thought to have adversely impacted the semi-arid climates within which Dromornis Stirtoni lived, as the ecosystem lacked the required resilience, and as such, food supplies were impacted.

Feeding and Diet
It is widely accepted that Dromornis Stirtoni was a herbivorous bird, predominantly eating seed pods and tough-skinned fruits. This has been deduced from various key features of the bird. One of these features is that the end of the bird’s bill, does not have a hook, and that the beak is instead wide, narrow and blunt, typical of a herbivore. The bird also had hoof-like feet, rather than ‘talons’, which are typically associated with carnivores or omnivores. Moreover, placement of the eyes on the side of the birds head prohibits it from seeing directly in front, which would limit its ability to hunt. Lastly, analysis of the amino acids within the egg shells of Dromornis stirtoni suggest that the species was herbivorous. Despite this however, there are various indicators that suggest the bird may have been carnivorous or omnivorous. The size and muscularity of the birds skull and beak would also suggest that they may not have been herbivores, as no source of vegetable food in their environment would have required such a powerful beak. 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.

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