Stirton's Thunder Bird

Dromornis stirtoni was the largest of the dromornithids, a group of huge flightless birds known only from Australia. The late Miocene Dromornis, from Alcoota Station in the Northern Territory, weighed up to 500kg and stood over three metres in height, making it heavier than the Giant Moa of New Zealand and taller than the Elephant Bird of Madagascar.

Mihirungs were large, flightless birds with deep lower jaws, a distinctly shaped quadrate bone (connecting upper and lower jaws), stubby wings, massive hind legs and hoof-like toes. They lacked a keeled sternum (breastbone), a specialization related to the reduction of flight muscles.

Dromornis stirtoni was over three metres in height, and from 450-500 kilograms in weight.

The Alcoota region was subtropical open woodland during the late Miocene.

Dromornis stirtoni is known only from Alcoota Station, central Northern Territory.

Related to: The closest relatives of dromornithids were once thought to be the flightless ratites (emus, ostriches, rheas and their kin) but it is now believed that dromornithids evolved from waterfowl (ducks, geese and their ancestors).

There is a great deal of debate over the diet of dromornithids. Many palaeontologists are convinced they were herbivores (eating mainly tough-skinned fruits and seed pods), but others think at least some dromornithids may have eaten meat, based on the shape and size of their skulls and beaks. All dromornithids lack a hooked beak, as in raptors, and Genyornis newtoni, from the Pleistocene, had hoof-like rather than recurved claws on its feet. Both these features are herbivore-like, although the feet of the larger dromornithids are unknown. Analysis of eggshells (amino acid analysis) in Genyornis supports an herbivorous diet at least in this dromornithid. Genyornis has also been also found in large numbers in some deposits, unlike carnivores (at the top of the food chain, carnivores are generally very rare). Those holding the view that dromornithids were to some degree carnivorous cite the huge size of the beak ('a case of overdesign'). The skulls of the larger dromornithids (Bullockornis and Dromornis) had proportionately more massive beaks than other species, including Genyornis, and there may in fact have been a variety of dietary options for the relatively diverse dromornithids.

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

There is little direct evidence for the lifestyle of dromornithids. Eggs of the Pleistocene Genyornis newtoni have been found in sand dune deposits, suggesting that it nested in these dunes.

Dromornis stirtoni is known only from Alcoota Station in the Northern Territory, which would have when Dromornis was alive. Entire skeletons of Dromornis have been found at Alcoota Station.

The oldest evidence of dromornithids in Australia may be an impression (a natural cast) of articulated foot bones found near Brisbane in Queensland, which is Eocene in age and therefore some of the oldest evidence for Australian birds. The foot is that of a large, ground-dwelling bird similar to the Pleistocene Genyornis newtoni, the last known dromornithid and only one known from a complete, articulated foot. Genyornis and the Redbank bird have similar proportions of the foot bones (e.g., phalanx I of digit II is longer and thinner than phalanx I of digit III) and lack processes for flexor tendons. However, the Redbank bird does not have the hoof-like toes that the swifter Genyornis possessed.

Once thought to be ratites (the group to which emus, cassowaries, rheas and ostriches belong), dromornithids are now believed to be either within Anseriformes (the duck/goose group) or just basal to it. Recent revisions of the taxonomy of other large, flightless birds place these taxa (Gastornithidae, the family to which Diatryma belongs, and the Miocene Brontornis from Patagonia) within Anseriformes. All of these large birds, including the dromornithids, have a short dentary symphysis and a dorsally directed pterygoid process on the quadrate, unusual features not related to flightlessness. This revision is still debated, although many feel that the general placement of at least dromornithids somewhere near the base of the anseriform radiation has merit.

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