Australia: The Land Where Time Began

A biography of the Australian continent 

Bullockornis ("Bullock Bird")

Bullockornis planei (Rich,1979), the type species, was named after the Australian palaeontologist Michael Plane, who co-reported and described the Bullock Creek fossil locality (Plane & Gatehouse, 1968). The original find, a right femur, indicated the specimen was about the height of a modern ostrich, but much more massively built. The Bullock Creek Local Fauna from the Middle Miocene Camfield Beds, in the Northern Territory, has produced many specimens, among which are crania and upper and lower mandibles. Fossils from Bullock Creek are among the best preserved fossils from Australia. Some have been stained black or bluish-grey by manganese oxide, others are white or cream. The B. planei crania and mandibles were the first well preserved material that supported the theory that the dromornithids were members of the Anseriformes.

In the book by Murray & Vickers-Rich, Magnificent Mihirungs, they give the dimensions of the short, broad cranium as about 118.0 mm wide across the orbits, and 174.0 mm from the tips of the craniomandibular ("paroccipital") processes. Behind the tympanic apertures, large, rounded post-tympanic crests flare out that are continuous with a low coronal (nuchal) crest. The neck muscles are attached to this crest high up on the crown.

In the B. planei skull, the orbits are small, C-shaped notches about 40 mm in height, between the base of the postorbital process and the anterolateral process. A complex joint surface that traverses the entire frontal seems to truncate them on the upper margins. The craniorostral joint is unique, being composed of several elements with reciprocal structures on the posterior of the upper mandible. Nasal salt gland fossae in the form of oval depressions are situated behind the orbital rims. One specimen has flat, oval basipterygoid processes. As is typical in Anseriformes, they are 26.5 mm long and 19.0 mm wide, and are elevated slightly above the parasphenoid rostrum. The quadratal fossae are large, as is the case in Dromornis, but they differ in being more oval than round, 21.0 mm long and 11.5 mm wide.

The endocranial fossae of broken Bullockornis neurocrania provide some knowledge of the surface morphology of dromornithid brains. B. planei has large cerebral hemispheres with considerable expansion of the ventrolateral lobes. Relative to the cerebral hemispheres, the optic lobe fossae are smaller, shallower and less distinct than in the duck, Anas platyrhynchus. In ?Bullockornis sp. the optic lobe fossae are deeper and more clearly defined than is the case in B. planei, but in comparison to the modern domestic duck, they are significantly smaller relative to the cerebral hemispheres. In B. planei, the cerebellum is large and transversely narrow. The cranial vault surrounding the endocranial cavity is 40 mm thick, and the bone is cancellous with large sinuses (air cells). The result is that compared to modern ducks and geese, the actual brain volume is much smaller in relation to the outer dimensions of the cranium.

Murray & Vickers-Rich include some detailed data on the known bones of Bullockornis in their book, Magnificent Mihirungs. There is a great deal of variability in the size, and some variability of the shape, of the femur and tibia bones of B. planei. Most femurs and tibias display a degree of lateral compression, though some lack the compression, in which case they more closely resemble individuals of Dromornis stirtoni from the Late Miocene. The authors suggest that these bones that lack the compression that was found in the Bullock Creek deposit could possibly be a species of Dromornis, though they think the variation appears to be more continuous than would be expected for a different species. A characteristic that is considered to consistently distinguish Bullockornis from Dromornis is a significantly shorter anteroposteriorly, and with a less rounded posterior margin.

Another species of dromornithid has been found in the Bullock Creek deposit that closely resembles Ilbandornis woodburnei from the Alcoota Local Fauna in the Camfield Beds from the Late Miocene. It has been tentatively called ?Bullockornis sp. The remains of this species are not as common in the Bullock Creek Despots as those of Bullockornis planei, the only fossil of this species to be found is a cranial wall fragment. The main feature of ?Bullockornis sp. distinguishing it from  Ilbandornis is slightly smaller size. Other distinguishing features of the ?Bullockornis sp. are a straight-sided femur shaft, a low, broad trochanter, and moderate width condyles and condylar crests that are slightly divergent. (Murray & Vickers-Rich, 2004).

Because of the combination of its large skull and possible carnivorous habits and its probable waterfowl relationships, Bullockornis was nicknamed 'The Demon Duck of Doom'.

It was assumed that the earlier and the smaller Bullockornis and Ilbandornis were also herbivorous. Then in 1997 a 38.5 cm long skull of Bullockornis planei was discovered. Birds have heads that are big enough to match the size of the food they normally eat, so if Bullockornis ate fruit they must have been enormous, Some think they ate tough plant material, possibly as tough as coconuts. It is thought by some that they may have been carnivorous. 

Birds from Riversleigh Fossil Deposits

Sources & Further reading

1. Dinosaurs of Australia and New Zealand, John A. Long, UNSW Press, 1998
2. Peter F. Murray & Patricia Vickers-Rich, Magnificent Mihirungs: The Colossal Flightless Birds of the Australian Dreamtime, Indiana University Press, 2004
3. Mary E White, After the Greening, The Browning of Australia, Kangaroo Press, 1994
4. Chris Johnson, Australia's Mammal Extinctions, a 50,000 year history, Cambridge University Press, 2006

Links

• Dromornithidae images
• Anseriformes