Australasian Science: Australia's authority on science since 1938

An Ancient Case of Mistaken Identity?

Credit: Peter Trusler

An image of a pair of Dromornis mihirungs to illustrate their enormous bills and great differences from waterfowl that these browsing herbivores show. Credit: Peter Trusler

By Trevor Worthy

The megafaunal bird Genyornis was six times larger than an emu. Why, then, was its egg the same size? Or was it?

Mihirungs are extinct giant birds of Australia. The smallest was about the size of an emu and the largest, the 9–7-million-year-old Dromornis stirtoni, is more than ten times the mass of an emu at an amazing 480 kg.

Until the 1990s, these birds were thought to be ratites, somehow related to the emu and cassowary. Since then, however, scientists have classified them among fowl (chickens, ducks and their kin), but it seems they are not close to any of the living forms.

Eight species of mihirungs lived in Australia during the last 26 million years, and while up to three occurred together in the Miocene, only one was left by the Pleistocene. This was Genyornis newtoni, and it’s one of the best-known species. It was discovered in the 1890s in Lake Callabonna, South Australia. From those excavations and others since, dozens of bones of it are known yet it remains rare. We do not even have a reasonable skull to know what shape its head was. Furthermore, while it is of Pleistocene age, all of the sites its bones have been recovered in are either undated or poorly dated, so they tell us little about when exactly this megafaunal bird went extinct.

This problem was thought to have been addressed by an analysis of eggshell that differed from that of emu and was first attributed to Genyornis newtoni in 1981. This kind of eggshell is widespread in sand dunes in the Murray Basin, in the Eyre Basin, and other places across Australia.

Most recently this eggshell type has been reported from the dunes in the far north-west of the continent. It has formed the basis of a significant body of work by Gifford Miller and associates, who concluded that Genyornis newtoni went extinct rapidly and Australia-wide about 45,000 years ago, soon after human arrival in the continent. Moreover, the same shell suggested differences in the diet of this bird compared with emus, whose shell co-occurred in the same deposits. Genyornis newtoni was apparently the most well-dated megafaunal species, and the only one comprehensively shown to have gone extinct in widespread locations soon after human arrival.

However, all of this inference rests on the correct identification of this eggshell, and when a complete egg of this egg-type was reconstructed from a collection of fragments of a single collapsed egg at the South Australian Museum a few years ago, it piqued my curiosity. Surely this could not be right – this supposed Genyornis egg was similar in size to that of an emu, slightly wider for sure, but slightly shorter too (126 x 97 mm). Emus are large birds but they only weigh about 40 kg or so. Genyornis newtoni was a 250 kg monster by comparison.

I was familiar with moa eggs from several species, large and small. The bigger moas, the 100–200 kg birds, laid large eggs and the largest species, the South Island giant moa (Dinornis robustus) with a mean weight of 150 kg and perhaps reaching 250 kg, was approaching the size of Genyornis newtoni yet had an egg 240 x 178 mm in size. This 126 by 97 mm egg seemed much too small to have been laid by Genyornis newtoni.

Thus I embarked on an investigation of the eggshell with Gerald Grellet-Tinner and Nigel Spooner ( Nigel had discovered this egg in 2001 and he dated it in our study to 54.7 ± 3.1 years ago. It was not the youngest but its age was close to the last ones known at about 45,000 years ago.

Comparisons of its size confirmed that the egg was six times too small for a bird the size of Genyornis newtoni. Yet, it was about average for the size of the eggs represented by the eggshell found in many localities – the old calculations of the 1980s had resulted in exaggerated size predictions.

We then examined the structure of the eggshell in detail, its physical morphology and its chemistry, and compared it with a range of fowl as well as other fossil eggs that were thought to represent more ancient species of dromornithids and mihirungs. In brief, the supposed Genyornis eggshell differed greatly from another mihirung eggshell, and was fairly similar to the eggshells of mound-building megapodes.

We knew that there were other giant birds in Australian prehistory – something overlooked by those that attributed the fossil shell to Genyornis. For instance, there were several species of giant megapodes currently classified in the genus Progura.

When one looks at megapode eggs, it’s readily apparent that they are somewhat unusual: all are rather bigger than expected for their mass compared with most birds. This is easily explained by the fact that megapode chicks hatch fully developed and dig their way out of the subterranean burial site of their egg, and are almost ready to fly upon reaching the surface. The large eggs facilitate this precocial development.

Hence we asked if this Genyornis egg had been laid by a giant species of Progura. We used Progura naracoortensis from South Australia, whose average mass has been estimated at 4.9 kg, and plotted these data with that of other megapodes. It was readily apparent that the supposed Genyornis egg was about the right size for one expected of a 4.9 kg megapode. We know that Progura naracoortensis was not the largest species of Progura, with Progura gallinacea of Queensland a larger bird and other similar-sized birds widely distributed in Australia.

We concluded that there had been a simple case of mistaken identity. All shell hitherto referred to Genyornis was in fact much more likely to have been laid by one or more species of giant extinct megapodes. As is the nature of science, we await testing of this hypothesis – perhaps with DNA.

Meanwhile, we now have enough evidence to conclude that the eggshell attributed to Genyornis newtoni in 1981 was in fact laid by the giant megapode Progura, which we would expect to have sought out sand dunes to nest in like other megapodes that are not mound-builders. This taxon became extinct about 45,000 years ago – about 10,000 years after the egg was laid.

As for the giant mihirung Genyornis newtoni, we know little about when it went extinct due to the rarity of its osseous remains. None of these remains are dated, and its eggshell has yet to be identified.

Trevor Worthy is Vice-Chancellor’s Postdoctoral Research Fellow at Flinders University’s School of Biological Sciences.