Huge rock mounds may have been made by a giant extinct bird

The mystery behind hundreds of large gravel mounds in inland Australia may have been solved – with an otherwise unknown extinct giant bird the cause.

More than 1000 of the peculiar mounds have been logged in the semi-arid western region of New South Wales. The largest are 40 metres in diameter, over two metres high and contain as much as 1250 cubic metres of variously sized pebbles. 

They are so big and solid that they will easily support the weight of a small truck. Their weathered look and the presence of vegetation on them suggests that they are ancient. {%recommended 5227%}

The mounds have long baffled geologists, because they are made of small stones – the largest weighing only about one kilogram – that reflect the make-up of the surrounding surface layer (or “regolith”) but do not contain any bits of the underlying bedrock. This rules out any plausible geologic origin, meaning some other agency must have been at work to create them.

Now, writing in the Australian Journal of Earth Sciences, geologist Leigh Schmidt suggests an intriguing theory. The mounds, he says, were the nests of an evidently very large, and, today, very extinct bird.

The theory is not without supporting evidence. Throughout much of Australia’s arid and semi-arid land lives a species called the mallee fowl (Leipoa ocellata), which does not build a conventional nest but instead uses its strong legs to construct a mound. At the top of the structure, the bird creates a hollow which it fills with leaf litter, on which it lays eggs. It adds or removes rotting vegetation to govern the heat at which the eggs incubate.

The birds are about the size of a domestic chicken. In 2017, researchers identified five fossil species thought to be related to the mallee fowl – collectively known as megapodes – all of them three to five times the size of the extant bird. 

However, these birds lacked the specialised foot structure of the present species and so are assumed to have created burrows to nest in, rather than mounds.

If they had been equipped to build mounds, however, even the largest would not have been able to create piles as enormous as the ones that today dot the landscape.

One potential explanation, says Schmidt, is an another, very much larger species of megapode, long extinct but thus far unfound in the fossil record.

Establishing the date of the mounds has proved challenging. However, Schmidt uses sand deposits to estimate their age to between 55,000 years and 10,000 years. This is significantly later than the suggested dates for the megapode fossil species already identified, which range from 200,000 to 400,000 years ago.

Other researchers have suggested that the mounds may represent either middens or burial sites constructed by indigenous comunities, but Schmidt finds that the evidence does not support these interpretations. Particularly, he notes, humans would have no cause to only deploy stones that weigh a kilogram or less.

“Identification of the large gravel mounds as megapode nests provides a reminder that not all geological features have a geological origin,” he writes.