The birds would eventually evolve into animals weighing up to 650 kilograms (1,430 pounds). Brian Choo
Josh Davis 24 Feb 2016, 16:14

They were the largest birds ever to have existed, and yet few people have heard of the mihirung, the “giant ducks” that once stomped around modern day Australia. Little is known about them or their origin, but now researchers have described the earliest and smallest member of this group, Dromornis murrayi, although it wasn’t exactly small compared to modern birds. The study is published in the Journal of Vertebrate Paleontology.

“Originally, it was the smallest, at a pretty hefty 250 kilograms [550 pounds], but by eight million years ago it had evolved into D. stirtoni, which averaged a whopping 450 kilograms [990 pounds] – with some individuals reaching 650 kilograms [1,430 pounds] – the largest birds the world has known,” explains Dr Trevor Worthy. The newest species is also thought to be the earliest, living around 26 million years ago during the late Oligocene and early Miocene, much before the largest birds were strutting around Australia.

Rather than being related to what are known as ratites, a group including all the giant birds still surviving today – emus, ostriches, rheas, and cassowaries – the mihirung were actually descended from waterfowl, and so are more closely related to ducks. While the birds originated in the Oligocene, during a period when land mammals grew in size and diversity around the world, the giant birds persisted for millions of years into the Pleistocene, when the last and largest species of the group, D. stirtoni finally became extinct around 50,000 years ago.

This point in time is characterized by a pattern of large animals being wiped out across much of the world. Australia was one of the regions worst hit by these widespread extinctions, with 15 out of the 16 animals weighing more than 40 kilograms (88 pounds) that lived there dying out. The reasons behind this are still hotly contested, but they do seem to match up with the arrival of humans in almost every case. It is generally assumed that the hunting by these novel predators, coupled with a period of dramatic climatic change is what eventually saw them off.

The fossil bones that have been described by the scientists as belonging to the new species of mihirung, D.murrayi, were found in the Riversleigh fossil deposits, northwest Queensland. According to the researchers, the bones from this species are actually quite common in this area. The study specifically analyzed bones from the skull, breastbones, and those from the leg and foot, although fragments from the rest of the animal were also looked at. “We even had some tiny bones of the wing, which showed this gigantic bird had already, by 26 million years ago, essentially lost its wings,” says Dr Worthy.

There is still debate as to the exact role these birds played in the ecosystem, with their distinct skulls confusing things. To some, the heavy bills are simply the wrong shape and size for an animal that fed on mainly plants, and instead seem to be adapted to crushing bones. However, others have pointed out that the lack of a hooked beak and talons on the feet suggest that it wasn’t a meat eater. This discovery of the earliest example of mihirung could help shed light on the origin of these giant birds. 

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