A team of vertebrate paleontologists from the University of Cape Town and Flinders University found big bones from the extinct mihirungs, also known as thunderbirds and demon ducks, in the northern part of the Flinders Ranges and near Alice Springs. The discovery yielded new insights into their slow-breeding patterns, which are believed to be a major factor in their extinction.
Science News reported that studies of the microstructure of these fossil bones include analysis of their size and breeding cycle that showed it changed over thousands of years and that they ultimately could not cope with the environmental changes around them.
Dromornis stirtoni, a flightless bird from the Late Miocene of Australia, pencil drawing
What are Mihirungs?
Mihirungs are giant flightless birds that were the biggest birds to ever walk in what is now Australia. They weighed up to hundreds of kilograms and disappeared about 40,000 years ago.
They have many names, including thunderbirds and demon ducks, because of their great size and close evolutionary relationship with modern-day ducks. They are flightless, herbivorous birds that have lived for more than 20 million years. Over that time, they grew larger than the 3-meter-tall Stirton's thunderbird (Dromornis stirtoni).
Most research on them has been about their anatomy and evolutionary relationship with birds, but little is known about their biology. Information about how long it took for them to grow and mature is still missing, says paleobiologist Anusuya Chinsamy-Turan from the University of Cape Town.
So, their team and colleagues at Flinders University in Australia took samples from the 20 fossilized leg bones to verify their life stages. Amazingly, the microscopic structure of the bones was still intact and could be used to decipher important clues about the animals in different life stages.
They examined growth marks in the bones that provided information on how fast the bone grew while the mihirungs were alive. Findings suggest that the birds took 15 years to reach their full size and most likely became sexually mature before that, based on the shift visible in the bone structure that is associated with reaching reproductive age.
The findings differed from previous analyses of the last known mihirung Genyornis newtoni that grew up faster than its giant relative. It only took two years to grow into its maximum height and a bit more in the following years to fully mature for them to be able to reproduce.
Researchers believe that the difference may have resulted from an evolved response after living in a drier climate for millions of years. That means growing and reproducing quickly is more advantageous when resources are unpredictable.
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Mihirungs Too Slow to Reproduce
Despite the faster rate of Genyornis newtoni in growing and reproducing, they are still slower than emus who lived alongside them. Emus grow up quickly in just a year and reproduce not long after, then lay a large number of eggs.
Researchers explained that the difference might explain why mihirungs went extinct shortly after hungry humans arrived in Australia, but emus continued to thrive until today, Phys.org reported. Researchers conclude that even though mihirungs seem to have adapted by growing and reproducing for millions of years, it was not enough for them to survive.
Moreso, the different strategies between the two birds, gave the emu an advantage when humans arrived 50,000 years ago, leading to the extinction of mihirungs 10,000 years later. In the end, the latter lost the evolutionary race, and the entire order of birds in Australia and the world was lost.
The full findings of the study, titled "Histovariability and Palaeobiological Implications of the Bone Histology of the Dromornithid, Genyornis newtoni," were published in the Anatomical Record.
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