Skippy: A 25 million year old success
New fossil research reveals that modern day kangaroos have hardly changed since their ancient ancestors roamed the Australian continent 25 million years ago.
La Trobe University palaeontologist, Dr Ben Kear and his colleague Neville Pledge of the South Australian Museum have discovered the fossil of one of the oldest ancestors of the modern kangaroo, which reveals the incredible resilience and flexibility of the animal in the face of constant environmental change.
The fossil, named Ngamaroo archeri after the renowned Australian paleo-animalogoist, Professor Michael Archer, is different, but startlingly similar to the modern kangaroo, and clearly shows the genesis of the iconic Australian marsupial 25 million years ago.
"This discovery is important because what we have found is the oldest direct ancestor of our modern Skippy," says Dr Kear.
"It didn't look all that different from today's kangaroos and it was hopping," he says.
The fossil, unearthed in 1981 in the Ngama Quarry in the Lake Eyre Basin of central Australia, was only recently examined using advanced techniques of analysis.
Ngamaroo's similarity with modern kangaroos tells us a number of things about the resilience and adaptability of kangaroos. It also gives us clues about their potential survival, especially in the face of drastic climate change.
"Australia has experienced enormous climate change in the past 25 million years, it has oscillated from wetlands to desert and back again," says Dr Kear.
"Very few marsupials have remained unchanged for that long but the kangaroo has because of its incredible flexibility," he adds.
However, there are differences between the Skippy we know and it's ancient ancestor. The Ngamaroo roamed an Australian landscape that was wetter, with more abundant and greener foliage. Its diet comprised softer types of vegetation than the tough grasses eaten by the modern kangaroo, which has grinding teeth, and processes its food in the gut in a similar way to the horse.
"This suggests an adaptation that perfectly suits the grasslands that dominate a harsher, dryer environment," says Dr Kear.
The Ngamaroo evolved side by side with another more distant relative, the dog-like and omnivorous Nambaroo, discovered in northern Queensland by Dr Kear and colleagues Dr Bernie Cooke from the Queensland Museum, Professor Archer (University of NSW) and Professor Tim Flannery (Macquarie University).
The discovery of the fang-toothed, galloping Nambaroo caused a sensation when it was announced to the world in November 2007, as it suggested that an extinct group of ancient ‘hop-less' kangaroos may have been replaced over time by the ancestors of the modern species.
"Ngamaroo was definitely herbivorous and definitely hopped. Conversely, its cousin the omnivorous, non-hopping Nambaroo didn't survive," says Dr Kear.
The process of evolution in living species is thought to work over hundreds and thousands of years bringing change in response to a variety of factors but most significantly the natural environment.
'Compared to how much other Australian mammal groups have changed in that time span, it shows that our hopping kangaroo really is a great survivor,' says Dr Kear. '
"Hopping is one of the keys to why the modern kangaroo lineage has been such a success; their specialised limb proportions have just remained the same. Ngamaroo basically has the same stance and form of locomotion as its modern relatives. If anything, the discovery of Ngamaroo proves that Australia holds one of evolution's great success stories," he adds.
Another recent study by Dr Kear and colleagues, Professor. Mike Lee, Wayne Gerdz and Professor Tim Flannery, examined the limb bone proportions of kangaroos through time and showed that hopping has been around for a lot longer than originally thought. The previously favoured hypothesis has been that the true kangaroo hop evolved in response to the aridification of Australia only around 10 million years ago. Examination of the Ngamaroo as well as other specimens now shows that some very early kangaroo species were hopping around 25 million years ago, while others still used a bounding gait – and it was these non-hoppers that eventually died out.
Dr Kear's paper on the Ngamaroo is published in the May edition of the Australian Journal of Zoology. The study on the evolution of kangaroo hopping has been released as a chapter in the new book Mammalian Evolutionary Morphology (Springer).
Dr Kear is available for interview, contact:Mark Pearce
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