• The Riversleigh giant carnivorous, toothed platypus, Obdurodon tharalkooschild, tenderizing a young short-necked tortoise. (Supplied)
Why have so many of the weirdest Aussie animal fossils shown up in the same place - Riversleigh?
By
Jacinta Bowler

10 Jun 2016 - 3:59 PM  UPDATED 10 Jun 2016 - 4:04 PM

Giant carnivorous platypuses, galloping fanged kangaroos and tree-climbing crocodiles sound like creatures from a strange dream, but all these animals were real a few million years ago.

And they have all been discovered at the same fossil site – Riversleigh, Queensland.  

Riversleigh World Heritage Fossil Site

David Attenborough has called Riversleigh one of the four most important fossil deposits in the world. Originally a cattle station, the site gained World Heritage listing in 1994 because of the amazing fossils found there.

“Riversleigh itself has at least 300 individual fossil deposits and we continue to find a mass of new sites every year.  The fossil deposits range in age from approximately 25 million years old to about 30,000 years old,” says palaeontologist Dr Karen Black from the University of New South Wales (UNSW).

Here, the combination of calcium carbonate, phosphates, and a rainforest full of animals has led to 25 million years of pristine and highly detailed fossils.

“Anything that landed in that water was guaranteed to become a fossil, because almost immediately limestone began to precipitate around the object... It could have been a leaf, we've even found butterfly wings,” says Professor Michael Archer, also from UNSW, who has been digging fossils at the site for over 35 years.

Here are a few of Riversleigh’s strangest discoveries. 

Snail Eating Marsupial - Malleodectes mirabilis

The snail eater is the newest weird animal found at Riversleigh. It’s distantly related to marsupial carnivores, but has a weird premolar which scientists think it used to crush up snails.

“It wasn't until we found the tooth that was part of a mammal jaw that we realised what it was, and how strange it was. There's no other mammal in the world that has teeth like this,” says Archer.

“It is speculation, but its informed speculation, based in part… [by] actual study of the teeth itself, that lead us to the conclusion that it is primarily a snail eater.”

The Giant Platypus - Obdurodon tharalkooschild

“The Giant Platypus was far bigger than any known platypus and at around 80cm to 1m in length. It also differs to the living platypus in that it has well developed teeth – whereas those of the modern species are poorly developed and absent in adult individuals,” says Black.

There were a wide variety of platypuses 25 million years ago - a far cry from the one lone species we see in rivers today.

The Fanged Kangaroo - Balbaroo fangaroo 

 

Kangaroos that have teeth like a wolf, are nearly the size of a human, and gallop instead of hopping, right towards you. Definite nightmare material.

“The carnivorous kangaroo was something that shocked us when we first found them. We didn't believe it, but we couldn't come to any other conclusion,” says Archer.

“There was a whole group of these [types of kangaroos] that would have stood up to about a human shoulder with teeth that show unmistakably that they are eating flesh and bone.”

These types of species would have made their home in lush rainforests, not open plains like today’s kangaroos.

The Cleaver-headed Crocodile - Baru darrowi

These crocodiles could grow up to 5 metres in length, and the first skull found at Riversleigh was discovered with the bones of a marsupial lion in its mouth.

These guys were not to be messed with.

Although this species may not have been able to climb trees, some other species like Mekosuchus whitehunterensis, which were goanna-like, may have been able to climb, possibly to eat bird eggs.

Marsupial Lions – Thylacoleo carnifex

These guys aren’t actually related to lions, but they are still the largest meat-eating mammal known to have existed in Australia.

So why did all these amazing species lay to rest in one place?

“Riversleigh 20-15 million years ago was a biodiversity hotspot because of the palaeoenvironment of temperate rainforest with rain all year round,” Black explains.

“The diversity of plants in the Riversleigh rainforests was able to support a high diversity of mammals. Mammals at this time were occupying niches no longer seen in the much drier environments of Australia today.” 

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