Fossils reveal prehistoric life cycle

By Nick Perry/AAP and AG Staff 20 July 2010
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Scientists have stumbled upon fascinating clues about an extinct marsupial.

SKULLS UNEARTHED IN A remote Queensland cave have allowed scientists to map, for the first time, the entire life cycle of an extinct marsupial species. The unprecedented discovery in the 15-million-year-old cave could also hint at the future impact of climate change on Australia’s flora and fauna.

A team of researchers from the University of New South Wales (UNSW) was exploring the world heritage Riversleigh fossil field, in north-western Queensland in 1990, when they literally stumbled upon the site. The findings were reported last week in the journal Vertebrate Paleontology.

Dr Karen Black, from UNSW’s School of Biological, Earth and Environmental Studies, says scientists have been exploring sites at Riversleigh for about 30 years but had never made such a groundbreaking discovery. “It’s not until we begin to crack these rocks open that you realise how many fossils are in there,” she added.

World first giant marsupial

Among the hundreds of beautifully preserved fossils found buried in the limestone cave floor were 26 skulls from the Nimbadon, a wombat-like marsupial and major herbivore group that existed before kangaroos dominated. Nimbadons lived about 15 million years ago and died out about 12 million years ago, largely from climate change induced losses in habitat, Karen says.

By comparing the intact skulls and other bones from varying stages of the marsupial’s life – including as babies in the pouch – scientists were able to map the Nimbadon’s life cycle from birth to death in a world-first study.

“We’ve got skulls representing pouch young all the way through to elderly adults, and that’s a first,” Karen says, emphasising that the marsupials played a significant ecological role in prehistoric Australia. “There is no other fossil deposit (in the world) that has that.”

The bones are already giving up clues to the development of early marsupials, the researchers say. “Interestingly…Nimbadon skulls developed in much the same way as those of modern marsupials like kangaroos and quolls,” Karen says.

The Nimbadons also appear to have exhibited the same birthing behaviour as today’s marsupials – where the underdeveloped young crawl into the pouch and suckle until they fully develop. “As in modern marsupials, advanced development of the facial region in Nimbadon pouch young appears to be an adaptation to hold the teat in place in the mouth,” Karen says.

With such a unique sample of skulls and corresponding skeletons, scientists will now use CT scans to conclude how marsupials’ brains developed over time and how this affected their behaviour, functionality and evolution.

More secrets to unlock for the nimbadon

The discovery of so many Nimbadon alongside galloping kangaroos, a fox-sized thylacine and thousands of tiny forest bats was “really unusual”, Karen says. This indicates the marsupials were found in large numbers when they fell through the vertical cave to their deaths.

“Today, we are left with the floor of the cave, the walls and roof having been eroded away over millions of years,” Karen says. “These subterranean caves would have been relatively common at Riversleigh. In fact, they still are today.”

This mob behaviour suggests the lush, dense vegetation synonymous with early Australia had begun to clear as the country underwent a transitional phase.

“The cave is 15 million years old and samples a period of time when Australia was changing from … a greenhouse phase to an icehouse phase,” Karen says. “That is particularly important. If we can get an understanding of what was going on at this cave then we would be able to… predict what’s going to happen with climate change in the future with Australia’s flora and fauna.”

The question that remains, however, is how many more secrets remain unlocked at the site. With bones literally jutting out from the limestone, discoveries made at Riversleigh could continue to answer the puzzles of the past while offering a glimpse into the future.

“At the moment we can’t really say (how many fossils are there). There are still a lot of bones exposed on the surface,” Karen says. “We’ve only just scratched the surface, so there are literally thousands of bones left waiting there.”

Giant marsupials similar to modern ones

The “solid study” showed “there were a lot of similarities in terms of how an old marsupials developed compared to a modern marsupials, says Dr Gavin Prideaux, a palaeontologist from Flinders University in South Australia, who was not involved in the research. 

One of the mysteries the new find uncovers is the development of sinuses – cavities – in the skulls of larger animals,Gavin says. As ancient species became bigger, their bones needed to grow with them – along with the problem of more weight to carry. The solution, Gavin says, was for animals to develop cavities in the bones to save weight.

Until now, scientists have not been able to study the development of the cavities within a species. The new research allow researchers to see the development of cavities “in the evolutionary sense, but also within age series, in terms of an individual sense,” he says.

Marsupials “were such an important part of the Australian landscape, but we understand so little about their anaotomy and ecology,” Gavin says. “It’s interesting for a really detailed study to be done on diprotodon development, full stop,” Gavin says. “This is one foundation of how we can get a better handle on understanding this really amazing group of animals in a detailed light.”