Extract from ABC News
A reconstruction of Phantasmodon travouilloni from the Early Miocene rainforests of north-western Queensland. (Supplied: Peter Schouten)
In short:
Fossils from Riversleigh in north-west Queensland have revealed a previously unknown lineage of insect-eating marsupials called Keeunamorphia.
The discovery helps fill a major gap in Australia's marsupial fossil record and reshapes understanding of their early evolution.
What's next?
Researchers expect further discoveries from central Australia and Riversleigh deposits could help determine if their origins predate the Australian continent.
Rogue teeth found in an outback fossil deposit have turned decades of thinking on marsupial evolution "on its head", researchers say.
Teeth and jaw fragments from Riversleigh in north-west Queensland have been used to describe three new insect-eating marsupial species from about 18 million years ago that belong to a newly identified and primitive order dubbed Keeunamorphia.
The discovery, led by UNSW Sydney researchers, was published in the Journal of Palaeontology last week.
Tim Churchill says the teeth are an important piece of the puzzle to working out marsupial lineages. (Supplied: UNSW Sydney)
Palaeontologist Tim Churchill said the basic teeth filled a huge blind spot in Australia's marsupial fossil record from 25 to 55 million years ago.
"We don't know which of these early marsupial ancestors actually led to all of our beloved Australian marsupial groups today," Dr Churchill said.
"Seeing one of these primitive marsupials after that 30-million-year gap in the Miocene, when all the Australian lineages are already established … showed me that these proto-marsupials … survived for 35 to 40 million years longer than we thought.
"The impact of this finding goes beyond Australian marsupials … what we thought has really had to turn on its head in terms of what lineages were doing, when."
Oldest known lineage
Australian marsupials are believed to have arrived from South America via Antarctica before the break-up of Gondwana about 55 million years ago.
When researchers analysed the relatively recent fossils, they found teeth like Djarthia murgonensis, Australia's oldest known marsupial, which appears around the time of the continental split.
Co-author Mike Archer said the teeth's shape and size distinguished them from known marsupials from the period.
"There's this weird cusp on the upper molars of all of them that aren't present in other groups," Dr Archer said.
"If we were standing in Australia 13 or 14 million years ago, we'd be describing these animals as a living fossil."
The central cusp present on the upper teeth of all known Keeunamophians helped researchers distinguish the marsupial fossils. (Supplied: Tim Churchill)
About 30 million years ago, Australia's interior was dominated by lush rainforest.
Ranging from 25 to 200 grams, the shrew-sized animals found in Keeunamorphia likely thrived in wet environments, occupying similar niches to antechinuses and dunnarts.
But as the continent warmed and rainforests were replaced with an arid interior, Dr Churchill believed modern-day carnivorous marsupials called dasyurids outcompeted the ancient lineage.
"When we think they go extinct, that's when … the phascogales, antechinus and the dunnarts, those small insectivorous marsupials start to really explode in diversity," he said.
"It's possible that … it was their demise that gave room for our modern dasyurids to really flourish."
The Julia Creek dunnart is a dasyurid adapted to the arid plains of north-west Queensland. (Supplied: QUT/Andrew Baker)
Limited by the sparse fossil record, Dr Archer believed future discoveries would show Keeunamorphia likely evolved in South America and could be older than Australidelphia, the order where all living Australian marsupials branched from.
"When we know a lot more about … this group, we may decide it falls outside that group," he said.
"I suspect we're going to find the group was established prior to Australia breaking off as an island."
Mike Archer believes fossil sites in South America could be key to uncovering the true history behind the Keeunamorphia. (Supplied: UNSW Sydney)
Western Australian Museum's terrestrial zoology curator, Kenny Travouillon, who was not involved in the research, agreed the findings were a breakthrough.
"The Riversleigh fossils continue to shed light on the past diversity of marsupials that once existed on this beautiful continent," Dr Travouillon said.
"This study revealed how little we know about the fossil record and how there is still so much to discover.
"The discovery of a new order of marsupials is a rare event and an extraordinary find. It helps tie together a few enigmatic species that have been puzzling palaeontologists for several decades."
Melting ice and new opportunities
The exact path that birthed Australia's marsupial diversity will likely never be known.
But with global warming opening inaccessible parts of Antarctica and South America, Dr Archer says breakthroughs are imminent.
"Palaeontologists are quietly rubbing their hands together to get at new fossil deposits that have never been seen before. It'll be under that ice," he said.
"We know there is a very big story here and we're just beginning to get the first inklings of just how big this story is.
"I think the only way forward now is for collaboration between Australian and South American palaeontologists to tackle this whole problem."
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