A newly identified branch of the marsupial family tree is adding a surprising twist to the history of Australia’s most distinctive mammals. The finding suggests that their evolution was more complicated, and more mysterious, than scientists previously understood.
Marsupials first reached Australia more than 55 million years ago. Since then, they have spread into an astonishing range of habitats and lifestyles. Today, they include about 160 species, from High Country animals (thumb-sized possums that sleep through the winter), to desert specialists in the Red Center, including tiny moles with pink fur and no eyes that spend their lives underground.
Yet despite their success, scientists still do not fully understand how marsupials diversified across Australia. Major gaps in the fossil record have left long stretches of their early history almost invisible.
Now, in a new paper published in the Journal of Paleontology, researchers from UNSW report three newly identified species that may belong to an ancient and previously unknown order of marsupials. The discovery offers a rare look at some of the earliest stages of marsupial evolution on the continent.
“Not only is it a new order, it could also be the most ancient lineage of all Australian marsupials,” says UNSW paleontologist Dr. Tim Churchill.
“It may be the early ancestor of all our marsupial carnivores.”
A New Branch of the Marsupial Family Tree
The standard explanation is that marsupials reached Australia after traveling from South America through Antarctica, before the ancient supercontinent Gondwana split apart.
The broad outline is widely accepted, but the details remain uncertain. Fossils dating back about 55 million years suggest that Australian marsupials may have come from a single early lineage that later divided into the marsupial groups alive today.
Those groups are currently classified into five orders within the superorder Australidelphia, which includes all living and extinct Australian marsupials (and one South American).
Dr. Churchill is now proposing a sixth order, called Keeunamorphia. According to his analysis, this group may have persisted for around 35 million years.
Members of Keeunamorphia were probably small insect eaters weighing between (25-200 grams). They lived in the forests of what is now northern Queensland before disappearing around 15 million years ago.
That landscape looked very different from the dry, open country seen there today. At the time, the region was likely covered by wet, dense rainforest and supported the ancestors of many animals still alive in Australia.
“Around 14 million years ago is when the region starts to cool again,” Dr. Churchill says.
“The dense forest disappears and becomes more open woodland, with more lakes and more grasslands.”
Fossils From Riversleigh
The three Keeunamorphia species described by Dr. Churchill lived around 18 million years ago. After they died, their remains ended up in shallow cave pools, where parts of their bodies were preserved at what is now the Riversleigh World Heritage Area, one of the world’s most important fossil sites.
Complete skeletons are uncommon in the fossil record, so the researchers relied on much smaller clues: teeth and pieces of jaw. From those fragments, they worked to determine where the animals fit in the marsupial family tree.
To do that, the team combined fossil evidence with genetic information from living species. This allowed them to build a phylogenetic tree, a model that maps relationships between species and estimates when different branches separated over time.
“We’re essentially trying to create a tree that shows both the relationships of all the different species in the tree, while also calculating when those branches probably diverged,” Dr. Churchill says.
Teeth Reveal an Evolutionary Puzzle
The analysis showed that these three species lived alongside several other marsupials that scientists had already studied. However, their teeth were unusual, and they did not seem to be closely related to the other marsupials around them.
Instead, the teeth resembled those of Djarthia murgonensis, an extinct marsupial that lived about 35 million years earlier and is often viewed as a prototype for Australian marsupials.
Dr. Churchill says this points to a distinct marsupial lineage that had not been recognized before. It may have split off early in marsupial history and then survived for millions of years while other groups evolved around it.
“Whatever these things were, they seemed to be primitive compared to other marsupials at the time, and they seem to have been doing their own thing and surviving well enough alongside them,” says Dr. Churchill.
Although phylogenetic trees often point to a single early group that later gave rise to modern Australian marsupials, the fossil evidence appears less tidy.
A More Complicated Origin Story
According to Dr. Churchill, early members of Keeunamorphia may have appeared not long after the first marsupials arrived in Australia from Antarctica around 55 million years ago.
If that is correct, Keeunamorphia could represent one of the earliest marsupial orders to branch off from the main lineage. That possibility challenges the simpler version of marsupial evolution, in which one ancestral group gave rise to the full diversity of Australian marsupials.
It also raises a puzzling question. If this primitive group split off so early, how did it survive for so long while remaining relatively unchanged?
“Evolutionary history is a lot more complex than just one group leading to all of Australia’s marsupials after being left behind when the continent broke off from Antarctica,” says Dr. Churchill.
“It’s more likely that when Australia was part of Gondwana it was swarming with all sorts of bizarre, primitive marsupial-like things, and that several of them survived and led to our modern lineages.”
Hidden Diversity in the Fossil Record
Much of that early diversity may still be missing from the scientific record. A nearly 20-million-year gap in the fossil history of Australian marsupials leaves room for many lineages that have not yet been found.
Some of these ancient animals may have shared a common ancestor. Others may have come from separate lineages that were left in Australia as the continents drifted apart.
Scientists may never be able to fully reconstruct the routes that early marsupials took as they spread and evolved. But each new fossil tooth pulled from Australia’s ancient deposits adds another clue, making the story of marsupial evolution more complex, and far richer.