The discovery of a new branch of the marsupial family tree suggests the story of Australia’s unique mammals is more complex and less well-understood than we thought.
In the more than 55 million years since marsupials arrived in Australia they’ve evolved to fill just about every ecological niche you can think of.
You find them everywhere from the High Country (thumb-sized possums that sleep through the winter), to the Red Centre (little moles with pink hair and no eyes that live underground) – around 160 species in all, each specially adapted to its environment.
Despite that success, it’s not clear how marsupials spread through Australia, with gaps in the fossil record rendering tens of millions of years of evolution invisible.
But in a new paper published today in the Journal of Paleontology, a team of UNSW researchers has found three new species, suggesting evidence of a new, ancient order, and offering a glimpse into the earliest stages of marsupial evolution in Australia.
“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.”
Continental drift
The generally accepted story is that marsupials walked to Australia from South America via Antarctica before Gondwana broke apart.
The specifics are murky, but fossils from around 55 million years ago suggest there might be a single lineage of Australian marsupials that split apart into the marsupial orders we see today.
There are five of those orders making up the Australidelphia superorder, which includes all living and extinct Australian marsupials (and one South American).
Dr Churchill is proposing a sixth – Keeunamorphia – and says it existed for around 35 million years.
They were likely small (25-200 grams) insect eaters that roamed the forests of what is now north Queensland before dying out about 15 million years ago.
The region is now dry, open country with only coarse grass and a few scrappy trees, but in those days would have been home to a lush, wet rainforest and the ancestors of many species alive today.
“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.”
When the members of three species of Keeunamorphia Dr Churchill describes died around 18 million years ago, their bodies were submerged inside shallow cave pools and partly preserved in what is now one of the best fossil sites in the world, the Riversleigh World Heritage Area.
Fully preserved skeletons are rare, so Dr Churchill and the team make do with a few teeth and jaw fragments instead, piecing together scraps of evidence to figure out how these creatures are related.
They combine fossil finds with genetic data from living species to build what’s known as a phylogenetic tree, a model that shows how things are related and estimates when lineages split in the past.
“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.
That analysis showed that while these three species lived alongside a range of previously studied marsupials, their teeth were different, and they didn’t appear to be closely related to anything else around at the time.
The teeth they found looked a lot like the teeth of another extinct marsupial that lived 35 million years before, Djarthia murgonensis, which is known as the prototypical Australian marsupial.
Dr Churchill says this is evidence of a distinct lineage of marsupials previously unknown to science, one that had branched off early and survived for millions of years.
“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.
And while phylogenetic trees consistently suggest one early group that spread out into what we see today, the fossils aren’t nearly as clear.
Common ancestors?
Dr Churchill says that early members of the Keeunamorphia order lived just after the first marsupials hopped over here from Antarctica around 55 million years ago.
The researchers think this could represent one of the first marsupial orders to branch off from the main trunk all those years ago, and it calls into question the “neat” story of marsupial evolution.
If this primitive, ancestral marsupial did split so early, how can it have persisted so long 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”
A lot of this diversity is probably hidden from us, lurking somewhere in the nearly 20-million-year blank space in the fossil record.
These different species may well share a common ancestor, but could also be the descendants of several different lineages that were stranded here when the continents broke up.
We will perhaps never know for certain what routes early marsupials took on their evolutionary journey, but with every tooth excavated from Australia’s ancient mud, the story gets a little more complicated, and a little richer.
