A newly identified branch of the marsupial family tree adds a surprising twist to the history of Australia’s most distinctive mammals. The discovery suggests that their evolution was more complex and more mysterious than scientists previously understood.
Marsupials first arrived in Australia over 55 million years ago. Since then, they have spread into an amazingly wide range of habitats and lifestyles. Today, they include about 160 species, from highland animals (like thumb-sized possums that sleep through the winter) to desert specialists in the Red Center, such as tiny moles with pink fur and eyes that spend their entire lives underground.
But despite their success, scientists still don’t fully understand how marsupials diversified across Australia. Large gaps in the fossil record make long parts of its early history largely invisible.
Now, in a new paper, paleontology journalUNSW researchers reported three newly identified species that may belong to an ancient and previously unknown class of marsupials. This discovery provides some rare observations of the early stages of marsupial evolution on the continent.
“Not only is it a new order, it may also be the oldest lineage of all Australian marsupials,” says UNSW palaeontologist Dr Tim Churchill.
“It could be an early ancestor of all our marsupial carnivores.”
A new branch of the marsupial family tree
The standard explanation is that marsupials reached Australia from South America through Antarctica before the breakup of the ancient supercontinent Gondwana.
Although the broad outlines are widely accepted, the details remain unclear. Fossils dating back some 55 million years suggest that Australian marsupials may have descended from a single early lineage that later split into the groups of marsupials living today.
These groups are now classified into five orders within the Superorder Audelphia, which includes all extant and extinct Australian marsupials (and one South American marsupial).
Dr. Churchill is currently proposing a Sixth Order called Keenamorphia. According to his analysis, the group may have existed for about 35 million years.
Members of Keeunamorphia were probably small insect-eating animals weighing (25-200 grams). They lived in the forests of what is now northern Queensland until they disappeared about 15 million years ago.
The landscape was quite different from the dry, open countryside seen there today. At that time, this region was probably covered in dense, moist rainforest and supported the ancestors of many of Australia’s animals still living today.
“About 14 million years ago is when the region started to get cold again,” Dr Churchill said.
“Dense forests will disappear, giving way to more open woodlands, lakes and grasslands.”
River sled fossil
The three Keenamorphia species described by Dr. Churchill lived approximately 18 million years ago. After their deaths, their bodies were submerged in shallow cave pools, where their body parts were preserved in what is now the Riversleigh World Heritage Site, one of the world’s most important fossil sites.
Complete skeletons are rare in the fossil record, so researchers relied on much smaller clues, such as teeth and parts of the jaw. From these fragments, they determined where the animal fit into the marsupial family tree.
To do so, the research team combined fossil evidence with genetic information from modern species. This allowed them to build phylogenetic trees, models that map relationships between species and estimate when different branches separate over time.
“We’re basically trying to create a tree that can show the relationships between all the different species in the tree, while also calculating when those branches diverged,” Dr. Churchill says.
Teeth reveal evolutionary puzzles
Analysis revealed that these three species coexisted with several other marsupials that scientists had already studied. However, their teeth were unusual and they did not appear to be closely related to other marsupials around them.
Instead, the teeth resembled those of Jarsia murgonensis, an extinct marsupial that lived about 35 million years ago and is considered the prototype of Australian marsupials.
Dr Churchill says this points to a distinct marsupial lineage that was previously unrecognized. It likely split early in marsupial history and then survived for millions of years while other groups evolved around it.
“Whatever these were, they seemed to be primitive compared to other marsupials of the time and had unique behaviors that allowed them to survive well alongside them,” Dr Churchill says.
Although phylogenetic trees often show a single initial group that later gave rise to modern Australian marsupials, the fossil evidence appears to be less orderly.
A more complex origin story
Dr Churchill said the earliest members of Keenamorphia may have emerged shortly after the first marsupials arrived in Australia from Antarctica around 55 million years ago.
If that’s correct, Keenamorphia may represent one of the earliest marsupials to diverge from the main lineage. This possibility calls into question a simpler version of marsupial evolution, in which one ancestral group gave rise to the complete diversity of Australian marsupials.
It also raises puzzling questions. If this primitive group split up so early, how did it survive so long relatively unchanged?
“The evolutionary history is much more complex than a single group left behind when the continent separated from Antarctica, leading to all of Australia’s marsupials,” Dr Churchill says.
“When Australia was part of Gondwana, it was populated by all sorts of strange, primitive marsupial-like things, and it’s likely that some of them survived and led to our modern lineage.”
Diversity hidden in the fossil record
Much of that early diversity may still be missing from the scientific record. There is a gap of nearly 20 million years in the fossil history of Australian marsupials, leaving room for many lineages yet to be discovered.
Some of these ancient animals may have had a common ancestor. Some may have come from other lineages left behind in Australia when the continent separated.
Scientists may never be able to fully reconstruct the paths that early marsupials took as they spread and evolved. But each new tooth fossil pulled from ancient Australian deposits adds another clue, making the story of marsupial evolution more complex and rich.
