Scientists Uncover a Previously Unknown Lineage of Ancient Marsupials

The discovery of a new branch on the marsupial family tree suggests that the history of Australia’s unique mammals is more complex than once thought.

Marsupials arrived in Australia over 55 million years ago. Since then, they have adapted to almost every environment. Today, Australia has about 160 marsupial species. Each one has been shaped by its surroundings.

However, their early history is hard to trace. Many gaps in the fossil record hide millions of years of marsupial evolution. This leaves scientists unsure how these animals spread and diversified.

Fossils Reveal a Missing Branch

A new study in the Journal of Paleontology offers a rare look into this missing history. Scientists from UNSW have described three new species. These species may belong to an ancient, unknown group of marsupials. This gives researchers new clues about early marsupial evolution in Australia.

"Not only is it a new order, it could also be the most ancient lineage of all Australian marsupials," said UNSW paleontologist Dr. Tim Churchill. "It may be the early ancestor of all our marsupial carnivores."

Ancient Australia Was Changing

The common idea is that marsupials reached Australia from South America. They crossed Antarctica before the supercontinent Gondwana broke apart.

The exact details are still unclear. Fossils from about 55 million years ago suggest Australian marsupials might have come from a single line. This line then branched into the marsupial groups we see today.

These groups are part of the superorder Australidelphia. This includes all living and extinct Australian marsupials, plus Monodelphis (a South American opossum).

Dr. Churchill now proposes a sixth order called Keeunamorphia. He believes this group survived for about 35 million years.

These animals were likely small insect-eaters, weighing about 25 to 200 grams. They lived in the forests of what is now north Queensland. They disappeared around 15 million years ago.

That part of Queensland is now dry, open country. But back then, it was a wet rainforest. It was home to the ancestors of many species still alive today.

"Around 14 million years ago is when the region starts to cool again," Dr. Churchill said. "The dense forest disappears and becomes more open woodland, with more lakes and more grasslands."

Teeth Redraw the Family Tree

The three Keeunamorphia species Dr. Churchill described died about 18 million years ago. Their remains were washed into shallow cave pools. They were partly preserved at Riversleigh World Heritage Area, a rich fossil site.

Complete skeletons are rare. So, Dr. Churchill and his team used teeth and jaw fragments. From these small pieces, they worked to understand where the animals fit in the marsupial family tree.

The group combined fossil evidence with genetic data from living species. They created a phylogenetic tree. This model shows how species are related and when different lines split.

"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 explained.

The analysis showed these three species lived alongside other known marsupials. But their teeth were different. They did not seem closely related to other animals from the same period.

Their teeth closely matched those of Djarthia murgonensis. This extinct marsupial lived 35 million years earlier and is often seen as a model for Australian marsupials.

Dr. Churchill said the evidence points to a separate marsupial line that science had not recognized before. This line branched off early and lasted for millions of years.

"Whatever these things were, they seemed to be primitive compared to other marsupials at the time," Dr. Churchill noted. "They seem to have been doing their own thing and surviving well enough alongside them."

Phylogenetic trees often suggest one early group led to today's marsupials. But the fossil record tells a more complex story.

Evolution Looks Less Tidy

Dr. Churchill believes the earliest Keeunamorphia lived soon after marsupials first arrived from Antarctica about 55 million years ago.

The UNSW group thinks Keeunamorphia might have been among the first marsupial orders to split from the main evolutionary path. If so, it complicates the simple idea of how Australian marsupials evolved.

A big question remains: if this primitive marsupial separated so early, how did it stay relatively unchanged for so long?

"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," Dr. Churchill said.

"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."

Much of that ancient diversity may still be hidden. There is a nearly 20-million-year gap in the fossil record.

The species might have shared one common ancestor. But they could also come from several different lines that became isolated in Australia as the continents separated.

Scientists may never reconstruct every path taken by Australia’s early marsupials. But each tooth found adds another detail to a story that is becoming more complex and more revealing.

Deep Dive & References

A new metatherian order from Australia (Keeunamorphia, Metatheria), and new Early Miocene species from the Riversleigh World Heritage Area, northwestern Queensland - Journal of Paleontology, 2026