A new family of ancient marsupials has been discovered, relatives of modern-day wombats and koalas.
The partial fossilised skeleton of a 25 million-year-old animal was studied by researchers in the UK and Australia.
It was found in the 1970s in the clay floor of Lake Pinpa, a remote, dry salt lake east of the Flinders Ranges in South Australia. It was taken to the American Museum of Natural History in New York City and after puzzling taxonomists for years, it has finally been formally described by researchers.
The species has been named Mukupirna, meaning 'big bones' in Dieri, the Aboriginal language spoken in the region where the fossil was found. It lived in Australia 25 million years ago and grew as large as a black bear, about five times heavier than modern wombats.
An analysis of evolutionary relationships shows that Mukupirna is most closely related to wombats, but it has several unique features that show it's the only known member of the Mukupirnida, a new family of marsupials that was previously unknown to science.
It is hoped that the skeleton can shed light on the evolutionary history of marsupials, which has proven difficult to piece together.
Pip Brewer, Senior Curator of Fossil Mammals at the Natural History Museum, London
, studied the specimen alongside colleagues from the University of Salford, Griffith University, the University of New South Wales and the American Museum of Natural History.
She says, 'This skeleton is so important because mammal fossils, particularly skeletons, are rare in Australia. Remains from this region and from deposits this old are particularly valuable, and this one tells us something interesting about the diversity of marsupials throughout their evolutionary history.
'These relatives of wombats are such fascinating and enigmatic animals, but we know hardly anything about their early evolution and habits because of gaps in the fossil record.'
Australia's koala and three species of wombat are the only survivors in a group of animals called Vombatiformes. This group is different to (although closely related to) other marsupials including possums, kangaroos and wallabies.
Vombatiforms formed a diverse group which included cow-sized herbivores, giant sloth-like animals, marsupial lions and koalas. Most of these eventually went extinct for reasons which are currently unclear.
Even when it comes to wombats, which still exist, we have a lot to learn about how they evolved, including how and why they became such expert diggers.
The ancient Mukupirna was also a vombatiform and a digger, adapted to taking advantage of food that was just below the soil's surface.
Being able to dig when food is short has been critical to the long-term survival of wombats. Their modern diet of grass and their subterranean lifestyle seem to have been the eventual result of this ancient adaptation.
Associate Professor Julien Louys of Griffith University, who co-authored the study, says, 'The description of this new family adds a huge new piece to the puzzle about the diversity of ancient and often seriously strange marsupials that preceded those that rule the continent today.'
Mukupirna probably lived in an open forest environment, with trees but no grass. Its large size was common for its group. Body weights of 100 kilogrammes or more evolved at least six times in vombatiforms over the last 25 million years. The largest known vombatifom was Diprotodon, which weighed more than two tonnes and survived until approximately 50,000 years ago.
Pip adds, 'Mukupirna resembles all of the extinct marsupials in its group, and also none of them. Its discovery just goes to highlight how diverse ancient marsupials were, and how differently they all lived. There is still so much work to be done to figure out where this group emerged from.'