By Paul Rincon
BBC News Online science staff
A mouse-sized fossil found in China may be the oldest ancestor of modern marsupials - the mammal family that includes kangaroos and koalas.
Sinodelphys szalayi was adapted to tree-climbing
The creature, which was unearthed in Liaoning province, extends the ancestry of marsupials by 50 million years.
The stunning specimen preserves an imprint of the animal's coat of hair and analysis of its feet suggests it was adapted to climbing in trees.
Details of the find are reported in the latest edition of the journal Science.
Sinodelphys szalayi, as the new species has been named, lived alongside the dinosaurs in the early Cretaceous Period.
The 125-million-year-old creature has close affinities with the family of mammals known as metatherians, which includes the marsupials.
The fossil is astonishingly complete and thus provides scientists with a rich insight into the early evolution of mammals.
Co-author of the Science paper Dr Zhe-Xi Luo, curator of vertebrate palaeontology at the Carnegie Museum of Natural History in Pittsburgh, US, spoke to BBC News Online about the discovery.
"In the past, most mammal fossils from dinosaur times are isolated teeth," he said.
"As a result, we know what they ate, but not how they moved. So many mammal fossils have come out of this area of China, we now have the first glimpses of how these mammals moved."
Sinodelphys' claws have similar proportions to modern-day climbing animals and are curved, perhaps for grasping the branches. The suggestion, certainly, is that the creature spent significant amounts of time in trees.
Luo and his colleagues propose that the ability to climb trees kick-started the diversification of mammals into different types, such as placentals and marsupials.
"The ground was dominated by the dinosaurs. Since the jaws of early mammals have been found in meat-eating dinosaurs, it's possible that mammals took to the trees to avoid predation by them," Dr Luo added.
The authors claim the discovery of Sinodelphys supports theories that the ancestors of marsupials diverged from the ancestors of placental mammals in Asia.
In 2002, Luo and colleagues described the earliest fossil placental mammal - Eomaia scansoria - which was discovered in the same quarry in northeastern China as Sinodelphys.
The remnants of the mammal's coat of hair can still be seen
"At the earliest evolutionary split between marsupial relatives and placentals, you would expect two fossils like this to be geographically close.
"What this tells us is that the split occurred no later than 125 million years ago. At this time, we already have definite features that can be assigned to either type," Dr Luo told BBC News Online.
Perhaps the best known difference between marsupials and placental mammals is in the way they reproduce.
Marsupial young are born relatively underdeveloped. In some but not all marsupial species, the mother develops a special pouch on her body in which to nurse her young.
In placental mammals, the young develop in the uterus, nourished by a specialised organ attached to the uterus wall - the placenta.