Image caption What the animal did with those impressive canines is not exactly clear

An extraordinary looking, mouse-sized, fossil animal is shedding new light on the ancient history of mammals.

With a thin snout, beady eyes and long canines, the creature would have looked remarkably like that fictional sabre-toothed squirrel of Ice Age movie-fame.

But Cronopio dentiacutus is one of the very few mammal specimens to come out of South America from the era when dinosaurs ruled the Earth.

The 100-million-year-old animal is reported in the journal Nature.

We know so little about the actual diversity of mammals that even some very wild guesses might come through; they might actually be present in the fossil record Prof Guillermo Rougier, University of Louisville

It was discovered in sandstone sediments at Cipolletti, Rıo Negro Province, Argentina.

Those ancient river sediments reveal a lot about what the local environment was like in the Late Cretaceous, but scientists are struggling to pin down the details of Cronopio's lifestyle.

The animal displays a host of features that appear to have no parallel among living or extinct mammals, says Prof Guillermo Rougier from the University of Louisville, Kentucky, US.

"The back teeth, the molars, are the kind of teeth that you will find in an insectivore, an animal that eats insects of different kinds, and even very small invertebrates, or perhaps small lizards, which were present in the same place," he told BBC News.

"But we have no idea why he needed such huge canines. Those tusks are a big surprise."

It is possible Cronopio used them to skewer certain insect prey, but it is clear the canines could not have been deployed with much force.

The slender nature of the snout and of the teeth themselves mean that to have bitten down hard, or to have wrestled another creature with its mouth, would have invited almost certain injury.

Cronopio is what is termed a dryolestoid. These were a group of primitive, extinct mammals belonging to the lineage leading to modern marsupials and placental mammals.

They are known mainly from teeth and jaws found in North America and Europe from the Jurassic Period (145-200 million years ago). To now have relatively complete dryolestoid skulls form South America in the form of Cronopio is therefore a boon to scientists trying to study the spread and diversity of mammals through Earth history.

Image caption There are so few well-preserved South American mammals from the time of the dinosaurs

"In the northern continents, there is a longer tradition of palaeontology and so they are well represented," observed Prof Rougier.

"In South America, Africa and Australia - not so much work has been done proportionately, and so we know very little; and that's why Cronopio is so important.

"Instead of having a picture that is so heavily biased to what happened in the North, we're starting to get some information about what happened in the southern continents, and fortunately in this case the quality of the specimens is very good."

As to that likeness with Scrat, the acorn-obsessed squirrel in the animated Ice Age features films, Prof Rougier finds the association highly amusing.

"I remember when I saw the movie I thought, 'why have they done this ridiculous animal - there is no such thing?'. And then we find something that kind of looks like it. But it just goes to show - we know so little about the actual diversity of mammals that even some very wild guesses might come through; they might actually be present in the fossil record."

Jonathan.Amos-INTERNET@bbc.co.uk