Finding fossils of species that are no longer alive gives us an insight to the incredible diversity of nature and life, but it also often raises a pertinent question: why did the species go extinct? And as science progresses, theories are often formed and just as often, discredited.

Such a question has cropped up again, with the discovery of three extinct marsupial species (and indications of a fourth) in South America. The fossil find tells us that a little-known marsupial family, the Palaeothentidae, was diverse and widespread across the continent before it disappeared in entirety about 12 million years ago.

In a statement Tuesday, Russell Engelman, a biology student at Case Western Reserve University in Cleveland, Ohio, and lead author of a new study on the Palaeothentidae, said: “It was previously assumed this group slowly went extinct over a long time period, but that’s probably not the case. They were doing very well at the time they were supposedly on death’s door.”

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The paper, published in the Journal of Systematic Palaeontology, describes the animals and where they fit in the family, as well as their paleoecology and paleobiology.

Fossils of the three species — called Palaeothentes serratus, Palaeothentes relictus and Chimeralestes ambiguus — are about 13 million years old, making them among the youngest known fossils from their family. They were found at the high elevation fossil site of Quebrada Honda in southern Bolivia.

All three species had long snouts but differed in other features, like body size and diet.

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The mouse-sized P. serratus ate insects and had well-developed slicing premolars. The slightly larger P. relictus had large grinding molars and probably ate fruits and seeds, along with insects. C. ambiguus, like its name indicates, was a somewhat ambiguous creature with an uncertain evolutionary relationship to its other family members. It was probably the same size as P. serratus but had a diet similar to P. relictus.

A more generalist member of the family, which was identified on the basis of only a lower jaw, is Acdestis maddeni. It had a shorter snout than the other three species described in the paper, and given its teeth, probably ate anything, including small vertebrates.

But why did the whole group go extinct? The only hope to an answer lies in finding more fossil sites, researchers said in the statement.