PhD student Shimona Kealy with the skull of a Tasmanian devil. Credit:Stuart Hay "There used to be a heap of different tigers – bigger and smaller," says Shimona Kealy, a PhD scholar at the Australian National University who led the study. "Some were as small as a cat." For the study, published in BMC Evolutionary Biology in December, a team led by Ms Kealy combed through the fossil record of the six known species of Australian tiger, as well as running DNA analysis on their modern ancestors. They discovered two important clues to their disappearance. First, most tiger species went extinct at the same time. Second, that time coincided perfectly with the mid-Miocene disruption, a period of global cooling that quickly dropped temperatures worldwide by about 7 degrees. "When we saw the numbers come in and line up, we were stunned," Ms Kealy says.

Tasmanian devil bones. Credit:Stuart Hay Why would global cooling lead to the downfall of the tigers? More research will need to be done, but Ms Kealy's study offers one tantalising possibility – the rise of the devils. Global cooling led to the lush rainforest that used to sprawl across mainland Australia giving way to dry open scrubland similar to the Australia we know today. The tigers were not well adapted to hunt in those types of environment, Ms Kealy suggests. Credit:Stuart Hay "We think the structure of tigers' feet and ankles might have made them better suited to closed forests with uneven surfaces, such as roots and logs, and less well suited to open woodlands," she says.

As it happened, another group of native carnivores were extremely well suited to Australia's new environments: Dasyuridae, a family of small marsupials that includes Tasmanian devils, quolls and dunnarts. A modern-day devil. Dasyuridae have always fascinated Ms Kealy. As a child growing up in Bulahdelah, on NSW's North Coast, her house was frequently visited by brown antechines, tiny carnivorous hopping mice from the Dasyuridae family. "I never knew why nobody else had heard of them, they were so common in my house," she says. "My mum used to hate them. They used to invade the house all the time, but they are great for controlling cockroaches. They look like little hopping mice, but they've got a pouch and they hop around – and they are carnivorous."

Dasyuridae, Ms Kealy's study showed, had traits perfectly suited to life in open woodlands, including an ear structure that enhanced their hearing over long distances – important for hunting prey. "We think they might have actually been better adapted in the first place to these new open spaces that replaced the rainforests that were there, and they were then able to out-compete the tigers," says Ms Kealy. Could it be possible devils and quolls, who were so well suited to the new Australia, managed to out-compete tigers and eat them off the land? That is one possibility raised by the study. It's backed up by another possible clue trapped in the fossil record: as the number of tigers shrunk, the number of Dasyuridae bloomed. University of Melbourne thylacine expert Associate Professor Andrew Pask says Ms Kealy's theory makes sense. "There is a lot of evidence in the fossil record showing there were huge changes in the environment, massive climate shifts. And that fits really nicely with the fossil record data showing when the tigers dropped in number.

"This is the first time that it's been linked to a particular event in time. And that pointed to climate change, which is really the only thing that leads to those sort of mass extinctions."