The discovery of Australia’s oldest known bilby fossil shows the marsupial evolved from having short teeth in the rainforest to long teeth in the desert.

A 15 million-year-old bilby fossil is very old but it certainly isn’t long in the tooth.

The discovery of Australia’s oldest known bilby fossil shows the shy, nocturnal marsupial originally lived in rainforest before becoming a desert dweller.

Unlike their present-day descendants, the bilby from 15 million years ago also had short teeth for eating soft fruits.

Bilbies have since developed long teeth to dig holes for insects and worms, and chew through desert sand.

University of Queensland lead researcher Dr Kenny Travouillon says the discovery of the oldest known bilby fossil has taught scientists about how they have evolved to cope with Australia becoming a more arid environment.

“The animal had a long period of time to evolve through the changes,” he told AAP.

“The teeth of the fossil bilby are not very tall … because they only ate really soft fruits.

“If you compare it to the bilby today, the crown of their teeth are really, really tall and it’s basically evolved that feature so that as it wears down, it still has teeth available for the whole time that it lives.”

The 15 million-year-old fossil was unearthed from northwest Queensland’s World Heritage-listed Riversleigh region about five years ago by palaeontologists from the University of Queensland and the University of NSW.

The area near the Queensland-Northern Territory border is a well-known fossil site and has pockets of rainforest.

Dr Travouillon said the rock containing the fossil was dissolved about two years ago, leading to the bilby fossil’s discovery.

During the 1950s, a five million-year-old desert fossil was discovered by American palaeontologists at Lake Palankarinna in South Australia.

That bilby fossil also had short teeth.

Bilbies have been on earth for about 25 million years and are closely related to the bandicoot.