Researchers in Australia have unearthed a new species of platypus which scientists have dubbed the “platypus-zilla”, because it could have measured up to one metre long.

Scientists at the University of New South Wales (UNSW) estimated the new species - Obdurodon tharalkooschild - would have been at least twice as large as the living species of platypus after unearthing a single fossilised tooth in the Riversleigh fossil beds in northwest Queensland.

However, finding just one molar tooth has made it difficult for researchers to determine exactly what the platypus-zilla would have looked like fifteen million years ago.

In the study, published in the Journal of Vertebrate Palaeontology, the team found that unlike the living species of platypus today, it had fully functional teeth they suspect would have been used to consume a wide range of animals living in near them in pools and lakes.

In contrast, the modern platypus completely lacks teeth as an adult and instead bears horny pads in its mouth, meaning the new extinct species is unlikely to have been its immediate ancestor. The team now believe suggesting the platypus' evolutionary past was more complicated than previously thought.

“Discovery of this new species was a shock to us because prior to this, the fossil record suggested that the evolutionary tree of platypuses was a relatively linear one,” UNSW Professor Mike Archer said.

“Now we realize that there were unanticipated side branches on this tree, some of which became gigantic.”

The area the fossil was found in is now a desert but would have been covered in forest millions of years ago, leading researchers to believe the animal may have lived in or near freshwater ponds.

Associate Professor at the University Suzanne Hand said it probably fed not only on crayfish and other freshwater crustaceans, but "also on small vertebrates including the lungfish, frogs, and small turtles that are preserved with it in the Two Tree Site fossil deposit."