A giant, carnivorous platypus with fearsome teeth once roamed the waterways of Australia, researchers have discovered.

The newly identified species, dubbed Obdurodon tharalkooschild, measured about a metre long, double the size of its modern-day equivalent.

Within the distinctive platypus beak, the animal had rows of sharp teeth, which scientists believe they used to slice and chew crayfish, frogs and small turtles. However, for such prey this deadly threat has passed – and will never interact with river-wading humans' ankles – since the ancient platypus is extinct. A highly distinctive tooth, dated between 5m and 15m years old, was discovered by a team from the University of NSW in the Riversleigh world heritage area in north-west Queensland.

The tooth, found embedded in limestone, revealed an entirely unknown branch of the platypus's evolution. Until now, the idiosyncratic mammal, which lays eggs and is venomous, was considered to have a single direct evolutionary lineage. "It's not every day you find such a thing, it was quite a surprise," UNSW associate professor Suzanne Hand told Guardian Australia. "Young platypus have teeth but they then lose them in favour of horny pads in their mouths.

"This platypus had a fully functional set of teeth. It would have been quite a sight, living in the rainforest. A platypus of that size would have been fairly fearsome and could have given you a bit of a nip." Hand said that platypuses were present in South America until 61m years ago, as well as Antarctica, when the two continents were joined to Australia's land mass. The discovery of a new branch of the platypus family illustrates the shrinking distribution of the animal. "We once had them in South America, and then central Australia and north-west Queensland," Hand said. "They are now limited to the river systems of eastern Australia, so it's a bit of a worry that they've shrunk from three continents to the margins of one country. "This older platypus had functional teeth and a much broader diet than the modern one. It should strike a chord of caution in us that the platypus appears to have become more specialised. Sometimes it can be rather dangerous to be out on your own, evolutionarily speaking." The name Obdurodon tharalkooschild is a combination of the Greek for "lasting tooth" and an indigenous Australian creation story about the origin of the platypus, where a strong-willed female duck ignored her parents' warnings and was set upon by Bigoon, a water-rat, leading to unusual-looking offspring.

• On 5 November 2013 this article was amended to correct an error in the standfirst