Australian researchers have identified a new species of marsupial lion that roamed the rainforests of northern Australia 25m years ago.

Weighing in at just over 22kg and boasting a squat, flat head, the new species, named Wakaleo schouteni after wildlife illustrator and paleoartist Peter Schouten, is the fifth known species of dog-sized marsupial lion discovered at Riversleigh, located within Boodjamulla national park, near the Queensland-Northern Territory border.

The formal announcement of discovery, made in the Journal of Systematic Palaeontology on Thursday, followed two decades of careful checking and cross-referencing with fragments of other marsupial lions scattered throughout Australia.

The first piece of the puzzle was spotted by a keen-eyed volunteer in the 1980s, who saw some weathered bone and teeth protruding from the rocks in a steep gully in Riversleigh.

Palaeontologists later discovered a skull, which was cleaved in two halves, and fragments of the postcranial skeleton – the humerus, sacrum and a few hand bones.

Lead author Dr Anna Gillespie, who began working on the fossils as part of her PhD with the University of New South Wales in the 1990s, was working on the back half of the skull when she got a call from a laboratory in Mt Isa. Both parts of the skull were found at Riversleigh, a fossil-rich limestone deposit that received Unesco world heritage listing in 1994.

It is unique in Australia for capturing a broad sweep of fossils dating from nine to 24 million years old, allowing palaeontologists to see the evolutionary history of certain species.

“[The lab] contacted me and said, ‘We’ve got the front half of a marsupial lion,’ and I said, ‘Well, I’ve got the back’, and it was from the same site, and they sent it down to me and lo and behold it matched beautifully,” Gillespie told Guardian Australia. “That’s the best specimen we have actually, this beautiful skull.”

The skull placed the animal in the Wakaleo family, an early ancestor of the largest known carnivorous marsupial, the 130kg Thylacoleo carnifex, which hunted megafauna between 1.9m and 30,000 years ago.

Most of the fossils were found in an area of limestone dated as the early Miocene period while others are believed to date back to the late Oligocene, putting the animal’s time on earth at between 18 million and 29 million years ago.

It’s believed to be a contemporary of Priscileo pitikantensis, a slightly smaller marsupial lion discovered at Lake Pitikanta, near Lake Eyre, in remote South Australia in 1961. The similarities between the two species saw the later reclassified as Wakaleo pitikantensis.

Gillespie said the difference between the two could be due to differences in habitat or geographic isolation. It’s also possible pitikantensis is slightly older. Fossils of the three previously-discovered Wakaleo suggest that each iteration of the carnivores was larger than the last.

Gillespie has not found enough examples of the W. schouteni’s arm and leg bones to be sure but she said fossils of other animals preserved alongside it suggests it may have led a tree-climbing existence, scurrying up trunks with the help of koala-like opposable thumbs.

Arm bones collected so far suggest “a fairly sturdy shoulder, halfway between the koala and a brushtail possum”, she said.

“The diversity of the mammal faunas at the sites at Riversleigh have indicated that it’s likely that the environment up there in the late Oligocene was forested, probably open forested, but was we move into the early miocene, Australia’s climate got hotter and wetter and also we can see at Riversleigh that the diversity increases again, so it looks like the forest is more closed, maybe possibly rainforest,” Gillespie said. “The likelihood is that this animal was scampering after its prey, up trees and through the treetops.”

Its size, roughly that of a border collie, would have lent itself to an arboreal existence.

“They weren’t heavy animals, they could easily have scampered up trees after prey,” Gillespie said.

The limestone caves and ridges of Riversleigh, which continually open up new sinkholes and dissolve in puddles, trapping and preserving new fossils for scientists to study in another 100,000 years, may yet yield more information.

“We continue to look for more bits of it to answer more questions,” Gillespie said. “Everybody else gets excited when they find skulls and teeth, I get excited when I find limb bones because I really like to fit these animal in their environments and see how they’re actually living.”

Schouten, a highly respected illustrator whose imagination has given shape to most of the prehistoric animals found at Riversleigh, said he was “flabbergasted” a species had been named after him.

“After scribbling away at what I absolutely love to do for the past 40 years it is somewhat strange to realise that people have noticed,” he told Guardian Australia. “It is just as great an honour to be recognised by my peers by having a species named after me. And what a species! I would have been happy with a lowly nematode, but a marsupial lion - wow!”