The remains of a new species of tiny marsupial lion which prowled the lush rainforests of northern Australia 18 million years ago have been unearthed in the Riversleigh World Heritage Area.

The UNSW discovery team has named the new species Microleo attenboroughi for its small size and to honour the famous broadcaster and naturalist Sir David Attenborough, in recognition of his support for Riversleigh, which he said was one of the four most important fossil areas in the world.

The new species was much smaller than the other members of this extinct marsupial lion family, including its most famous but younger relative – the lion-sized Thylacoleo carnifex.

“Microleo attenboroughi would have been more like the cute but still feisty kitten of the family,” says study lead author UNSW’s Dr Anna Gillespie. "It was not lion-size or even bob-cat-size. Weighing only about 600 grams, it was more like a ringtail possum.”

The news species is described in the journal Palaeontologia Electronica, based on a fossil specimen of part of its skull and teeth.

Its dentition includes an elongate, lethally sharp, knife-like premolar in front of basined molars – specialised features which are common to all members of this extraordinary family of marsupial carnivores, the Thylacoleonidae.

Microleo attenboroughi was recovered from a limestone deposit believed to have formed in a pool within a rainforest landscape about 19 million years ago, during the Miocene.

The fossil deposit, named Neville's Garden Site, has already revealed a rich fauna including six kinds of bandicoots, many kinds of possums and kangaroos, toothed platypuses, diminutive koalas, thousands of bats, fish, turtles, lizards, pythons and birds including storks, logrunners and the earliest-known Australian parrot.

“Despite its relatively small size compared with the Pleistocene Thylacoleo carnifex – the last surviving megafaunal marsupial lion – the new species was one of the larger flesh-eaters existing in its ancient community of rainforest creatures at Riversleigh,” says team member UNSW Professor Mike Archer.

The diversity of marsupial lions alive at this time at Riversleigh is unmatched in the fossil record from anywhere else on the continent.

“Microleo shared these northern Miocene rainforests with two larger species of marsupial lion, one cat-sized and the other dog-sized,” says Dr Gillespie.

“Although it is possible they competed with one other, the size differences probably means they each specialised on a different size range of prey.

"It’s likely that Microleo scampered amongst the tree-tops, gobbling insects as well as small vertebrates such as lizards and birds while simultaneously trying to avoid becoming a prey item for its larger relatives,” she says.