Ancient marsupial had lizard-like teeth

Fossil remains of a new type of extinct snail-munching marsupial identified by Australian palaeontologists, more closely resemble a modern-day lizard than a mammal.

The find sheds light on how different species of animals evolved similar features and may have competed for survival on the continent 10 million years ago, say the scientists from the University of New South Wales and Queensland Museum.

"This is the first time we've found evidence of [evolutionary] convergence between a marsupial and a lizard," says lead author, Rick Arena from UNSW's School of Biological, Earth and Environmental Sciences.

The group's findings are reported in this week's Proceedings of the Royal Society B: Biological Sciences.

The scientists identified teeth and upper jaw fragments from two specimens, found in 10 to 17 million-year-old deposits in Riversleigh World Heritage Area in North West Queensland, that came from a small marsupial about the size of a ferret.

During this time a diverse range of marsupials flourished in the wet rainforests of Riversleigh.

"The combination of teeth present was one that distinguished the fossils from other types of mammals, so we were able to narrow it down to a marsupial," says Arena.

But the tooth pattern and shape was unlike any other species of marsupial previously identified.

The upper jaw contained enormous hammer-like third pre-molar teeth and comparatively smaller premolar and molar teeth.

The hammer-teeth were anchored into the jaw by four roots suggesting the teeth were used to crush hard objects.

"Unlike the corresponding teeth in all other marsupials, [the hammer tooth] was broad and blunt. It didn't have any cutting or piercing parts, it was shaped almost like a dome," says Arena.

"We were unable to find any close matches to this particular tooth shape among other kinds of marsupials or even other mammals."

As a result, the scientists concluded that the two specimens were from different species of a new genus of marsupial named Malleodectes.

Resemblance to skink teeth

The researchers found the fossil teeth pattern bore a striking resemblance to that of the pink-tongued skink.

This modern-day lizard, which grows to a length of up to 40 centimetres, inhabits the east Australian rainforest, and dines on snails and slugs.

"The [skink's] teeth are roughly the same size as the fossil," says Arena.

"[The skink] uses its giant dome-like tooth to crush, smash open snails and then swallow them whole."

Cenozoic rock fossils from the Riversleigh area contain an abundance of freshwater and terrestrial snails, a likely food source for the Malleodectes. And the marsupial is likely to have had this feast all to itself until ancestors of the pinked-tongued skink also developed a taste for snails.

"We know from the fossil record there were not any similar lizards that had hammer teeth at that time. In fact, we've found fossils of the ancestors of pink-tongued skinks or relatives of their ancestors that do not have these special snail-eating teeth," says Arena.

"But since then they've evolved."

The scientists suggest that the ancestors of the pink-tongued skink gained the upper-hand sometime during the Late Miocene when the wet rainforest climate collapsed.

"At this time, the climate became more unpredictable and the balance might have shifted from favouring mammals to favouring lizards — this might have put the hammer-tooth snail-eating marsupial at a disadvantage," says Arena.

"We're not sure exactly what the sequence of events was. Whether one died out first and the other arose to fill that role, however, we can't rule out the possibility that they may have competed sometime in the last 10 million years.

"We don't know of an overlap, but it is possible because they both occupied the same niche."