New marsupial fossil has lizard teeth

A fossil of a snail-eating marsupial discovered by Australian palaeontologists has teeth that resemble a lizard's, providing a striking snapshot of convergent evolution. Gemma Conroy reports.

Pink-tongued skink (Cyclodomorphus gerrardii) foaming at the mouth while crunching a snail.

A fossil of a snail-eating marsupial discovered by Australian palaeontologists has teeth that resemble a lizard’s, providing a striking snapshot of convergent evolution.

Malleodectes, a small ferret-like marsupial that lived 10 to 17 million years ago, had a distinctive hammer-shaped tooth on each side of its upper jaw, said Rick Arena, lead author and palaeontologist at the University of New South Wales in Sydney.

These findings, published this week in Proceedings of the Royal Society B, are an example of convergent evolution, which occurs when two unrelated species have similar physical features.

“It’s a striking example of evolutionary convergence, and the first example of a marsupial with teeth that resemble a lizard’s,” said Arena, who discovered the jawbones in the famous Riversleigh World Heritage fossil field in Queensland.

Teeth show convergence

Although there have been many examples of convergent evolution between marsupials and unrelated mammals – such as the extinct thylacine, a dog-like marsupial – this is the first time it has been seen between a lizard and a marsupial.

After examining the skulls of two Malleodectes specimens, Arena and his research team were baffled by the enormous domed teeth in the marsupial’s upper jaw. “No other mammal with those kind of teeth exists,” Arena said.

The ball-peen hammer-like tooth in Malleodectes (left) and the pink-tongued skink (right) are examples of convergent evolution, suggesting both were highly-specialised snail-eaters.

Hammer crushes snails

But Scott Hocknull, co-author and senior curator at the Queensland Museum in Brisbane, noticed that this blunt ‘hammer-tooth’ showed a striking similarity to the teeth of the pink-tongued skink (Cyclodomorphus gerrardii), a modern snail-eating lizard inhabiting forests along Australia’s east coast.

“I immediately recognised the large bulbous teeth that are often found in small lizards,” said Hocknull, a palaeontologist whose research focuses on reptiles. “Marsupials were doing evolutionary experiments and filling every niche they could.”

The similarity between the pair of enormous teeth in the extinct marsupials and the modern skinks are a sign that these two very different species had each adapted to crushing snail shells whilst feeding.

Artist's illustration of a forest at Riversleigh in prehistoric Miocene times.

Lizards win snail competition?

The newly discovered Malleodectes, which means ‘hammer-biter’, most likely went extinct about 10 million years when Australia was undergoing rapid climate shifts.

As the marsupials declined with the diminishing forests, the hammer-toothed skinks may have exploited the sudden increase in snails.

But Gavin Prideaux, a palaeontologist specialising in marsupials at Flinders University in Adelaide, said that the marsupial’s demise mightn’t have been caused by competition with the skink, and they may have not even lived in the same environment.

“It’s definitely a unique animal and a very interesting discovery,” he said, but added that the jawbone is only a tiny fragment of the whole picture. “There’s not enough evidence for competition at present.”

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