Fossil find challenges marsupial mole evolution

By Amanda James November 12, 2010
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The discovery of a 20-million-year-old fossil suggests marsupial moles evolved in rainforests, not deserts.

THE DISCOVERY OF A 20 million year-old fossil has shed new light on the origins of the mysterious marsupial mole that burrows beneath the Australian desert floor.

A fossilised mole relative, discovered at the Riversleigh World Heritage fossil site in north Queensland, appears to have lived in rainforest – suggesting these elusive creatures did not evolve in the desert, as was previously thought.

Modern-day marsupial moles, which are completely blind, almost never surface from their underground dwellings in the desert areas of the Northern Territory, Western Australia and South Australia. 

“I had made the assumption that there was some unknown desert on the continent that must have been there to explain the special desert adaptations of the marsupial moles,” says Professor Mike Archer of the University of New South Wales in Sydney. “I was dead wrong.”

Until the discovery by Mike and his team of researchers, no fossil of the marsupial moles were known. The team publish their findings this month in the journal Proceedings of the Royal Society B.

Mole mystery

With a conical skull and a tube-shaped skeleton, the two modern species of marsupial moles, Notoryctes typhlops and Notoryctes caurinus are built for burrowing in the desert, where they can spend their entire lives underground. This suggests the moles, which weigh just 40-60 g, are uniquely adapted to the desert; but the new find turns this theory on its head.

There is no evidence of a desert in Australia when the earliest form of the species lived here, which appears to have been 20 million years ago, says Dr Joe Benshemesh, a Wildlife Biologist at La Trobe University in Melbourne, was not involved in the research. The earliest known desert in Australia, he says, was around one million years ago – not long enough for the mole to have adapted its special characteristics.

The mole simply applied its features to a new environment, says Joe. “The adaptations the mole developed in the rainforests suited them well in deserts. Over time, the environment probably dried up around them and they were preadapted and moved into the desert.” The fossils suggest they became mole-like while burrowing through the mossy floors of those ancient rainforests.

An elusive marsupial mole crawling on the desert floor.
(Photo: Mike Gillam/Auscape)

Solving the relationship puzzle

The finding helps experts better understand a second mystery about the relationship between the marsupial mole in Australia and the Cape golden mole in Africa (Chrysocloris asiatica). Like the marsupial moles here in Australia, the Cape golden mole is blind, earless and has spade-like feet, a stumpy tail, and a camel-coloured coat

Both the marsupial mole and the Cape golden mole have highly specialised V-shaped teeth- a striking similarity considering that the dental structures are used for identification in these mammals in the same way that fingerprints are used for distinguishing humans, says Mike.

Despite the physical similarities, the mole in Africa is a placental and is more closely related to humans than it is to the mole in Australia, which is a marsupial, like kangaroos and koalas.

It is unusual for two species on different continents to be so similar. Mike and his team concluded that the marsupial moles in Australia and the Cape golden mole in Africa evolved via two very different pathways to arrive at the same point – a true lesson in what biologists call convergence.