By Julie Ryan

A major paleontological find in south-eastern Victoria by husband-and-wife team Dr Tom Rich and Professor Pat Vickers-Rich has challenged scientists' traditional understanding of the evolution of mammals.

A tiny fossilised jaw bone discovered by a joint Monash University and Museum of Victoria research team has cast doubt on current theories surrounding the evolutionary process of mammals.

Unearthed in an excavation site near Inverloch in south-eastern Victoria, the fossil, measuring only 16 mm in length, is believed to be the remains of a 115 million-year-old placental mammal.

The significance of this tiny jaw bone, which also has four teeth, may be lost on many. But for husband-and-wife team Dr Tom Rich and Professor Pat Vickers-Rich, the discovery is the find of a lifetime.

While Dr Rich, from the Museum of Victoria, and Professor Vickers-Rich, director of Monash University's Science Centre, are no strangers to major paleontological finds (they have been unearthing dinosaurs in Australia for more than 25 years), this particular find is one of the most significant.

Oldest mammal found

"This jaw bone possibly represents the oldest placental mammal ever found in the world," Dr Rich explained.

A similar fossil, discovered in Mongolia some years ago, indicated that placental mammals originated in Asia, and with the exception of bats, only found their way to Australia in the last five to six million years.

"It is now plausible, owing to this discovery, that placental mammals may have been widespread on all land masses on earth 115 million years ago, suggesting that the way we currently look at the evolutionary process of mammals may be fundamentally wrong," Dr Rich said.

The current theory is that placentals and marsupials, which evolved from the same ancestors, were confined to the Northern Hemisphere until about 65 to 75 million years ago. Around that time, it is thought, an island chain may have allowed both types of mammals to enter South America, with marsupials continuing south to Gondwana and Australia. It was not until Australia had broken away from Gondwana and drifted closer to Southeast Asia that placental mammals supposedly arrived in this country.

"If this fossil is a placental mammal, it changes our whole perception of how these creatures originally dispersed around the world," Dr Rich said.

The scientific name given to this new mammal is Ausktribosphenos nyktos or A. nyktos, meaning 'the Australian Cretaceous tribosphenic mammal that lived by night'.

It is believed that the little mammal to which this jaw belonged was an insect-eating, shrew-like creature -- much like a marsupial mouse -- which lived alongside polar dinosaurs during the Early Cretaceous period.

Professor Vickers-Rich said the mammal lived during a time when Australia was still connected to Antarctica and was situated within the Antarctic circle.

"The environment would have been cool but very lush, much like the landscape in the high country of Tasmania and New Zealand today but with an average annual temperature of about zero degrees," she said.

Many may wonder how so much can be discovered from a 16 mm piece of bone found embedded in a rock.

According to Dr Rich, it's all in the teeth. "This little fellow's molars are specialised for both cutting and crushing -- like the teeth of most placentals and marsupials," he said.

"However, it lacks other features seen in the jaws of marsupials, making it more common to a placental mammal."

As well, there are some discrepancies in the jaw's features that are neither placental nor marsupial, which begs another question. If the fossil is not the jaw of a placental mammal, then it represents an entirely new kind of mammal.

Either way, Dr Rich believes the discovery presents scientists with an interesting challenge. "We don't really know as much as we thought we did about the evolution and distribution of mammals on earth."

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