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.
"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
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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