Melbourne
A TINY fossil jawbone, unearthed on an Australian beach, could turn our
picture of mammalian evolution upside down. The fossil鈥檚 discoverers claim it is
115 million years old and comes from a shrew-like placental mammal. Yet nothing
of the sort is supposed to have existed in Australia until 5 million years
ago.
Most modern mammals鈥攊ncluding ourselves鈥攁re placentals, in which
the developing young are nurtured inside the uterus. They are believed to have
originated in the northern hemisphere more than 100 million years ago and then
slowly spread throughout the globe.
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The other two mammalian groups, the egg-laying monotremes and the marsupials,
in which the young mature in a pouch, struggled to compete with the advancing
placentals. Australia, into which the first terrestrial placentals were thought
to have migrated only 5 million years ago, is their main remaining
stronghold.
At least, that is the established view. But the new find suggests that
placentals arose simultaneously in both the southern and northern hemispheres.
If so, the textbooks must be rewritten. 鈥淭o find a placental mammal in Australia
that ancient fits nothing we know,鈥� says David Archibald of San Diego State
University.
The jaw was discovered in March near Inverloch, about 150 kilometres
southeast of Melbourne, by Nicola Barton, a volunteer from London working on a
dig organised by Tom Rich of the Museum of Victoria in Melbourne and his wife,
Pat Vickers-Rich of Monash University in Melbourne.
Rich, Vickers-Rich and their colleagues describe the jaw, from an animal they
have named Ausktribosphenos nyktos, in Science(vol 278, p
1438). They believe A. nyktos was an insectivore that lived on the
landmass of Gondwana that later split up into Australia, South America, southern
Africa and Antarctica. Its home then lay at a latitude of about 70掳
south鈥攃old and plunged into darkness for three months of the year.
The bone is 16 millimetres long with teeth less than 1.8 millimetres high
that are adapted for slicing and crushing food鈥攁 feature not found in
monotremes. The jaw has three molars and five premolars, which is typical of
placentals. Marsupials usually have four molars and three premolars.
One premolar also seems to be a hybrid between a simple, triangular premolar
and a more complex molar. This pattern is found in placentals, but not in
marsupials. 鈥淥n the way we recognise placentals in 1997, this thing fits the
bill,鈥� says Rich.
But some palaeontologists are not convinced. The main objection is the
structure of the talonid basin, a depression at the back of each molar. In
placentals this is a single structure, but in A. nyktosit is divided
into two. 鈥淭he talonid basin is unlike anything you would expect of a primitive
placental mammal,鈥� says Alfred Crompton of Harvard University.
鈥淔rankly, I think it is something new鈥攏ot a monotreme, not a placental
and not a marsupial,鈥� says William Clemens of the University of California at
Berkeley. 鈥淚t鈥檚 a new group that was either converging with other mammals or
running parallel with them, eventually dying out.鈥�
Rich maintains that similar doubts could be raised about several fossils from
the northern hemisphere that are generally assumed to be primitive placentals.
鈥淲e can鈥檛 assume it is not a placental because we know so little about the
evolutionary history of mammals on Gondwana,鈥� agrees David Krause of the State
University of New York in Stony Brook.
This ignorance will now have to be addressed, Rich argues. Whether or not
A. nyktos proves to be a placental, he says, its discovery means
palaeontologists must give more thought to the importance of the southern
hemisphere in mammalian evolution.
