New light on evolution

Thursday 15 January 1998

An ancient jawbone may change our understanding of mammals

A FOSSIL jawbone, which was discovered in March near Inverloch, about 150 kilometres southeast of Melbourne, Australia, might change all that we know about the evolution of mammals.

The established view is that almost all mammals - including humans - are placentals which originated in the northern hemisphere more than 100 million years ago and spread across the world. The two other groups among mammals - the egg-laying monotremes and the marsupials (in which the young mature in a pouch) - struggled to compete with the fast-paced placentals. Australia is their main remaining stronghold where they are thought to have migrated only five million years ago.

But the new evidence suggests that placentals arose simultaneously in both the southern and the northern hemispheres. Nicola Barton, a volunteer from London working on a site organised by Tom Rich of the Museum of Victoria in Melbourne and his wife, Pat Vickers-Rich of the Monash University, Melbourne, claim that the fossil is 115 million years old and comes from a shrew- like placental. But nothing of this sort was supposed to have existed in Australia until five million years ago.

Ausktribosphenos nyktos (A nyktos), as has been named by Rich, Vickers-Rich and their colleagues believe that it was an insectivore that lived on the landmass of Gondwana. They are said to have split up into Australia, South America, southern Africa and Antarctica. Its home then lay at an latitude of about 70 south - cold and plunged into darkness for three months of the year.

The fossil bone which was discovered is 16 millimetres long with teeth less than 1.8 millimetres high that are adapted for slicing and crushing food - a feature not found in monotremes. The jaw has three molars and five premolars, which is typical of placentals. Marsupials, however, usually have four molars and three premolars.

Rich feels that the discovery of this fossil jawbone is important to paleontology. He says that all the doubts that have cropped up with this discovery must be addressed to. He says that whether A nyktos proves to be a placental or not, its discovery means paleontologists must give more thought to the importance of the southern hemisphere in mammalian evolution.

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