Published online 4 January 2001 | Nature | doi:10.1038/news010104-5

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Poles apart, molars together

Mammals' complex teeth may have evolved twice, on either side of a great divide.

Living monotremes have ditched their tribosphenic teethLiving monotremes have ditched their tribosphenic teeth

The teeth that might have allowed mammals to develop from their tiny shrew-like ancestors into today's relative giants arose twice on different continents. So a new study reported in Nature1 suggests, challenging previous ideas on how advanced mammals first came about more than 100 million years ago.

Early mammals' teeth simply sliced past each other. But the molars in most living mammals interlock precisely and have small facets that come together with a scissor action. These 'tribosphenic' teeth are perfect for shearing and grinding food.

Zhe-Xi Lou, of the Carnegie Museum of Natural History, Pittsburgh, and co-workers looked at the jaws, teeth and skeletons of over 20 mammal species, ranging from 165-million-year-old fossils to those living today. The team's analysis revealed two distinct lineages of mammals with the same specialized teeth.

"Tribosphenic teeth have such a complex and intricate assembly, people thought it inconceivable that they would have a multiple origin," explains Luo.

One lineage came out of the ancient southern continent, Gondwana, that included South America, Africa, southern India, Antarctica and Australia. The second emerged from the northern landmass, Laurasia, which comprised northern Europe, Asia and North America. Previous studies claimed that advanced mammals radiated from one or other continent.

The team suggests that the Gondwanan lineage gave rise to the egg-laying mammals known as monotremes. These survive only as the platypus and echidna. The rest of today's mammals -- marsupials and placentals -- stemmed from Laurasia.

Living monotremes have ditched their tribosphenic teeth, although fossil forms retain clues to their Gonwanan ancestors, claims Luo.

It all adds up to the living monotremes being the only survivors of a group of Gondwanan mammals. Today's southern mammals travelled down from the north between 84 and 65 million years ago.

Other researchers, although excited by the findings, have some concerns. Chief among them is that the path to living monotremes rests on just a couple of platypus-like fossils.

"Whether this theory will stand the test of time depends on finding new material that can help to fill in the many gaps," says Bob Presley, who worked on mammal development and evolution at Cardiff University.

But the beauty of Luo's approach, as Presley acknowledges, is that, it can to some extent predict what's missing. Luo agrees: "The future test is whether new finds will fit effortlessly into our existing framework, and corroborate our theory."

But palaeontologists have only just begun to look in the right areas of what was once Gondwana. Further fossil finds are certain to give them even more to chew on. 

  • References

    1. Luo,Z.-X.,Cifelli, R.L. & Kielan-Jaworowska, Z. Dual origin of tribosphenic mammals. Nature 409, 53 - 57. | PubMed | ChemPort |