'Secret of man's success lies in his perfect teeth'

A dual-purpose back tooth was so important for the success of warm-blooded mammals that it was invented at least twice during their early evolution, scientists have found.

A dual-purpose back tooth was so important for the success of warm-blooded mammals that it was invented at least twice during their early evolution, scientists have found.

A molar that was capable of cutting and grinding food in one easy chewing action allowed the small shrew-like ancestors of man to exploit the demise of the dinosaurs and become one of the most diverse groups of animals on the planet.

The "tribosphenic" tooth has a highly distinctive triangular shape that is perfectly formed to break up hard, edible material, allowing the early mammals to fuel their body's large appetite for energy-rich food. The upper and lower molar teeth contain small cutting facets that interlock precisely in a scissor-like motion when the animals chew, making mastication more efficient than the primitive chomping action of the dinosaurs.

Richard Cifelli of the Oklahoma Museum of Natural History examined more than 20 fossilised mammals dating from 200 million and 64 million years back and found that the tribosphenic molar tooth arose independently in two groups of mammals living separately on two ancient continents.

The scientists found the distinctive tooth in the fossilised ancestors of placental mammals, which now live predominately in the northern hemisphere, and those of the marsupials, whose descendants now live in South America, Madagascar and Australia.

They suggest in a study published in Nature that the innovative tooth arose twice, once on the ancient northern continent of Laurasia, and again in the south on the land mass known as Gondwana.

Anne Weil, a biologist at Duke University in Durham, North Carolina, said that the discovery underlines the crucial importance of developing an efficient tooth for a warm-blooded animal with high metabolic needs. "Living shrews, which face the same constraints, have prodigious appetites. So survival probably depended on efficient food processing, and the tribosphenic dentition provides just that," Dr Weil said.

"As a tribosphenic mammal bites down, a large cusp on the upper molar settles, mortar-like, into a pestle-like basin on the lower molar," she said. "This combination of shearing and grinding has long been considered the innovation that was possibly the most significant in the spread and diversification of mammals."

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