CARLOS GOLDIN / SCIENCE PHOTO LIBRARY CARLOS GOLDIN / SCIENCE PHOTO LIBRARY
Comparison of a monotreme fossil tooth, and teeth in a skull of the living duck-billed platypus Ornithorhynchus. These molar teeth show a similar structure. The 63 million-year-old fossil tooth was discovered in 1991 in Patagonia by Argentine palaeontologist, Professor Rosendo Pascual. It is the first evidence of a link between living mono- tremes and South American ancestors. Living mono- tremes (platypus, spiny anteater) are primitive egg-laying mammals found exclusively in Australia and New Guinea. This fossil tooth, however, shows that monotremes originated in South America and crossed to Australia at the end of the Cretaceous era when these continents were joined.
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