1 october 2004

What really happened to the dinosaurs?

The theory that dinosaurs became extinct primarily due to an asteroid hitting the Earth needs urgent re-examination, according to a recently published paper co-authored by Dr Matt Phillips of Oxford's Ancient Biomolecules Centre. Along with Professor David Penny of Massey University, New Zealand, he is calling for scientists to look again at the fossil and genetic evidence to find out what really happened to the dinosaurs.

According to the popular theory, birds and mammals were only able to proliferate and diversify on Earth once an asteroid impact wiped out the dominant dinosaurs and pterosaurs at the end of the Cretaceous period 65 million years ago. But in a paper published this month in Trends in Ecology and Evolution Dr Phillips and Professor Penny argue that although there is overwhelming evidence for an asteroid impact at the end of the Cretaceous period, the decline of dinosaurs and rise of birds and mammals had begun millions of years earlier.

The authors argue that the popular theory of the extinction of dinosaurs has been confused by the linkage of two unrelated ideas - that an asteroid impact occurred at the end of the Cretaceous period and that dinosaurs were suddenly and unexpectedly wiped out at this time. It is also linked with the assumption that the normal processes of evolution are insufficient to cause major evolutionary changes; that is, extraordinary circumstances are needed to bring about change. This is a concept that has come under increasing scrutiny from evolutionary biologists over the past three decades.

'A major theme of our work is that the ideas of an asteroid impact marking the end of the Cretaceous, and the demise of dinosaurs, are independent ideas which have been linked,' said Dr Phillips. 'The evidence for or against each must be evaluated separately.'

The authors call for scientists to take a closer look at the fossil and genetic evidence to critically test the theory. Fossils can show when the different species of dinosaurs, birds and mammals roamed the Earth, and therefore provide evidence of when dinosaurs started their demise, and when birds and mammals began to proliferate and diversify. Genetic analysis also provides an important tool for scientists to find out what happened to animals around the end of the Cretaceous and the beginning of the Tertiary. By studying the genetics of living birds and mammals, it is possible to determine whether their ancestors arose very quickly after the asteroid hit the Earth, thus supporting the popular theory, or whether their appearance was more gradual.

So what did cause the decline and ultimate extinction of dinosaurs? 'It's just so hard to know and is little more than speculation at the moment,' said Dr Phillips. 'A number of aspects of mammalian and avian biology could have been keys to tipping the balance in their favour in competition for niches with dinosaurs and pterosaurs in a world in which prey were diversifying and the climate was becoming less mild. I expect that the asteroid impact, its associated abiotic effects and resultant disruption to ecosystems probably was the nail in the coffin for non-avian dinosaurs, but I also suspect that dinosaurs and pterosaurs would have survived well into the Tertiary had it not been for the prior elimination of most of their smaller sized members.'