Sea change to blame for end of dinosaur era

The dinosaurs were already dying off when a massive asteroid crashed into the Earth about 65 million years ago, according to a leading group of British scientists.

The real cause of their decline was probably a combination of climate changes, such as volcanic eruptions, combined with a gradual but significant drop in sea levels around the world, said a team of 22 experts.

The dinosaurs were not the only creatures to suffer as global climate change altered habitats, said Dr Norman McLeod, who led the research over the past 10 years.

Millions of species also disappeared over the course of almost 11 million years before and after the asteroid impact - an event geologists call the "Cretaceous-Tertiary biotic transition" - or, more pronounceably, the "K-T boundary"

The asteroid impact in the Yucatan peninsula in the Gulf of Mexico was confirmed last month by geological evidence that was finally tied together by an American team.

But the British team, including scientists at the Natural History Museum, University College, London, and Birkbeck College reckon it was in fact only the coup de grace for a huge number of species which disappeared from the fossil record soon afterwards.

"There are other ways than an asteroid to produce extinctions," said Dr McLeod, from the Natural History Museum. "The sea level now is, historically, low." That is because "mid-ocean ridges" have fallen as the Earth's crust has shifted, enlarging the ocean basins and expanding their volume.

The effect was to lower the sea level around the world. It would have fallen by about 50 to 100 metres over the course of millions of years. But that has the effect of changing the climate, making summer and winter more extreme, and so in turn altering and fragmenting the habitats that species were used to.

Eventually animals and plants would be marooned in "islands" of favourable climate, and so become vulnerable to any change in conditions. Only the more adaptable would survive. Among those which did were mammals. However, millions of plant and animal species at all levels of complexity became extinct as sea levels fell and weather systems changed.

The latest findings are published this week in the Journal of the Geological Society.

The asteroid impact theory had suggested that a huge extra-terrestrial rock hit the Earth, throwing up enough dust to cut out sunlight and cause the equivalent of a "nuclear winter" which wiped out the dinosaurs.

But Professor Alan Lord of UCL, one of the paper's authors, said: "We can't say that the meteorite didn't deal the final blow, but they seemed to be fading anyway.

"Maybe it was something like genetic exhaustion - they couldn't adapt."