By Paul Rincon
BBC News Online science staff
The idea that dinosaurs survived for some time after the asteroid impact blamed for wiping them out 65 million years ago has been dealt a blow.
Convention dictates dinosaurs were extinct before the Tertiary
Dinosaur egg fragments dug out of rocks in China seem to postdate the dramatic extinction event popularly believed to have extinguished the creatures.
But new data suggests the egg pieces got mixed up in later deposits through the action of mud and debris flows.
Details of the latest findings are published in the Journal of Geology.
Dinosaurs survived until the end of the Cretaceous Period of Earth history. But by the beginning of the Tertiary Period, about 65 million years ago, they had apparently vanished.
At numerous sites around the world, a clay layer separates rocks laid down in the Cretaceous from those deposited in the Tertiary. This is known as the K-T boundary.
The boundary contains high concentrations of the element iridium, commonly found in meteorites. Researchers have proposed that a meteorite impact which produced a huge crater at Chicxulub in Mexico, could have been responsible for the demise of the creatures.
Discoveries of dinosaur egg fragments in deposits from Nanxiong Basin, southern China, which contain Tertiary animal remains and pollen, suggested dinosaurs there could have survived until about 62 million years ago.
But US and Chinese researchers now dispute this.
They claim the egg pieces originated in Cretaceous deposits and were swept up in mud and debris flows during the Tertiary. This jumbled material was then re-deposited.
Dr Brenda Buck of the University of Nevada, Las Vegas, US, said she came upon the idea while examining palaeosols, ancient soils that have been buried and later exposed in Nanxiong.
"During the dry season you had these big open cracks," she explains.
Fragments of fossilised dinosaur eggs were found at the site
"Mudflows would come down and fill in those cracks. All those mudflows are in the [rock] sections where the flora and fauna are mixed."
Dr Buck suggests the presence of several iridium layers at Nanxiong supports a view that Cretaceous rocks were reworked in the Tertiary.
There have been other claims for the survival of dinosaurs into Tertiary times at sites in Montana and New Mexico in the US, in Bolivia and in India.
All of these claims have been questioned by other researchers.
"The only really well documented dinosaur remains are from the American west. We actually have no idea what's happening anywhere else in the world," Dr Norman MacLeod, keeper of palaeontology at the Natural History Museum in London, told BBC News Online.
"We know that they lived on other continents, so there's no particular reason to suppose that that western US population was the last population.
"It could well be that they went above the K-T boundary in other parts of the world, especially parts that were remote from the Chicxulub impact."