Many of the best fossil specimens have been unearthed in the Liaoning province in China. These include the 'feathered' dinosaurs Sinosauropteryx, Protarchaeopteryx, Caudipteryx and Beipiaosaurus, birds such as Confuciusornis, primitive mammals, what is thought to be the oldest flowering plant, and a wide range of other animals and plants.

They were all pulled from ancient lake beds, from rocks in what is known as the Yixian Formation. Scientists had estimated the diverse fauna found in these sediments to be from the late Jurassic period - about 140 million years ago.

Now, after the application of new dating methods, it appears the sediments are from the Cretaceous period, 20 million years later than the previous estimates.

Big questions

Accurate dating is important because it will help answer some of the big questions in evolutionary biology, such as the timing of the origin of birds and flowering plants, and the relationship of birds to dinosaurs.

"Probably no other story has created so much attention in the last couple of years as the feathered dinosaurs or the world's oldest flowering plant or some other aspect of this fauna from China," says Carl Swisher III, of the Berkeley Geochronology Center in California, and lead author of the new research published in the journal Nature.

"Most people thought it was quite old but these dates directly associated with the fauna suggest it is not as old as originally thought."