For the past few years, a village in northeastern China has been the fossil-hunting capital of the world. Quarries around the village of Sihetun, in Liaoning Province, north of Beijing, have produced spectacular fossils of birds complete with feathers and beaks, exquisite skeletons of tiny mammals, as well as the remains of fishes, amphibians, insects, molluscs and plants.

But Sihetun's stars have undoubtedly been dinosaurs preserved complete with feathers, or feather-like downy skin-coverings. Four species of 'feathered' dinosaur have been described so far: they have set the palaeontological world alight with debate about the relationship between dinosaurs and birds.

Most palaeontologists now agree that the presence of feathers, among other things, in non-flying dinosaurs shows that birds and dinosaurs are close relatives. But there is still a cloud over the whole affair – nobody has been really sure precisely how old the Sihetun fossils are. This debate has now been resolved thanks to work reported in the 1 July issue of Nature. The answer is that the feathered dinosaurs are at least 20 million years younger than some had thought – confirming some earlier suspicions that the at least some of the feathered dinosaurs were the 'living fossils' of their time, living long after the rest of the living world had changed.

In the report, Carl Swisher III of the Berkeley Geochronology Center, Berkeley, California, with Yuan-qing Wang, Xing Xu and Yuan Wang of the Institute of Vertebrate Paleontology and Paleoanthropology, Beijing, China, present the first dates unambiguously associated with the Sihetun fossils. The results indicate an age of around 124 million years, well within the early part of the Cretaceous Period.

There have been several attempts to establish an age for the fossil beds around Sihetun, but Swisher and colleagues point to problems with earlier analyses. For example, earlier work concentrated on rocks not directly associated with the fossils; the quality of the rocks used to make the measurements may not have been high enough, or that the rocks were of the wrong sort.

Swisher and colleagues use a method based on the steady radioactive decay of a variety (or 'isotope') of the element argon, found in volcanic minerals deposited at the time the fossil-beds were established.

In the days of the dinosaurs, the region was occupied by a lake close to active volcanoes. Animal carcasses were deposited on the floor of the lake. Being free of oxygen, they did not decay readily – this explains the preservation of feathers, beaks and other 'soft' structures. Every so often, ash from a volcanic eruption would carpet the region, forming a thin layer on the lake floor. 124 million years later, the lake has long vanished – what we see today are layers of fossils, interbedded with old volcanic ash. Swisher and colleagues can establish the date of the ash layers by careful measurements of the argon contained in crystals of certain minerals contained in the ash.

The method is extremely accurate – but the crystals have to be just of the right type. To be useful for dating the fossils, the ash layers have to have been deposited in a clear sequence with the fossils (rather than having intruded from rocks formed elsewhere.) Most of all, the ash layers and the fossils should ideally be recovered from the same place. The new work is unprecedented in that it succeeds on all counts.

Until now, estimates of the age of the Sihetun rocks have varied widely, from as long ago as 147 million years ago, to as young as 121 million years ago. This age range is important, as it straddles a watershed in the history of life – the boundary between the Jurassic and the succeeding Cretaceous Periods, 144 million years ago. At the end of the Jurassic, the world's fauna underwent some important changes, with many elements of the Jurassic fauna become extinct and being replaced by new creatures.

One element of the Jurassic fauna was the earliest-known bird, Archaeopteryx. This is known only from a few fossils from small area of Bavaria in southern Germany, and is known to have lived at the very end of the Jurassic Period. Archaeopteryx is very primitive-looking for a bird, and had jaws full of teeth rather than a beak. It makes quite a contrast with the Sihetun bird Confuciusornis, which had a horny beak. If Confuciusornis were Jurassic, it would be as old as Archaeopteryx, making Archaeopteryx look like a relic that had outlived its time – and pushing the origin of birds way back into the Jurassic. Matters would be simpler were Confuciusornis much younger.

On the other hand, the feathered dinosaur Sinosauropteryx from Sihetun looks – apart from the clothing of down-like fibres – very like a dinosaur called Compsognathus, which comes from the same Jurassic beds that have yielded Archaeopteryx. Detailed examination of many other elements of the Sihetun fauna, such as the insects, and the archaic-looking flying reptile Eosipterus, suggest that it has a Jurassic 'look' about it.

The establishment of the Sihetun fauna as not only Cretaceous, but a full 20 million years later than the Jurassic-Cretacous boundary, solves a problem but leaves another. That is, several of the creatures in the fauna, such as Sinosauropteryx and Eosipterus, look like relics of an earlier age. Could Sihetun have been the site of the first Jurassic Park, a living museum in the Cretaceous Period?