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. 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. Matters would be simpler were Confuciusornis much younger. 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.