By the 1990s, it was clear that the so-called Yixian Formation contained particularly well-preserved remains of dinosaurs, birds, mammals, insects, frogs, turtles and other creatures. Unlike the skeletal and often fragmentary fossils found in most other places, many animals came with internal organs, feathers, scales, fur, and the contents of their stomachs. This suggested some sort of sudden and unusual preservation process at work.

Until now, the main hypothesis for the perfect fossils has been sudden burial by volcanism, perhaps like the waves of hot ash from Mount Vesuvius that buried many of Pompeii’s citizens in AD 79. The Yixian deposits have been commonly nicknamed the “Chinese Pompeii”. . Although the idea of ​​Pompeii is very attractive, one study claims that it is completely false. Instead, the study says the creatures were preserved by more mundane events, including burrow collapses and periods of rain that accumulated sediment that buried the dead in oxygen-free pockets. The current study used newly sophisticated technology to date fossils over a compact period of less than 93,000 years during which nothing special happened.