By the 1990s, it was clear that the so-called Yixian Formation contained uniquely well preserved remains of dinosaurs, birds, mammals, insects, frogs, turtles and other creatures.

Unlike the skeletal and often fragmentary fossils unearthed in most other places, many animals came complete with internal organs, feathers, scales, fur and stomach contents. It suggested some kind of sudden, unusual preservation process at work. The finds even included a cat-size mammal and a small dinosaur locked in mortal combat, stopped in mid-action when they died.

The world’s first known non-avian feathered dinosaurs showed up—some so intact that scientists worked out the feathers’ colors. The discoveries revolutionized paleontology, clarifying the evolution of feathered dinosaurs, and proving without a doubt that modern birds are descended from them.

How did these fossils come to be so perfect? The leading hypothesis up to now has been sudden burial by volcanism, perhaps like the waves of hot ash from Mt. Vesuvius that entombed many citizens of Pompeii in A.D. 79. The Yixian deposits have been popularly dubbed the “Chinese Pompeii.”

A new study says the Pompeii idea is highly appealing―and totally wrong. Instead, the creatures were preserved by more mundane events including collapses of burrows and rainy periods that built up sediments that buried the dead in oxygen-free pockets. Earlier studies have suggested that multiple Pompeii-type events took place in pulses over about a million years.