Scientists have long been captivated by the fossil beds of China's Liaoning province, where an incredibly diverse and well-preserved collection of animals was fossilized about 125 million years ago in a mysterious mass death. Now, they think they finally have an explanation for the ecosystem that became known as the "Jehol Biota": All those lower Cretaceous era creatures, from dinosaurs to mammals to birds, fish, and insects, were likely killed by volcanic eruptions in an event similar to the destruction of Pompeii, the BBC reports. Specifically, researchers believe a "pyroclastic density current," a wave of extremely hot and fast-moving gas, was responsible for the deaths.
Scientists analyzed 14 specimens and found that all the bones had traces of volcanic materials, Smithsonian reports. Such an eruption, the same type that destroyed Pompeii, would have killed the animals instantly, and their bodies would have then been preserved in ash. Plus, the creatures appeared to be mid-movement when they died in postures that match those of other animals killed in volcanic events, and the area where they lived was surrounded by volcanoes. As for why the fossils are grouped together in what the BBC calls an "animal graveyard," scientists believe the "pyroclastic flow" of gas, ash, and rock pushed their bodies to the same areas—typical of a pyroclastic density current. The Jehol Biota has long been a boon to science, the Los Angeles Times reports: Some of the fossils are so well-preserved, researchers can tell what was in an animal's stomach when it died. Even flowering plants were preserved in the flow, the New Scientist reports.