According to the team of researchers behind the new study, the animals preserved in the ancient graveyard lived in a region of conifer forests and lakes, surrounded by volcanoes. Layers of sediment found in the beds indicate there were probably several small but intense eruptions during the early Cretaceous. When these volcanoes erupted, they believe, surges of hot gas and volcanic rock surged across the landscape. These surges, known as pyroclastic flows or pyroclastic density currents (PDCs), can move at speeds of up to 450 mph, while the temperature of the poisonous gas spewing from the volcano can reach more than 1,000 degrees. Such deadly blasts would have killed the animals instantly, encasing their bodies in layers of ash.
Like the humans killed in Pompeii, the animals were frozen in mid-movement, with backbones extended and limbs flexed–typical, researchers say, of victims killed by PDCs. According to lead researcher Baoyu Jiang: “All the studied fossils are directly embedded within pyroclastic flows. And the preserved animals are characterized by entombment poses and showed evidence of charring, similar to those associated with victims at Pompeii.”
Because both freshwater and terrestrial animal fossils have been found in the Jehol Biota beds, the researchers believe the animals were likely swept into an ancient lake. They claim the animals were all found together because the PDCs picked them up and deposited them all in the same location. Professor Mike Benton, a paleontologist from the University of Bristol who is not involved in the study, calls this conclusion “quite a radical, new idea,” and one that poses a direct challenge to the prevailing view that it was ancient rivers that carried the animals into the lakes. While saying that the basis of the new study is sound, he believes it is “unlikely” that the pyroclastic flows moved the animals, pointing out that at Pompeii, people were killed but not transported.