The curious juxtaposition of exceptionally preserved dinosaurs, mammals and early birds found together in the Yixian and Juifotang rock formations in northern China is the result of lethal volcanic flows, according to a study published in Nature Communications. The finding helps to untangle the mystery behind the preservation history of the Jehol Biota - an ancient ecosystem of northern China, around 130 to 120 million years ago.

During this period, a mix of dinosaurs, mammals, early birds and lizards lived in a landscape of lakes and conifer forests, surrounded by volcanoes. Today these animals are found as incredibly well-preserved fossils in mixed graveyards in northern China, yet the cause of these mass mortalities and their preservation history remains a mystery.

Baoyu Jiang and colleagues collected samples from fossil birds and dinosaurs from several key locations and analysed them and the sediments in which they were preserved. The team found that each of the skeletons had been directly embedded within pyroclastic flows, incredibly fast moving currents of hot gas and rock that accompany some explosive volcanic eruptions. Each of the preserved animals was characterised by entombment poses and showed evidence of charring, similar to those associated with victims at Pompeii, Italy. The findings suggest that pyroclastic flows led to the mass mortality, entombment, transportation and ultimate preservation of the Jehol Biota.

Pyroclastic flows are lethal, have occurred throughout time and, based on these findings, the authors suggest that such flows could be responsible for the preservation of other fossil groups that are closely associated with ash deposits.