The dinosaurs, birds, and early mammals found in the fossil beds of northern China are famous—both for their exceptional preservation and for their incredible diversity. But no one knew how they died or why hundreds of creatures from different habitats were buried together on ancient lake floors.

Now researchers say they were likely killed by a series of volcanic eruptions more than 120 million years ago. The ash entombed and preserved them, much like the doomed victims of Pompeii. (See also "Pompeiians Flash-Heated to Death—No Time to Suffocate.")

After analyzing fossils and sediment, Baoyu Jiang of China's Nanjing University and his team concluded that lethal, sudden pyroclastic volcanic eruptions marked by air blasts, hot gas, and ground-hogging clouds of fine ash likely smothered, charred, and then carried forward everything in their path to create these bone beds, according to the study published in Nature Communications.

The finding explains why so many creatures would come to be buried on lake floors, and how they remained well preserved enough to retain signs of soft tissue features, such as feathers, tens of millions of years later.

View Images Backscattered electron-microscope images show carbonized soft tissues (dark materials) on the fossils (a and b, Psittacosaurus; c and d, Confuciusornis). PHOTOGRAPH BY BAOYU JIANG

While paleontologists have long suspected that volcanic eruptions played some role in China's Jehol fossils, Jiang says the pyroclastic eruptions explain everything about them. The blasts, he says, "not only caused major casualties as they [do] today, but also transported some of the remains into nearby lakes and rapidly buried the remains."

The report compares the charred bones in the study fossils to ones seen from the victims of the Roman town of Pompeii, killed in 79 A.D. by the eruption of Mount Vesuvius. The study authors also analyzed ashy sediments across the region to explain the preservation and accumulation of the fossils. (See video: "Volcanic Death.")

"The authors go a step further than had been done before in suggesting that all the Jehol animals were killed, transported, and exceptionally preserved by the pyroclastic flows," says paleontologist Michael Benton of the U.K.'s University of Bristol, by email. "This is quite a challenge to previous views that assumed most of the animals lived in and around the lakes in which they are found," he adds.

Mass Mortalities

The eruptions occurred across northern China from roughly 120 to 130 million years ago. In the study, the international team examined the ash encasing 14 well-preserved fossils from five bone beds, including the crow-size bird Confuciusornis and the parrot-faced dinosaur Psittacosaurus.

Ashes covering the fossils are fine-grained, covering charred bone, the researchers found, similar to pyroclastic ash seen in the massive 1883 eruption of the Indonesian volcano Krakatoa. The death poses of the creatures in the bone beds resemble those of other pyroclastic ash victims, with limbs extended. The bones have spiderweb cracks like those seen on the charred bones of Pompeii victims, according to the study.

Birds caught in the blasts over the 10-million-year period of eruptions would have suffocated and fell from the sky. "The eruptions must have been numerous and caused multiple mass mortalities of both terrestrial and aquatic animals," the study says. Rather than the mud coating that the bones would have if they floated into lake beds, the fine ash covering appears uniform, say the study authors.

View Images Photomicrographs of thin sections show abrasive pits, missing bone material (arrows), and cracks at bone edges (a, Psittacosaurus; b, Confuciusornis). PHOTOGRAPH BY BAOYU JIANG

Bones of Contention

"All of the evidence certainly points to this explanation," says paleontologist Mark Norell of the American Museum of Natural History in New York, who was not part of the study. "Looking at the exceptional preservation and the numbers [of fossils], it just seems a noxious gas cloud swept in and took them all out."

Benton is cautious about embracing this new explanation for how the animals ended up in the fossil beds, calling it "unlikely." "I think the basis of the work is good, but the evidence that the pyroclastic flows actually transported the carcasses in most cases seems unlikely," Benton says. "At Pompeii, people were overwhelmed and killed, but not transported."

Norell is more convinced, saying the idea of volcanic air blasts pushing carcasses into lake beds has been discussed among paleontologists for years. "At other sites, the bones end up jumbled and scavenged," Norell says. "Instead, these Jehol remains are just exquisitely preserved."

"This really is a dinosaur version of Pompeii, 125 million years old," says paleontologist Stephen Brusatte of Scotland's University of Edinburgh, who was not part of the study. "These volcanoes were agents of death, but also a godsend when it came to fossil preservation."

Brusatte adds: "Without such an unusual preservational setting, we would never get so many well-preserved fossils with details of skin, muscles, feathers, and other soft tissues. There's a reason these types of fossils are rare—because it takes extraordinary events like mega volcanic eruptions to preserve them."

The volcanic findings might also help point the way to future findings of fossil beds across Mongolia and northern China, Norell adds, perhaps pointing to more once-volcanic spots to investigate for dinosaur bones. "I think there is a lot of work left for us in finding fossils in China," he says.