One of the most extraordinary fossil beds of Cretaceous creatures in the world formed about 125 million years ago, in what’s now northeastern China.

Researchers have thought that the diverse members of this ancient community were abruptly buried by catastrophic volcanic flows of hot ash and rock.

But that volcano doomsday scenario — sometimes referred to as China’s Cretaceous Pompeii — didn’t happen, researchers contend November 3 in Proceedings of the National Academy of Sciences. Instead, the team says, the animals found in these rocks — including nonavian dinosaurs, birds, mammals, insects, frogs and turtles — were buried by a series of unfortunate, but not catastrophic, events.

These Cretaceous Period rock layers, known as the Yixian Formation, are famous for two types of fossils: A collection of still-perfectly articulated skeletons preserved in 3-D relief; and fossils that are flattened but bear exquisitely preserved details, such as feathers, pigments, soft tissues and even stomach contents. It was these feathery details that ultimately helped convince paleontologists that modern birds evolved from theropod dinosaurs (SN: 9/18/99).

In the new study, the researchers used a very precise geochemical dating technique to analyze tiny zircon minerals collected from the rocks containing the fossils, as well as from two dinosaur fossils that were originally from the site but are now in a museum. These dates revealed that both of the Yixian’s fossil beds date to within just 93,000 years of each other, a geological blip in time, say paleontologist Scott MacLennan of the University of the Witwatersrand, Johannesburg and colleagues.

But the two types of fossils didn’t form at the same time — which means that it wasn’t a single catastrophic event that did in all of the creatures, the researchers say. Cores drilled into the Yixian Formation at several different locations revealed the 3-D fossils to be older, lying beneath the rock layer containing the flattened fossils. A layer of hardened lava lies in between the two.

From these analyses, the team devised a new hypothesis for how all these creatures died. Instead of a dramatic sudden mass death, the researchers say, the Yixian Formation represents “a brief snapshot of normal life and death in an Early Cretaceous continental community.”

The 3-D fossils include skeletons of Psittacosaurus and other dinosaurs, apparently in nests (SN: 11/16/16). Researchers have previously noted that these fossils’ extraordinarily lifelike poses are reminiscent of those of humans found entombed in scalding ash and rock at Pompeii, an ancient Italian city that was catastrophically destroyed by the eruption of Vesuvius in A.D. 79 (SN: 2/4/14; SN: 8/7/24). That, and the presence of volcanic debris in the rocks containing the fossils, suggest that such pyroclastic flows may also have buried these creatures.

The new study proposes that, rather than being entombed by volcanic flows, Psittacosaurus were burrow-dwellers, and were buried when their burrows collapsed. The sediments immediately around and within the fossils were finer-grained than the surrounding rock; that, the team hypothesizes, could mean that there were voids in the rock formed by the bodies of the dinosaurs; over time, as the bodies decayed away, smaller grains of sediment seeped in to fill the voids and surround the skeletons.

As for volcanic evidence, the rocks containing these fossils do have some volcanic detritus, but there isn’t evidence of intense pyroclastic flows, the team says. Furthermore, the poses of many of the creatures suggest sleeping postures rather than fighting or fear, and there’s no evidence of crushed bones, as might be expected from being caught up and tumbled over and over in a swift volcanic flow.

While the 3-D fossils formed in a terrestrial setting, the rocks around the flattened fossils point to burial in deep, fine-grained lake sediments, MacLennan and colleagues say. An analysis of Earth’s orbital variations, known as Milankovich cycles, suggests that the period of these dino deaths coincided with intervals of heavy rain. The creatures may have died and washed into the lake, and been swiftly buried by thick layers of sediment; such rapid burial would mean a low-oxygen environment that is ideal for fossil preservation. The careful preservation of the details of these fossils, such as impressions of feathers, is also inconsistent with extreme heat conditions such as from volcanic flows, the team says.

Not all researchers are convinced. Baoyu Jiang, a paleontologist at Nanjing University in China who has previously worked on the Yixian fossils, says he doesn’t believe that the researchers have proven their case that the fossils weren’t buried by pyroclastic flows.

“The core finding of the paper … concludes that the sedimentation rate [when the fossils formed] was extremely high,” Jiang says. But that doesn’t necessarily mean that volcanism wasn’t the main culprit — and the team’s study analyzed only two individual specimens, not enough to draw that conclusion, he adds.

MacLennan and colleagues argue, however, that it’s a logical fallacy that a remarkable bonebed must have a remarkable origin. The volcanic debris at the site — bits of tuff, or hardened ashfall, and other volcanic rocks — may have thrown researchers off the scent of the true culprit behind the deaths.