A famous fossil formation in the Yixian region of northeast China has revealed some of the most striking and best-preserved dinosaur remains ever recorded. Over the past 60 years, archaeologists excavating the area have discovered dozens of whole, intact skeletons, seemingly frozen in time at the exact moment they died. Feathers and other intact soft tissue samples found in some fossils have even helped prove that modern birds are almost certainly descended from dinosaurs.

Until now, scientists largely attributed the unique preservation of these fossils to a series of sudden and violent volcanic eruptions, similar to those that transformed doomed residents of Pompeii in what we call “ash mummies.” Some researchers citing this theory have even colloquially referred to the training as “Chinese Pompeii.» But new archaeological evidence calls into question this supposed dramatic dinosaur catastrophe. In reality, according to a new study published in Proceedings of the National Academy of Sciences (PNAS), these animals could have perished following much more mundane events such as the collapse of burrows and particularly heavy seasonal rains.

“These (fossils) are just a snapshot of daily deaths under normal conditions over a relatively brief period of time,” Paul Olsen, a paleontologist at the Columbia Climate School and co-author of the study, said in a statement.

Some animals appeared to die in their sleep

The fossils found in the Yixian Formation fall broadly into two distinct categories. The first includes almost entirely intact 3D formations from deposits found primarily on land. One of the camp’s most famous fossils depicts a cat-sized mammal apparently engaged in a battle to the death with a small dinosaur. The second category of fossils are flattened, highly detailed carcasses typically found in lake sediments. Although this second group lacks the dimensionality of the others, they are considered very valuable because some of them contain soft tissues like internal organs, feathers, and scales that are not usually preserved in the fossil record .

Previous theories claimed that these creatures were essentially frozen in time after being exposed to extremely hot “pyroclastic flows” of volatile ash spewed by a nearby volcano. However, researchers noticed key differences between the samples taken in China and the human remains found in Pompeii. In the latter case, the burning ashes burned the hair and skin of the deceased. Humans from Pompeii were also found hunched over in “pugilistic positions,” suggesting a violent end marked by extreme agony. In contrast, many fossilized animals from Yixian have their tails and arms comfortably folded and nestled around their bodies. According to the study, some of these dinosaurs and mammals appeared to be sleeping deeply at the time of their death.

To get a better idea of ​​what happened, the researchers analyzed small grains of zircon taken from some fossil samples from the formation and surrounding rock. They then used an advanced technique called chemical abrasion isotope dilution thermal ionization mass spectroscopy, or CA-ID-TIMS, to date the samples. The fossil samples date back 125.8 million years and appear to be within 93,000 years of each other. Rather than a sudden disappearance event, the researchers linked the fossil to three periods during which variations in Earth’s orbit led to a wetter environment.

All that extra moisture may have caused sediment to build up in lakes and on land more quickly than previously thought, researchers say. These loose sediments may have quickly buried the animals and trapped the oxygen needed for bacteria and insects to survive and decompose. It is this absence of oxygen and bacteria that may have helped preserve the soft tissues of the lake’s flattened fossils. In contrast, the more 3D and vertical fossils found on land were not as sealed, which may explain why only their bones were preserved. At the same time, the rock cores surrounding the fossils tended to be made of coarse grains, while those immediately surrounding the fossils were much finer. These details, along with the wetness of the sediment, suggest that the animals were likely killed suddenly by the collapse of their burrows rather than by a major volcanic eruption.

“The preservation of the flattened, 3D fauna is not the result of a catastrophic series of volcanic processes à la Pompeii,” the researchers said. “Rather, these sediments are millennial-scale geological snapshots of diverse continental communities repeatedly sampled by cyclical environmental processes and normal community attrition.”

Although the exact causes of these burrow collapses in the region are not yet fully understood, the study suggests that it is likely much less dramatic than the previously proposed volcanic cataclysm. Burrow collapses, even today, are relatively common and can potentially result from solid or larger animals, in this case dinosaurs, trampling. This later theory would also help explain why well-preserved fossils from the Yixian Formation generally contain only smaller dinosaurs.

“What has been said about their method of preservation highlights a significant human bias,” Olsen added. “That is, attributing extraordinary causes, i.e. miracles, to ordinary events when we do not understand their origins.”