A famous fossil formation in the Yixian region of Northeast China has revealed some of the most striking, well-preserved dinosaur remains on record. Over the past 60 years, archaeologists excavating the area have uncovered scores of undisturbed, full-body skeletons seemingly frozen in time at the exact moment they died. Feathers and other intact soft tissue samples found in some of the fossils even helped prove that modern birds almost certainly descended from dinosaurs.
Until now, scientists largely attributed the unique preservation of these fossils to a series of sudden, violent volcanic eruptions similar to those that turned doomed residents of Pompeii into so-called “ash mummies.” Some researchers citing that theory even colloquially referred to the formation as “The Chinese Pompeii.” But new archaeological evidence is calling into question this supposed dramatic dinosaur catastrophe. In reality, according to a new study published in The Proceedings of the National Academy of Sciences (PNAS), these animals may have perished from far more mundane events like burrow collapses and particularly heavy seasonal rain.
“These [fossils] are just a snapshot of everyday deaths in normal conditions over a relatively brief time,” Columbia Climate School paleontologist and study coauthor Paul Olsen said in a statement.
Some animals appeared to die in their sleep
The fossils found in the Yixian formation broadly fall into two distinct categories. The first includes nearly fully intact 3D formations from deposits found mostly on land. One of the most famous fossils in the camp depicts a cat-sized mammal seemingly 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. While this second group lacks the dimensionality of the others, they are considered highly valuable because some of them contain soft tissue like internal organs, feathers, and scales that aren’t typically preserved in the fossil record.
Prior theories argued these creatures were essentially frozen in time after being exposed to extremely hot “pyroclastic flows” of volatile ash spewing from a nearby volcano. However, the researchers noticed some key differences between the samples in China and the human remains found in Pompeii. In the latter case, the scorching ash burned off the deceased’s hair and skin. The humans at Pompeii were also discovered hunched over in “pugilistic positions” suggesting a violent end marked by extreme agony. Many of the fossilized animals in Yixian, by contrast, have their tails and arms comfortably tucked and nestled around their bodies. Some of these dinosaurs and mammals, the study notes, look like they were sleeping soundly at their time of death.
To get a better sense of just what happend, the researchers analyzed small grains of zircon taken from some of the fossil samples in 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 ago and seem to occur within 93,000 years of each other. Rather than one sudden die-off event, the researchers linked the fossil to three periods where variations in the Earth’s orbit led to a wetter environment.
All that added moisture, the researchers argue, may have caused sediment to build up in the lakes and on land faster than once thought. That loose sediment may have quickly buried the animals and sealed out oxygen needed for bacteria and insects to survive and aid in decomposition. It’s that absence of oxygen and bacteria that may have helped preserve the soft tissue in the flattened lake fossils. The more 3D, upright fossils found on land, by contrast, weren’t as sealed off which may explain why only their bones were preserved. At the same time, core samples of rocks surrounding the fossils tended to be coarse grains, while those immediately surrounding the fossils were much finer. Those details, plus the moisture of the sediment, suggest the animals were likely killed suddenly by their burrows collapsing rather than any major volcanic eruption.
“The preservation of the flattened and the 3D fauna are not the result of a Pompeii-like catastrophic series of volcanic processes,” the researchers said. “Rather, these sediments are millennial-scale geological snapshots in time of diverse continental communities sampled repeatedly by cyclical environmental processes and normal attrition of the communities.”
Though the exact causes for these burrow collapses around the region still aren’t fully understood, the study suggests it’s likely far less dramatic than the previously proposed volcanic cataclysm. Burrow collapses, even today, are relatively common, and can potentially result from loose solid or larger animals, in this case, dinosaurs, stomping around overheard. That later theory would also help explain why the well-preserved fossils from the Yixian formation only tend to include smaller-sized dinosaurs.
“What was said about their method of preservation highlights an important human bias,” Olsen added. “That is, to ascribe extraordinary causes, i.e. miracles, to ordinary events when we don’t understand their origins.”
