A famous fossil formation in Northeast China’s Yixian region has revealed some of the most striking, well-preserved dinosaur remains ever recorded. Over the past 60 years, archaeologists excavating the area have uncovered dozens of undisturbed full-body skeletons that were seemingly frozen in time at the exact moment they died. In fact, feathers and other intact soft tissue samples found in some fossils helped prove that modern birds almost certainly descended from dinosaurs.
Until now, scientists attributed the unique preservation of these fossils largely to a series of sudden, violent volcanic eruptions, similar to those that turned the doomed inhabitants of Pompeii into so-called “ash mummies.” Some researchers who cited that theory even colloquially referred to the formation as “The Chinese Pompeii.But new archaeological evidence is casting doubt on 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 been killed by much more mundane events such as cave collapses and particularly heavy seasonal rains.
“This [fossils] are just a snapshot of daily deaths under normal conditions over a relatively short time,” Columbia Climate School paleontologist and co-author Paul Olsen said in a statement.
Some animals seemed to die in their sleep
The fossils found in the Yixian Formation broadly fall into two different categories. The first includes almost completely intact 3D formations of deposits that occur mainly on land. One of the most famous fossils in the camp shows 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 contain soft tissue, such as internal organs, feathers, and scales, that are generally not 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 spewing from a nearby volcano. However, the researchers noted some important differences between the samples in China and the human remains found in Pompeii. In the latter case, the scorching ash burned the hair and skin of the deceased. The people of Pompeii were also found bent over in ‘boxing positions’, suggesting a violent end marked by extreme pain. In contrast, many of the fossilized animals in Yixian have their tails and arms nestled comfortably around their bodies. Some of these dinosaurs and mammals, the study notes, look as if they were sound asleep at the time of death.
To get a better idea of what exactly happened, the researchers analyzed small zircon grains from some 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 from 125.8 million years ago and appear to have occurred within 93,000 years of each other. Instead of one sudden die-off event, the researchers linked the fossil to three periods when variations in Earth’s orbit led to a wetter environment.
All that added moisture may have caused sediment to accumulate in the lakes and on land faster than once thought, according to the researchers. That loose sediment may have quickly buried the animals and locked out the 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 lake’s flattened fossils. In contrast, the more upright 3D fossils found on land were not as occluded, which could explain why only their bones were preserved. At the same time, the core samples from the rocks surrounding the fossils were often coarse grained, while the samples immediately surrounding the fossils were much finer. These details, plus the moisture 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 and 3D fauna is not the result of a Pompeii-like catastrophic series of volcanic processes,” the researchers said. “Rather, these sediments are millennial-scale geological snapshots of diverse continental communities, repeatedly sampled through cyclical environmental processes and normal community depletion.”
Although the exact causes for the collapse of these caverns in the region are still not fully understood, the study suggests that it is likely much less dramatic than the previously proposed volcanic catastrophe. Burrow collapses are relatively common even today, and could possibly be the result of loose solid or larger animals, in this case dinosaurs, stomping around. That later theory would also help explain why the well-preserved fossils from the Yixian Formation tend to contain only smaller dinosaurs.
“What was said about their preservation method highlights an important human bias,” Olsen added. “That is, attributing extraordinary causes, that is, miracles, to ordinary events, while not understanding their origin.”