A new study published in the Proceedings of the National Academy of Sciences has refuted the long-standing hypothesis that volcanic eruptions were responsible for the exceptional preservation of fossils in the Yixian Formation of northeast China. The leading hypothesis for the exceptional preservation had been sudden burial by volcanism; however, the study suggests that the fossils were preserved over a relatively short period under stable conditions.
For decades, scientists believed the preservation of these fossils was due to volcanic eruptions, with some previous studies suggesting that creatures were encased by lahars. Experts had speculated that the key to this exceptional preservation might be related to the volcanism of the area, with some believing that eruptions occurred in waves that buried the dinosaurs under ash or mud. This explanation compared the Yixian fossils' preservation to how thousands of Romans were buried by ash at Pompeii.
However, the new study challenges this volcanic catastrophe theory. According to the study's results, "these fossils are just a snapshot of everyday deaths under normal conditions during a relatively brief period," suggesting they were preserved over a short timeframe under stable conditions. The researchers argue that the remarkable preservation of Yixian fossils was likely due to more common processes, such as periodic sediment accumulation from seasonal rains and the collapse of animal burrows, rather than volcanic eruptions.
Dr. Paul Olsen, a paleontologist at Columbia University's Lamont-Doherty Earth Observatory and co-author of the study, reflects that we tend to "attribute extraordinary causes, that is, miracles, to ordinary events when we do not understand their origins." He and his colleagues carefully analyzed the evidence and compared it to hypotheses related to volcanism. To assess the likelihood of volcanic activity, they used uranium-lead isotopes to date the Yixian fossils and surrounding material more precisely than had previously been possible.
Using this method, the researchers determined that all the specimens were laid down in three bursts in the space of less than 100,000 years approximately 125.8 million years ago. This timeframe contradicts earlier studies that had suggested multiple Pompeii-type events took place in pulses over about a million years. It also rules out the theory that there had been several eruptions millions of years apart.
Furthermore, the Yixian fossils do not show the signs of extreme conditions typical of pyroclastic flows. In contrast to the volcanic catastrophe theory, the exceptional preservation of feathers, one of the Yixian Formation's most noted features, is inconsistent with extreme heat. Feathers, fur, and everything else would almost certainly have been burned in a pyroclastic flow. Pyroclastic flows are known to burn, mangle, and twist bodies in a characteristic "pugilistic pose" due to intense heat, similar to the poses of victims in Pompeii. The Yixian fossils do not exhibit these twisted positions.
The researchers suggest that these creatures were preserved due to more mundane events like the collapse of burrows and periods of rain that accumulated sediments, burying the dead in pockets without oxygen. Rainy weather may have contributed to instability in the region, potentially causing sudden cave-ins that buried smaller animals alive. Such processes created oxygen-free environments conducive to fossilization, where many deceased creatures were quickly buried, and oxygen that normally would fuel decomposition was sealed out.
In these environments, the lack of oxygen further slowed decomposition. Sediments washed down in rainy periods then completed the process of sealing in the specimens. This process preserved not only the skeletons but, in some cases, soft tissues as well, with the sealing effect being fastest in lakes. The researchers interpret that there was enough oxygen around for a while for bacteria or insects to degrade at least the animals' skin and organs, but the oxygen supply was cut off by finer sediments before they could do the same to the bones.
The Yixian Formation became famous for its unparalleled fossil finds, representing the greatest collection of superbly preserved dinosaurs in the world, along with remains of birds, mammals, insects, frogs, and turtles. Among these treasures are the first known feathered non-avian dinosaurs, discovered in the Yixian Formation. The fossils include evidence of the last meals eaten by dinosaurs and other animals, and they preserved delicate details such as feathers, fur, scales, internal organs, and even stomach contents.
Scientists have been excited by the scientific riches of the Yixian fossils but have been divided over the question of why the preservation is so exceptional. Paul Olsen believes that similar fossil troves could exist elsewhere, particularly in the eastern United States, where several locations once had environments similar to Yixian. Sites in North Carolina, Virginia, Connecticut, and New Jersey have yielded well-preserved fossils in the past, but none have matched the scale of Yixian.
Paul Olsen said, "It's just that there is no place else where such intense collecting has been done in this kind of environment." He pointed out, "You have to dig out, say, 100,000 fish to find one feathered dinosaur, and no one is digging on the Yixian scale." The cost and effort required for systematic excavations are significant factors limiting such discoveries, especially in densely populated areas where land is highly valuable.
The study was coauthored by Sean Kinney and Clara Chang of Lamont-Doherty Earth Observatory, and researchers from the Nanjing Institute of Geology and Paleontology, the Institute of Paleontology and Paleoanthropology at the Chinese Academy of Sciences, and Princeton University.
Sources: Interesting Engineering, Phys.org, IFLScience, La Razon
This article was written in collaboration with generative AI company Alchemiq