Extraordinarily well-preserved fossils of feathered dinosaurs and other ancient life forms have long been thought to be the result of volcanic eruptions freezing them in time. However, a recent study challenges this notion.
Around 120 to 130 million years ago, during the era of dinosaurs, lush forests and lakes flourished in what is now northeast China. Fossils from that period remained largely untouched until the 1980s, when locals began to discover exceptionally preserved specimens, which they sold for high prices to collectors and museums. This sparked a fossil rush, with both residents and researchers excavating so extensively that their efforts are visible from space — possibly the largest paleontological diggings in the world.
In the 1990s, it became evident that the Yixian Formation contained an array of well-preserved fossils, including those of dinosaurs, birds, mammals, insects, frogs, turtles, and more. Unlike the fragmented and often broken fossils typically found elsewhere, many specimens were discovered with internal organs, feathers, scales, fur, and even stomach contents intact. This indicated some extraordinary method of preservation. Among the notable finds was a small mammal and a tiny dinosaur caught in the act of fighting, preserved in a moment of life. Additionally, the world’s first known feathered non-avian dinosaurs were uncovered, some so well-preserved that scientists could determine the colors of their feathers. These discoveries changed the field of paleontology, shedding light on the evolution of feathered dinosaurs and confirming that modern birds are direct descendants of these creatures.
The prevailing theory for the exceptional preservation of these fossils had been rapid burial by volcanic activity, similar to the hot ash flows from Mt. Vesuvius that buried the town of Pompeii in A.D. 79. This led to the nickname “Chinese Pompeii” for the Yixian deposits.
However, the new study argues that the Pompeii analogy is both appealing and incorrect. Instead, the creatures were preserved through more common events like the collapse of burrows and periods of rain that deposited sediments, burying the remains in oxygen-poor environments. Previous theories suggested that multiple volcanic events could have occurred over about a million years. In contrast, this latest research uses advanced technology to date the fossils to a much shorter period of less than 93,000 years, during which no significant volcanic activity occurred.
The findings were published in the journal Proceedings of the National Academy of Sciences.
“These discoveries are among the most significant in the last 120 years of dinosaur research,” stated Paul Olsen, a paleontologist from the Columbia Climate School’s Lamont-Doherty Earth Observatory and co-author of the study. “However, the previous preservation theories highlight a common human tendency to attribute extraordinary explanations to ordinary events when the origins are unclear. These fossils represent a snapshot of typical deaths occurring in normal environments over a relatively short timeframe.”
The fossils from the Yixian Formation can be categorized into two main types: intact 3D skeletons from terrestrial deposits and flattened yet detailed specimens found in lake sediments, some of which still contain soft tissues.
To determine the ages of the fossils, lead author Scott MacLennan from the University of the Witwatersrand in South Africa analyzed tiny zircon mineral grains from both the surrounding rocks and the fossils themselves. By measuring the ratio of radioactive uranium to lead using the precise method called chemical abrasion isotope dilution thermal ionization mass spectroscopy (CA-ID-TIMS), the fossils were consistently dated to around 125.8 million years ago, focusing on a period of less than 93,000 years, although the exact duration varies.
Further analysis revealed that this timeframe included three distinct wet periods linked to changes in the Earth’s orbit, leading to a more rapid buildup of sediments in lakes and on land than previously believed. Many organisms were quickly buried, with oxygen, which usually promotes decay, being cut off. This was particularly effective in lakes, which resulted in the preservation of soft tissues.
The researchers dismissed the volcanism theory for several reasons. Some earlier theories proposed that the fossils were encased by lahars, fast-moving mudflows from volcanic eruptions. However, Olsen noted that lahars are violent enough to destroy any life they encounter, making this explanation unfeasible.
Others suggested that pyroclastic flows, consisting of hot ash and toxic gases, were responsible. These flows covered individuals in protective layers, preserving them in their last moments. However, in the Yixian Formation, remains differ from those found in Pompeii; feathers, fur, and other organic materials would likely have been incinerated in a pyroclastic flow. Instead, many dinosaurs and animals were found with their limbs tucked comfortably around their bodies — appearing as if they were sleeping peacefully when they died.
Evidence suggests that sudden burrow collapses are more likely responsible for the preservation of these fossils. The surrounding rock cores mostly contain coarse grains, but those around and inside the skeletons are predominantly finer grains. This indicates that there was likely sufficient oxygen for bacteria or insects to degrade some of the skin and organs, filling the voids with finer grains as the decay occurred, while the bones remained intact. Even today, burrow collapses are a common cause of death in species like penguins, according to Olsen. The interesting fossilized confrontation might have occurred as the mammal entered the dinosaur’s burrow in search of food.
As for what triggered the burrow collapses, that’s still a matter of speculation. One possibility is that larger dinosaurs, not found among the remains, could have disturbed burrows simply by moving through the area. Additionally, particularly rainy periods may have destabilized the ground.
Olsen believes that the Yixian Formation is not one of a kind. “It’s just that there hasn’t been as extensive collection done in similar areas,” he explained. While China has made attempts to limit fossil sales for profit, the market continues to thrive, and significant government efforts are being directed towards developing tourism around fossil sites.
Olsen’s ultimate goal is to find feathered dinosaurs, but he pointed out that these are extremely rare, even in rich fossil deposits. “You might have to sift through 100,000 fish to find one feathered dinosaur, and the scale of digging done in Yixian is unmatched,” he noted. There are potential sites in the eastern United States that may yield similar fossils due to past environments akin to the Yixian. These include a rock quarry on the North Carolina-Virginia border with thousands of well-preserved insects, promising sites in Connecticut, and a former quarry in North Bergen, New Jersey, known for many preserved fish and reptiles. Excavations at such locations are more limited in scale, akin to the size of a bathroom,” he added.
“Conducting such excavations requires massive effort, which can be costly. Moreover, land in these areas is highly valuable,” he said, noting that no one is pursuing these projects currently, at least for now.
The study was co-authored by Sean Kinney and Clara Chang from the Lamont-Doherty Earth Observatory, along with researchers from the Nanjing Institute of Geology and Paleontology, the Institute of Paleontology and Paleoanthropology at the Chinese Academy of Sciences, and Princeton University.
