How did the world’s best-preserved dinosaurs die? 

Between 120 and 130 million years ago, during the age of the dinosaurs, northeast China hosted vibrant temperate forests and lakes that nurtured a diverse ecosystem. 

Fossils from this period stayed untouched for millions of years until villagers began to unearth exceptionally preserved specimens in the 1980s. 

These finds, often sold to collectors and museums, initiated a fossil rush that has since transformed the region into one of the most heavily excavated paleontological sites in the world, visible even from space.

Fossils of well-preserved dinosaurs

The dinosaur fossils discovered in the Yixian Formation stood out for being remarkably well-preserved.

Unlike the usual fragmented skeletal remains found at other sites, these fossils often included soft tissues, internal organs, feathers, fur, and even stomach contents. 

Notable among these finds was a cat-sized mammal locked in a struggle with a small dinosaur, both seemingly frozen in time.

Such discoveries confirmed that modern birds are descendants of feathered dinosaurs and provided crucial insights into their evolution, redefining paleontological understanding. 

Early theories suggested that a sudden volcanic event, similar to the eruption of Mt. Vesuvius that buried Pompeii, might have been responsible for preserving these fossils, leading to the formation’s nickname, “Chinese Pompeii.”

Refuting volcanic theories

A recent study published in Proceedings of the National Academy of Sciences challenges the long-held belief that volcanic activity was the primary reason for the pristine preservation of these fossils. 

The researchers, using precise dating techniques, argue that the fossils were preserved not by dramatic volcanic cataclysms but by more common occurrences such as burrow collapses and periods of heavy rainfall that quickly buried the remains. 

This new study pinpoints the age of these fossils to a compact period of less than 93,000 years, dismissing the notion that preservation was the result of multiple volcanic pulses over millions of years.

Well-preserved dinosaur discoveries

“These are probably the most important dinosaur discoveries of the last 120 years,” said study co-author Paul Olsen, a paleontologist at the Columbia Climate School’s Lamont-Doherty Earth Observatory. 

“But what was said about their method of preservation highlights an important human bias. That is, to ascribe extraordinary causes, i.e. miracles, to ordinary events when we don’t understand their origins. These [fossils] are just a snapshot of everyday deaths in normal conditions over a relatively brief time.”

Evidence against volcanic theories

The research team dismissed the possibility of preservation by volcanic lahars — fast-moving mudflows — due to their violent nature, which would have obliterated delicate remains.

Similarly, the theory of pyroclastic flows, like those that buried Pompeii, was ruled out. 

Such flows would have incinerated feathers and fur, and left the bodies in the characteristic “pugilistic” posture seen in victims of extreme heat.

Instead, the animals in the Yixian Formation are found in more relaxed positions, suggesting a peaceful end.

Theory of sudden burrow collapses 

Instead of volcanic cataclysms, the researchers propose that many of the preserved fossils were the result of sudden burrow collapses. Evidence for this theory came from the composition of the rock surrounding the fossils. 

While the general rock formations were made of coarse grains, the material immediately around and within the skeletons was finer, indicating that fine sediment seeped in as bacteria or insects degraded the soft tissues, preserving the bones. 

Olsen noted that even today, burrow collapses can be fatal for animals like penguins, illustrating how this process may have played a role in the preservation of ancient creatures.

The extraordinary find of a mammal and dinosaur locked in combat could potentially be explained by such collapses.

The mammal may have been invading the dinosaur’s burrow in search of food or safety when both met their end.

What triggered burrow collapses?

While the researchers propose burrow collapses as the primary cause of death, the exact triggers remain speculative. 

One possibility is that larger dinosaurs, which do not appear in the fossil record but likely roamed the area, may have inadvertently crushed the burrows while moving through the landscape.

Additionally, periods of intense rainfall could have destabilized the ground, contributing to collapses.

Potential for future dinosaur discoveries

Olsen believes that the Yixian Formation’s unique preservation is not exclusive and that other areas around the world could yield similarly well-preserved dinosaur fossils if excavated with the same intensity. 

“It’s just that there is no place else where such intense collecting has been done in this kind of environment,” he said.

While China has taken measures to regulate fossil sales, the demand remains robust, and significant investments have been made to promote tourism around these fossil sites.

Olsen mentioned sites in the eastern United States that share environmental similarities with Yixian, such as a quarry straddling the North Carolina-Virginia border, sites in Connecticut, and a former quarry in New Jersey. 

However, large-scale excavation in these areas has yet to happen due to the high cost and the value of the land.

“It takes enormous effort, which is expensive. And land is valuable in these areas. So no one is doing it. At least not yet,” Olsen concluded.

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Image Credit: Jun Liu, Institute of Paleontology and Paleoanthropology, Chinese Academy of Sciences

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