Around 120 to 130 million years ago, northeast China was home to lush forests, lakes, and an active ecosystem filled with dinosaurs, birds, mammals, insects, and other creatures.
Today, this ancient world, now known as the Yixian Formation, is one of the world’s richest fossil sites. In the 1980s, villagers began finding incredibly well-preserved fossils, sparking a “fossil gold rush.”
These discoveries were so widespread that the extent of excavations can be seen from space.
By the 1990s, scientists confirmed that this region held a treasure trove of fossils unlike most other places.
Many of the fossils were astonishingly detailed, showing internal organs, feathers, fur, scales, and even stomach contents.
Among the incredible finds was a fossil showing a small dinosaur and a cat-sized mammal locked in a battle, frozen mid-action when they died.
The discoveries included the world’s first feathered dinosaurs outside of birds, proving that feathers evolved in dinosaurs long before birds did.
These fossils revolutionized our understanding of dinosaur evolution and confirmed that modern birds are direct descendants of these feathered dinosaurs.
For years, scientists believed these fossils were preserved by a volcanic eruption, similar to the one that buried Pompeii in 79 AD.
This theory suggested that waves of hot ash buried the animals suddenly, preserving them in an instant. This led to the nickname “Chinese Pompeii” for the Yixian Formation.
However, a new study published in Proceedings of the National Academy of Sciences challenges this theory.
Led by Scott MacLennan from the University of the Witwatersrand in South Africa, the study suggests that these fossils were not preserved by a dramatic volcanic event but by more ordinary processes like burrow collapses and seasonal rains that built up layers of sediment over time.
Using advanced technology, the researchers dated the fossils to a specific period lasting less than 93,000 years, around 125.8 million years ago.
This relatively short timeframe indicates that the fossilization process was not a series of sudden volcanic events, as previously thought. Instead, the team believes that regular rain and sediment buildup played a much larger role in preserving the fossils.
MacLennan’s team used a method called CA-ID-TIMS (chemical abrasion isotope dilution thermal ionization mass spectroscopy) to measure tiny zircon grains in the fossils.
By analyzing the ratios of uranium to lead in these minerals, they could determine the precise ages of the rocks and fossils, showing that fossilization happened during ordinary, stable conditions rather than repeated volcanic disasters.
Instead of volcanic ash entombing the creatures in an instant, it appears that sediments slowly accumulated, creating oxygen-free pockets around the remains. This lack of oxygen prevented decay, preserving delicate details like feathers and soft tissues. This process likely occurred faster in lake environments, which may explain why some fossils are found in beautifully preserved, flattened forms in lake sediments.
The researchers also found evidence of fine sediment grains around the fossils, suggesting that animals may have died from events like burrow collapses. As their bodies decomposed, finer sediments seeped in and preserved the bones, while burrowing creatures or bacteria may have helped remove the softer tissues over time.
Study co-author Paul Olsen, a paleontologist at Columbia University’s Lamont-Doherty Earth Observatory, points out that many animals in the Yixian Formation are found in natural positions, as if they were resting, with arms and tails tucked around them.
This contrasts with the twisted “pugilistic” poses seen in Pompeii victims, whose bodies contorted from the intense heat of pyroclastic flows. If pyroclastic flows had buried the Yixian fossils, delicate feathers and fur would likely have been destroyed.
Olsen believes the preservation seen in the Yixian Formation might also be found elsewhere but on a smaller scale. In the eastern United States, he has found sites in places like North Carolina, Connecticut, and New Jersey where similar fossils could be hidden. However, he notes that excavations are challenging due to the high cost and limited space.
The Yixian fossils remain one of the most significant dinosaur discoveries in the past century, revealing details about ancient ecosystems and evolution.
While the popular “Chinese Pompeii” idea is captivating, this study reminds us that extraordinary fossils don’t always need extraordinary events to form. Instead, ordinary processes, repeated over time, can create remarkable snapshots of life from millions of years ago.
Source: Columbia University
