The Yixian Formation, a 120 to 130 million-year-old geological formation outcropping in the Liaoning Province in northeast China, for more than 20 years has produced exceptionally preserved fossils of insects, plants, shells, fish and several hundred skeletons of feathered dinosaurs.
How did these fossils come to be so perfect? The leading hypothesis up to now has been sudden burial by volcanism, perhaps like the waves of hot ash from Vesuvius that entombed many citizens of Pompeii over 2,000 years ago, and the Yixian deposits have been popularly dubbed the “Chinese Pompeii.”
Earlier studies have suggested that multiple volcanic events took place in pulses over about a million years, repeatedly burying animals and preserving their remains. But a new study completely refutes this theory, showing that the fossils date to a period of less than 93,000 years when there was no volcanic activity at all. The creatures were preserved by more mundane events including collapses of burrows during rainy periods.
“These are probably the most important dinosaur discoveries of the last 120 years,” says study coauthor 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.”
Previous studies have suggested that dinosaurs were encased by lahars, fast-moving concrete-like slurries of mud that flow off volcanoes following eruptions. But lahars are extremely violent, argues Olsen, and apt to rip apart any living or dead thing they encounter, so this explanation does not work for the intact, perfectly articulated skeletons.
An alternative explanations involved pyroclastic flows, fast-moving clouds of hot ash and poisonous gases. These struck down residents of Pompeii, then wrapped the bodies in protective layers of material that preserved them as they were at the moment of death. Even when remains decayed, voids in the ashes remained, from which investigators have made lifelike plaster casts. The remains characteristically are curled in so-called pugilistic positions, torturously doubled over and with limbs severely drawn up, as blood boiled and muscles crumpled in the explosive heat. Victims of modern fires exhibit similar poses.
While there are in fact layers of volcanic ash, lava and intrusions of magma in the Yixian Formation, the remains there don’t match those of the unfortunate Pompeiians. For one thing, feathers, fur and everything else would almost certainly have been burned in a pyroclastic flow. For another, the dinosaurs and other animals are not in pugilistic positions; rather, many are found with arms and tails tucked cozily around their bodies, as if they were sleeping ( a dinosaur described in 2020 was even named Changmiania liaoningensis meaning “Eternal Sleeper from Liaoning”).
The evidence points instead to sudden burrow collapses, say the researchers. Cores of rock surrounding the skeletonized fossils generally consist of coarse grains, but grains immediately around and within the skeletons tend to be much finer. The researchers interpret this to mean that there was enough oxygen around for a while for bacteria or insects to degrade at least the animals’ skin and organs, and as this happened, whatever fine grains were in the surrounding material preferentially seeped in and filled the voids; the more decay-resistant bones remained intact. Even today, burrow collapses are a common cause of death for animals, so Olsen. This new theory could also explain the fossil showing an early mammal and small dinosaur seemingly frozen in time during a fierce battle. This may well have happened as the mammal invaded the dinosaur’s burrow, when the burrow collapsed sealing the fate of both animals.
What caused the collapses remains speculation. Sedimentary features suggest that the Yixian Formation was deposited during a wet period in Earth’s history. Heavy rain could have helped destabilize the ground, Olson notes, burying the dinosaurs sleeping in their burrows.
To come up with fossil ages, the study’s lead author, Scott MacLennan of South Africa’s University of the Witwatersrand, analyzed tiny grains of the mineral zircon, taken from both surrounding rocks and the fossils themselves. Within these, he measured ratios of radioactive uranium against lead, using a new, extremely precise method called chemical abrasion isotope dilution thermal ionization mass spectroscopy, or CA-ID-TIMS. The fossils and surrounding material consistently dated to 125.8 million years ago, centered around a period of less than 93,000 years, and embedded between volcanic deposits.
Olsen believes the Yixian Formation is not unique. “It’s just that there is no place else where such intense collecting has been done in this kind of environment,” he concludes.
Just in the eastern United States, several places that once had environments similar to the Yixian could yield similar fossils. These include a rock quarry straddling the North Carolina-Virginia border where he has found thousands of perfectly preserved insects; sites in Connecticut where small excavations have shown promise; and a former quarry in North Bergen, N.J., now sandwiched between a highway and a strip mall that in the past yielded fabulously preserved fish and reptiles.
The full study, “Extremely rapid, yet noncatastrophic, preservation of the flattened-feathered and 3D dinosaurs of the Early Cretaceous of China,” was published in the journal Proceedings of the National Academy of Sciences and can be found here.
Additional material and interviews provided by Kevin Krajick, Columbia Climate School.
