The Mystery of the Yixian Formation

Sapeornis chaoyangensis from the Yixian Formation

Post Author - Ellis Nolan

With new technologies and techniques constantly being developed, our knowledge of the past is ever-changing. This is particularly true in the case of paleontology: dinosaurs can’t exactly tell us what they were doing when they were fossilized, and as such, scientists are often making the best of an incomplete set of facts. Such is the case with the Yixian Formation in northeastern China, also known as the “Chinese Pompeii.”

The Yixian Formation is a geological formation in China's Liaoning province. It is estimated to be between 120 and 130 million years old, and has yielded hundreds of uncommonly well preserved fossil specimens, including skeletons of feathered dinosaurs. The formation has been paramount to numerous paleontological studies, but a question still remains: how are these creatures so well preserved?

Scientists’ initial explanation gave way to the formation’s nickname. It was originally thought that the creatures and critters fossilized in the Yixian Formation perished because of volcanism, buried in ash like those who perished after Mount Vesuvius erupted in 79 CE, hence the “Chinese Pompeii.” Unlike Pompeii, whose citizens’ skeletons posed in expressions that bring to mind Edvard Munch, the fossils of the Yixian Formation bear no such appearance. Such seemingly calm fossil poses were in direct contradiction to a theory of death-by-lava or ash.

Psittacosaurus lujiatunensis and Repenomamus robustus fossilized in the midst of combat

To further compound the confusion, a recent study suggests many of the fossils in the formation originate from an era where there was little volcanic activity. Using a new dating technique called chemical abrasion isotope dilution thermal ionization mass spectroscopy or CA-ID-TIMS, the researchers were able to determine the fossils originated 125.8 million years ago, and were consistently centered around a time period of 93,000 years, too short to coincide with known major volcanic activity.

The researchers have a different hypothesis for how these fossils got to be so well preserved, and that accounts for the lack of volcanic activity at the time. Since many of the fossilized skeletons appear to be in sleeping positions (with one even appearing to be a surprise attack in one animal’s den), scientists now believe burrow collapses might be how these animals perished. Common even in modern day, the researchers’ hypothesis is compounded by other geological features of the Yixian Formation, which suggest the fossils formed during a particularly wet period in the region, making collapses even more common. Furthermore, unusually well-preserved fossils have also been found in regions of similar climate.

We’ll probably never know for certain how these dinosaurs perished, or how they were fossilized so intact. In this case, time, new techniques, and some good old fashioned deductive reasoning hopefully brought scientists closer to understanding the world dinosaurs inhabited. Hopefully time will bring a snappier name for CA-ID-TIMS, too.