One of the most remarkable fossil beds of Cretaceous creatures, formed approximately 125 million years ago in present-day northeastern China, has long been thought to have been rapidly buried by catastrophic volcanic flows of hot ash and rock. However, a study published on November 3 in the Proceedings of the National Academy of Sciences challenges this 'China’s Cretaceous Pompeii' scenario. Instead, the researchers propose that the diverse ancient community, including nonavian dinosaurs, birds, mammals, insects, frogs, and turtles, met their end through a series of unfortunate, yet non-catastrophic, events.
The Yixian Formation, renowned for its two types of fossils—perfectly articulated skeletons preserved in 3-D relief and flattened fossils with exquisitely preserved details such as feathers, pigments, soft tissues, and stomach contents—played a crucial role in convincing paleontologists of modern birds' evolution from theropod dinosaurs. The new study employed a precise geochemical dating technique on tiny zircon minerals from the fossil-containing rocks and two dinosaur fossils originally from the site but now housed in a museum. The results revealed that the two Yixian fossil beds are separated by only 93,000 years, a geological short span.
The study found that the two types of fossils did not form simultaneously, suggesting that a single catastrophic event did not cause the deaths of all creatures. Core samples from the Yixian Formation indicated that the 3-D fossils are older, lying beneath the layer containing the flattened fossils, with a layer of hardened lava in between. This led the researchers to hypothesize that the Yixian Formation represents a brief snapshot of normal life and death in an Early Cretaceous continental community, rather than a dramatic mass death.
The 3-D fossils include skeletons of Psittacosaurus and other dinosaurs, often found in nests. The lifelike poses of these fossils, reminiscent of human remains entombed in scalding ash at Pompeii, and the presence of volcanic debris in the rocks, initially suggested pyroclastic flows as the burial mechanism. However, the new study proposes that Psittacosaurus were burrow-dwellers and were buried when their burrows collapsed. The finer-grained sediments around and within the fossils suggest voids formed by the decaying bodies of the dinosaurs, which were later filled with smaller sediment grains.
While the 3-D fossils formed in a terrestrial setting, the rocks around the flattened fossils indicate burial in deep, fine-grained lake sediments. Earth’s orbital variations suggest that the period of these dinosaur deaths coincided with intervals of heavy rain, leading to swift burial by thick layers of sediment in a low-oxygen environment ideal for fossil preservation. The careful preservation of details like feathers contradicts extreme heat conditions from volcanic flows.
Not all researchers are convinced. Baoyu Jiang, a paleontologist at Nanjing University, argues that the study’s conclusion of an extremely high sedimentation rate does not exclude volcanism as the main cause. He also points out that the study analyzed only two specimens, insufficient to draw definitive conclusions.
MacLennan and colleagues counter that it is a logical fallacy to assume a remarkable bonebed must have a remarkable origin. The volcanic debris at the site may have misled researchers about the true cause of the deaths.
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