(USA TODAY) In northeast China lies one of the world's richest fossil beds, a glorious jumble of birds, dinosaurs, mammals, lizards and other fauna from more than 100 million years ago. Now new data suggest a gruesome origin for the exquisitely preserved remains: They are an animal Pompeii, cooked and preserved by avalanches of superheated volcanic gas and debris.
The trove of fossils, known collectively as the Jehol Biota, was first excavated more than a century ago. But only in the last few decades have researchers revealed its full splendors. Layers of rock have yielded finely detailed fossils displaying delicate feathers, coloration, even stomach contents. The wealth of feathered dinosaurs, early birds and primitive mammals, all roughly 120 million to 130 million years old, have been crucial to understanding life during the Early Cretaceous.
But the fossils also raise their own questions: Why did so many creatures die at once, and why did so many different kinds of animals all end up buried in a series of ancient lakes? To find out, researchers examined fossils from five locations scattered across the Jehol formations, which sprawl over an area some 180 miles long and 60 miles wide.
The sediments containing fossils seem to have come from pyroclastic flows, terrifying rivers of volcanic gas and bits of rock that usually travel at 50 mph or faster, incinerating almost every living thing in their path. When Vesuvius erupted in 79 A.D., pyroclastic flows of more than 900 degrees instantly vaporized the flesh of the residents of the nearby town of Herculaneum, while the people of Pompeii perished in flows of 500 degrees.
The Jehol fossils also show signs of having been roasted in a pyroclastic flow. Remains examined by the scientists have a fine, dark coating that is probably carbonized skin and muscle, and bones bear the same cracks and charred surfaces seen in skeletons from Pompeii, the researchers report in this week'sNature Communications.
What's more, many animals found in the Jehol formations were found with bent limbs, a pose typical of pyroclastic-flow victims, the study's authors contend. Death by volcanic avalanche would even explain the orientation of the fossils, the researchers say. The volcanic eruptions both killed and preserved, encasing the animals in a protective mold of hot, dry ash.
"All the evidence supports this hypothesis," says study co-author Jin Meng, a paleontologist at the American Museum of Natural History in New York. "Many or most of the vertebrate fossils were preserved this way," he says, though other animals probably died and ended up in the lake without being hit by the volcanic flows.
The findings are "intriguing" while also raising many questions, says University College Dublin geologist Patrick Orr, a specialist in the preservation of fossils. Orr praises the scrupulous observations in the paper, but he also wonders how animal skeletons could have stayed together so nicely when swept away by pyroclastic flows.
"I'm not saying the authors are incorrect," he says, "but there are other plausible mechanisms (for such fossil preservation) that have been reported elsewhere. ... Like all good, provocative papers, this challenges existing ideas and sets up a whole new set of questions."
Meng replies that not all the fossils at Jehol are intact skeletons, but he agrees that his team needs to examine more fossils.
"I don't think that this is the end of the whole thing," he says. "I think this is just the beginning."
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