Photos show the typical entombing poses of the Jehol terrestrial vertebrate fossils (a: Psittacosaurus; b-c: Confuciusornis). This boxer-like pose is typical of victims of pyroclastic density currents, resulting from postmortem tendons and muscles shortening.
Some of the finest fossils of the early Cretaceous were victims of Pompeii-like death by torrential rain of hot ash and deadly volcanic gases, according to a new study by American and Chinese researchers.
Fossils of early mammals, dinosaurs, pterosaurs, lizards, salamanders, birds, turtles, frogs, fishes and other freshwater and land animals have been found so well preserved in the rocks of the Yixian and Jiufotang rocks of southeastern Mongolia that even muscles, skin and scales are sometimes discernible after 130 million years.
The secret to that detailed preservation of what is called the Jehol Biota is that the animals were literally baked to a crisp by volcanic eruptions as they were buried.
"Fresh, hot, dry, acid volcanic ash promoted burning, charring or mummifying of soft tissues, which, as a result, became more resistant to decay and better preserved," explains Baoyu Jiang of Nanjing University who along with colleagues published their discovery in the Feb. 4 issue of the journal Nature Communication.
"The nature of the events and the mechanisms behind the exceptional preservation of the fossils, however, are poorly understood."
A closer look at the rocks themselves, along with the massive collection of animals that appear to have died at once, points to a sudden eruption of the kind that sends a tower of hot ash into the sky, which then collapses and creates a murderous landslide of volcanic ash that burns as it buries everything in its path.
Even the poses of the animals match what would be expected if they were killed suddenly, like the unfortunate Pompeii residents who died in the Mount Vesuvius eruption in 79 A.D.
"These Early Cretaceous strata in northeast China are riddled with volcanic debris, so the presence of some mass death assemblages of fossils in pyroclastic flows comes as no surprise," said paleontologist Spencer Lucas of the New Mexico Museum of Natural History.
"Sudden death and rapid burial by volcaniclastic processes certainly accounts for some of the exceptional preservation seen in the Early Cretaceous fossil assemblages in NE China, which represent one of the most remarkable records of life on land during that interval of geologic time."
The burnt, charred or mummified tissues probably served as molds for the extremely fine-grained ashes that coated them, Jiang reports. These formed the flattened body outlines in the same way as in the famous Burgess Shale of Canada.
Unlike that shale, however, volcanic eruptions are sudden and kill a lot of things as once -- thereby providing a remarkable snapshot of life at that time in the history of the Earth.