"Xu has had a bounty of fossils to work on, particularly from Liaoning. The creatures unearthed there are remarkably well preserved, perhaps because they were entombed quickly during volcanic eruptions and mudslides between 160 million and 120 million years ago. The rocks record fine details including the imprints of feathers, which allowed Xu to determine that a fierce 9-metre-long tyrannosaurid, which he named Yutyrannus, had a coat of long feathers"
http://www.nature.com/news/china-s-dinosaur-hunter-the-groun...
"Fossils of big animals are rare—and fossils of small organisms are rarer still, because small, delicate bodies decompose more readily than larger ones. However, in Liaoning even small animals and insects were well preserved.
This is because when plants and animals died there, they often washed into one of the many streams and lakes in the region. More importantly, they were quickly covered in volcanic ash soon after death. Because the remains were buried so rapidly, many of the Liaoning fossils preserve much of the entire organism in remarkable detail."
http://img.washingtonpost.com/news/morning-mix/wp-content/up...