When Yale University paleontologist Nicholas Longrich began studying a prehistoric bird called Archaeopteryx, he noticed something unexpected.

For starters, the bird had small feathers on its hind legs. When he began doing a feather-by-feather reconstruction of its wings, working from a fossil in Germany, he found it didn’t look like a modern bird at all. Instead of the single layer of feathers that give modern birds their dextrous flight abilities, the fossil appeared to have layers of feathers stacked on top of each other, almost like two-ply tissues.