One hundred and fifty million years ago, in what is now China, a creature lived and died in the space between two worlds — neither fully dinosaur nor fully bird — and the stone that swallowed it has now returned a rare gift to science. Researchers have identified a Jurassic fossil that documents, in bone, the gradual shortening of the tail that would eventually become the pygostyle: the fused vertebral anchor that gives modern birds their capacity for flight. Where the fossil record once offered only endpoints — the long-tailed dinosaur on one side, the compact-tailed bird on the other — this