It's in the article, along with photographs and CT scans: "We can be sure of the source because the vertebrae are not fused into a rod or pygostyle as in moder

It's in the article, along with photographs and CT scans: "We can be sure of the source because the vertebrae are not fused into a rod or pygostyle as in modern birds and their closest relatives," he explained. "Instead, the tail is long and flexible, with keels of feathers running down each side." ... The feathers lack the well-developed central shaft - a rachis - known from modern birds. Their structure suggests that the two finest tiers of branching in modern feathers, known as barbs and barbules, arose before the rachis formed.