Mark Puttick and colleagues investigated the rates of evolution of the two key characteristics that preceded flight: body size and forelimb length. In order to fly, hulking meat-eating dinosaurs had to shrink in size and grow much longer arms to support their feathered wings.
"We were really surprised to discover that the key size shifts happened at the same time, at the origin of Paraves," said Mr Puttick of Bristol's School of Earth Sciences. "This was at least 20 million years before the first bird, the famous Archaeopteryx, and it shows that flight in birds arose through several evolutionary steps."
Being small and light is important for a flyer, and it now seems a whole group of dozens of little dinosaurs were lightweight and had wings of one sort or another. Most were gliders or parachutists, spreading their feathered wings, but not flapping them.
'High rates of evolution preceded the origin of birds' by Puttick, M.N., Thomas, G.H., and Benton, M.J. in Evolution: DOI: 10.1111/evo.12363
Notice that the changes appear for the first time at the origin of Paraves (not earlier).
There are basal coelurosaur dinosaur fossils and there are basal feathered flying Paraves fossils.
The basal coelurosaur fossils show all the characteristics of basal coelurosaur dinosaurs.
The basal Paraves fossils show all the characteristics of basal feathered flying Paraves.
And nothing in between.