We can stop looking for the missing link in the dinosaur-bird transition now because it was seamless. Birds are a continuum of millions of years of evolution, and there was no great jump between non-birds to birds.

Researchers assembling the most comprehensive family tree of two-legged, meat-eating dinosaurs to date reveal that familiar bird features – like feathers, wings, wishbones, hollow bones, and bills -- evolved over millions and millions of years, slowly accumulating small shifts in shape and function. But once these pieces were in place, an evolutionary explosion began. The gradual assembly of the bird body plan as we now know it, culminated in rapid rates of evolution. The work was published in Current Biology this week.

“There was no moment in time when a dinosaur became a bird, and there is no single missing link between them,” says University of Edinburgh’s Steve Brusatte in a university statement. “What we think of as the classic bird skeleton was pieced together gradually over tens of millions of years. Once it came together fully, it unlocked great evolutionary potential that allowed birds to evolve at a super-charged rate.”

To examine the evolutionary links, tempo, and pace of the dinosaur-bird transition, Brusatte and colleagues analyzed the anatomical make-up of 853 features (ranging from air sacs to wristbones, or lack thereof) in 152 extinct birds and their closest non-avian dinosaur relatives. Then they used statistical methods to assemble a family tree called a phylogeny (pictured below).

The team found that the emergence of birds 150 million years ago was a gradual process that began pretty much when dinosaurs appeared on Earth around 230 million years ago. With some dinosaurs becoming evermore “birdy” over time, it’s very difficult to draw a dividing line between them. “This process was so gradual that if you traveled back in time to the Jurassic, you’d find that the earliest birds looked indistinguishable from many other dinosaurs,” study coauthor Steve Wang of Swarthmore says in a news release. When all these pieces forming the archetypal bird skeleton were in place, “birds then evolved rapidly, eventually leading to the great diversity of species we know today.”

The famous early bird, 150-million-year-old Archaeopteryx, was thought to represent a great evolutionary leap from dinosaurs, National Geographic explains, but these findings show that its avian traits evolved in dinosaur forebears long before. This 80-million-year transition was capped by unusually elevated bursts of evolution.

“It is particularly cool that it is evidence from the fossil record that shows how an oddball offshoot of the dinosaurs paved the way for the spectacular variety of bird species we see today,” says study coauthor Graeme Lloyd from Oxford. Here’s a simplified version of their phylogenetic relationships:

Images: Jason Brougham (top), Steve Brusatte (bottom)