Whether or not the world's earliest birds — as they evolved from dinosaurs — could fly or were still earthbound has long puzzled scientists, but a new fossil find sheds light on that puzzle, researchers say.

An analysis of the 125-million-year-old fossilized wing of a primitive bird suggests it could have been capable of flight in a manner similar to modern birds, paleontologists are reporting.

The fossil, discovered in central Spain, shows not only the articulated bones within the wing but also the remains of its feathers and the soft tissue within the wing, they say in a study published in the journal Scientific Reports.

Of particular interest was an intricate network of fibers that are an anatomical match for the arrangement of muscles, ligaments and tendons found in modern birds that helps position and control the wings' main feathers.

It is that positioning and control that allows modern-day birds to fly, and to do so efficiently, the researchers point out.

Finding similar structures in an early, primitive bird suggests it was capable of aerodynamic feats similar to those performed by modern birds, says University of Bristol doctoral student Guillermo Navalón, lead author on the study.

"It's very surprising that despite being skeletally quite different from their modern counterparts, these primitive birds show striking similarities in their soft anatomy," he says.

The evolution of birds from dinosaurs occurred around 150 million years ago but happened slowly, with features such as wings covered in feathers taking many millions of years to develop, the researchers note.

While the fossil from Spain strengthens the argument that even some of the earliest birds could fly, it still remains unknown just how accomplished they may have been in the sky, whether they could fly high over the heads of their dinosaur relatives or could only manage short flights close to the ground.

"The new fossil provides us with a unique glimpse into the anatomy of the wing of the birds that lived amongst some of the largest dinosaurs," says study co-author Luis Chiappe at the National History Museum of Los Angeles County. "Fossils such as this are allowing scientists to dissect the most intricate aspects of the early evolution of the flight of birds."