Evolution of Bird Flight Linked to Dinosaurs

Evolution of flight among vertebrates has always been mystifying to researchers. Though this facet evolved some million years ago, understanding the mechanism has been challenging.

However, according to the evidence traced by academics at the Universities of Bristol, Yale and Calgary, who studied the structure of feathers in bird-like dinosaurs, the prehistoric birds had a much more primitive version of the wings we see today. They had rigid layers of feathers acting as simple airfoils for gliding.

The researchers studied the earliest theropod dinosaurs. It was recently revealed that birds are actually the descendants of small flightless theropods. Despite the group's name, which means 'beast foot,' theropod feet usually resembled those of birds. Among the features linking theropod dinosaurs to birds are the three-toed foot, a furcula (wishbone), hollow bones, brooding of the eggs and (in some cases) feathers. On studying the theropod dinosaur it was found that feathers that were arranged in multiple layers in order to preserve heat were initially developed for insulation, before their shape evolved for display and camouflage.

As evolution occurred, the pattern of the feathers also changed. What became more evident was their key role in aerodynamics and mechanics of flight.

Natural selection over millions of years ultimately modified dinosaurs' forelimbs into highly-efficient, feathered wings that could rapidly change their span, shape and area, a key innovation that allowed dinosaurs to rule the skies.

For 130 years the pattern of the wings never altered, with bird wings having a layer of long, asymmetrical flight feathers with short covert feathers on top. They were able to separate and rotate these flight feathers to gain height, change direction and even hover.

This permitted the bird to travel in a manner so to produce both lift and thrust simultaneously.

This study focused on Anchiornis huxleyi and Jurassic bird Archaeopteryx Lithographica. The latter is 155-million-years old and widely considered to be the earliest known bird, presenting a combination of dinosaur and bird characteristics. Their wings differed from modern day birds in being composed of multiple layers of long feathers, appearing to represent early experiments in the evolution of the wing. Although individual feathers were relatively weak due to slender feather shafts, the layering of these wing feathers is likely to have produced a strong airfoil.

The inability to separate feathers indicate taking off and flying at low speeds may have been limited. Meaning the wings were primarily used in high speed gliding or flapping flight.

Dr Jakob Vinther of University of Bristol's Schools of Biological and Earth Sciences, said, "We are starting to get an intricate picture of how feathers and birds evolved from within the dinosaurs. We now seem to see that feathers evolved initially for insulation. Later in evolution, more complex vaned or pinnate feathers evolved for display.

"These display feathers turned out to be excellent membranes that could have been utilized for aerial locomotion, which only very late in bird evolution became what we consider flapping flight. This new research is shedding light not just on how birds came to fly, but more specifically on how feathers came to be the way they are today -- one of the most amazing and highly specialized structures in nature," explained Dr Nicholas Longrich of Yale University.

He further said, "By studying fossils carefully, we are now able to start piecing together how the wings evolved. Before, it seemed that we had more or less modern wings from the Jurassic onwards. Now it's clear that early birds were more primitive and represented transitional forms linking birds to dinosaurs. We can see the wing slowly becoming more advanced as we move from Anchiornis, to Archaeopteryx to later birds."