How birds got their wings: Part I
When you think of flight, you think of planes, birds. Maybe you even think of bats or insects, but I’m prepared to bet dinosaurs aren’t the first thing you thought of… The story of how birds got their wings goes back a long way. So long in fact that I have decided to break this blog series into 3 posts.
Birds are marvellously well adapted to flying and believe it or not, many of these adaptations are relics of their reptilian past. Not all though; by the time birds could fly they had moved away from the Dinosauria clade and the way they did this was through subsequent adaptations that I will explore in part II of this mini series. Finally, I will look into the evolutionary theories for flight, in other words, the reason birds left the ground in the first place. But for now, let’s take a look at birds’ shared adaptations with dinosaurs!
Fossils from the late Triassic archosaurs and more particularly the clade Avemetatarsalia, show evidence for hollow bones. Avemetatarsalia is one of the main branches of archosaurs and includes birds, dinosaurs and pterosaurs. Although pneumatic bones were present in some Avemetatarsalia, it is believed that this is a trait that evolved multiple times independently amongst this clade. The purpose of this in early birds was obviously to make flying more achievable and much the same can be said for pterosaurs (flying reptiles). As for the flightless dinosaurs such as the massive sauropods, pneumatization might have evolved to save energy as these animals moved about.
Flight is very metabolically costly, as one might expect. An efficient breathing system is needed! Unlike mammals, birds, and dinosaurs for that matter, do not waste oxygen. Their secret; a unidirectional pulmonary system. This consists of a one way flow of air, where lungs retain a constant volume, quite different to human lungs that inflate and deflate, (a pretty poor show in comparison). The genius of this pulmonary system lies in the two cycles occurring simultaneously. Around the lungs, are 9 air sacs that function like bellows. During inhalation, fresh air enters through the trachea and fills the lungs and posterior sacs. At the same time, stale air from the lungs is pushed into the anterior sacs. During exhalation, air from the posterior sacs fills the lungs while the air in the anterior sacs is now pushed out through the trachea. Without this mechanism, the muscles involved in flight would not be supplied enough oxygen to make something as energetic as flight possible.
Feathers are also an archaic feature and by now you will probably not be surprised to hear that many dinosaurs also had them. In fact, all theropods (the dinosaur group birds evolved from) are believed to have had feathers of some sort, although these were probably just tufts or “dinofuzz” in paleontologist jargon. It is thought that feathers first evolved to keep dinosaurs warm. It was previously thought that these evolved directly from reptilian scales. This hypothesis was proved to be incorrect however and we now know that feathers are an extension of the dermal papilla (which is the base of the hair follicle on humans).
These adaptations were still not enough to enable a bird to fly; birds needed more than what dinosaurs had! In my next post, I will be delving deeper in what subsequent adaptations pathed the road to flight…
