Tales of Evolution: When dinosaurs took flight

In part 2 of this series we looked at the evolution of early reptiles, and the group which gave rise to mammals. Here we look at a different group of reptiles – the dinosaurs – and how one particular group of them gave rise to birds...

The early reptiles were split into two groups: the synapsids and the sauropsids. The synapsids gave rise to the mammals, whereas the sauropsids gave rise to all modern reptiles and birds. Dinosaurs began to evolve from their sauropsid ancestors around 230 million years ago (mya), and quickly became the dominant animals on land. Although there is some debate over exactly what the dinosaur family tree looks like, we know that birds evolved from a group called the theropods. Many theropod fossils have been found which have some dinosaur-features, and some bird-like features. Archaeopteryx is one example. It lived about 150 mya, and had feathered wings like a bird – but also had teeth and a long bony tail, like dinosaurs. So how exactly did birds evolve from dinosaurs? How did all the complexities of wings and flight evolve? The answer, as is almost always the case in evolution, is that it was a very long and gradual process.

Theropod origins

The theropods, which included Tyrannosaurus rex and Velociraptor amongst others, were almost all bipedal - meaning they walked on two legs rather than on all fours. As a result of this, their arms were able to be used for purposes other than support. In the case of theropods like T. rex, the arms were probably not very useful, and became smaller over time. But in the case of other theropods, including the ancestors of birds, the arms were adapted for entirely new functions as they evolved into wings. Bird wings differ from the arms of most tetrapods (land vertebrates) in two main ways. Firstly, bird wings only have three digits (fingers), whereas most other tetrapod arms – including ours – have five digits. In birds, the first digit is very small, and the second and third digits are much longer and are fused together. Secondly, bird wings possess unique, highly specialised flight structures - feathers. The flight feathers of the wing are anchored to the digits and to the forearm bones by ligaments. Whereas other flying animals like bats (and the extinct pterosaurs) have skin stretching from the arms to the body to form the wing, birds do not – the wing is entirely composed of feathers which project from the skin.

Wings.png

^{Wings of bats, pterosaurs and birds: In bats and the extinct pterosaurs, the wing is formed by skin which stretches from the digits to the body. In birds, however, the wing is formed purely by feathers which project from the skin. Bat wings have five digits, four of which are elongated. Pterosaur wings have four digits, of which only the fourth is greatly elongated. Birds have three digits, of which the second and third digits are elongated and fused together. Image redrawn from reference.}

Wings and feathers

So how did these unique characteristics of bird wings evolve? The early theropods, which lived around 230 mya, had five digits on each hand. However, over time the fourth and fifth digits became smaller and smaller, and by the time birds evolved (around 120 mya), they had been completely gotten rid of altogether. It’s not entirely clear why theropods got rid of these digits, but presumably they were simply no longer necessary.

Whilst it seems straightforward for digits to be lost in evolution, it seems much more difficult for feathers to evolve. Flight feathers are highly complex structures, containing a central, hollow shaft (‘rachis’), and many ‘barbs’ projecting from the rachis. These barbs also have their own little projections called ‘barbules’ which cross-link the barbs, allowing the feathers to maintain their rigid structure, which is essential for flight. How could such complex structures evolve? Well, birds contain many types of feathers, and some of these are far simpler. As well as the complex flight feathers, there is another kind, called ‘down’ feathers. These are the fluffy feathers which cover chicks, and the belly and thighs of adult birds – in both cases to provide insulation, rather than flight capabilities. Down feathers differ from flight feathers in that their barbules don’t cross-link with each other. This means the barbs are free to move around, and the feather has a much less rigid structure. Even simpler are the ‘bristles’ found around the beak and eyes of many birds. These bristles, which are very similar in appearance and function to the whiskers of mammals, are each composed of a rachis with very few, small barbs projecting from it. It’s reasonable to suggest that bristles – simple hollow shafts – appeared first for the purpose of sensing movement. Then later in evolution, these shafts developed more barbs, which would help provide insulation, and these down feathers then spread to other parts of the body. Then, only later did cross-linking barbules evolve on the wing feathers, which would help the wings remain rigid and strong. These feathers may have originally evolved for functions other than flight, such as for mating or aggression displays, or even to shelter their young. Only later would they have been used for flight. Although feathers are usually not preserved in fossils, the fossil data we do have seems to support this progression. Simpler down feathers are found in many theropod groups – including the group which T. rex belonged to! The more complex flight feathers are also found in several theropod groups, including the group which Velociraptor belonged to – the dromaeosaurids. One fossil dromaeosaurid even had flight feathers on both its wings and its legs, suggesting that it could have used both its wings and its legs to fly – or at least glide.

Learning to fly

But what about flight itself? How did the first feathery dinosaurs take to the skies? It’s likely that wings were used for different things before they could be used for proper flight. As the ancestors of birds would have been quite small, it’s possible that wings could have been used to help them climb up into the branches of trees to escape larger predators. Chicks of the chukar partridge aren’t able to fly, but they can flap their wings as they run up steep surfaces (like tree trunks) to help them climb what would otherwise too steep to climb. Perhaps the ancestors of birds evolved a similar strategy. Once in the trees they could get back to the ground using their wings, either by gliding down, or by flapping to slow their fall - as some chicks do when they leave the nest for the first time. Over time, these behaviours (and the wings themselves) evolved to allow proper flight. 

Around 65 mya there was a mass extinction event which wiped out all dinosaurs – except one group: birds. It’s not clear why these small, feathery dinosaurs survived when the rest did not. Some people have suggested that the ability to fly may have been the factor – but pterosaurs were also wiped out in the extinction event. Whatever the reason, birds survived, and over the last 65 million years have diversified and spread all over the world. With around ten thousand species, they are the most species-rich group of tetrapods alive today. Arguably, dinosaurs never really lost their dominance.