150 million years ago, dinosaurs roamed the earth. They were more abundant, stronger and prominent than any other animal, but after the fatal meteorite struck, they disappeared. However, during the long period they walked around they gave rise to another animal group, that we all know and see every day. The mighty dinosaurs gave birth to the birds.
Text: Flore Van Maldeghem
The modern birds as we know them are the descendants of a group of two-legged dinosaurs, called the Theropoda. The famous Tyrannosaurus rex and velociraptors are probably the best known species in this group. So how did these enormous creatures evolve into pigeons?
Covered with feathers
For a very long time, the only known fossil link between birds and dinosaurs was Archaeopteryx, a creature with feathered wings but the teeth and tail of a dinosaur. This fascinating animal appeared in a time span of barely 10 million years and had almost all the characteristics of modern birds. Of course the dinosaur didn’t turn into a bird overnight. In fact, it was a seamless transition in which the classic features of birds evolved one by one. Prove for this was found in the 1990’s with the discovery of new dinosaur fossils in China. The animals found here, were covered in feathers, even though they lacked wings. This means that feathers evolved long before the birds developed.
Shrinking to a flying size
When we imagine a T. rex, we think of a huge, monstrous animal and when we picture a bird, we see a little flying creature. The size was obviously a problem. Almost 200 million years ago (50 million years before Archaeopteryx appeared), the coelurosaurs, the group birds evolved from, started shrinking, while most other dinosaur lineages grew larger. At a certain point this shrinking process accelerated. They shrank 160 times faster than other dinosaurs were growing. This suggests that the smaller animals had an advantage over larger ones (new habitats, new ways of life). Whatever the reason was, this small stature was probably a useful precursor to flight.
Changing the skull
After studying alligator eggs, it was discovered that the head of the embryos inside the eggs looked quite similar to birds. To test this, fossilized eggs with embryos of dinosaurs were examined and they showed the same similarities to birds. This leads to the hypothesis of birds evolving from dinosaurs by stopping the development early in life. So birds resemble tiny, infantile dinosaurs. This process is known as paedomorphosis and is an efficient evolutionary route. It was important in the evolution because it might have helped drive the miniaturization or vice versa. It also helped to make the skull a blank slate on which selection could create new structures.
Appearance of the beak
Another interesting characteristic of the bird is its beak. It uses it to find food, clean himself, make a nest and so on. In modern beaks, two bones (the premaxillary bones) fuse to form the beak, while in dinosaurs, ancient birds and most other vertebrates, these remain separate. To understand how this change occurred, two genes expressed in these bones were mapped out. They found that reptiles and mammals had two patches of activity, on the sides of the nasal cavity, while birds have a much larger single patch spanning the front of the face. When they undid the bird-specific pattern in chicken embryos they saw that these embryos developed a more dinosaur-like face. This is a good example on how simple molecular tweaks can trigger major structural changes.
The evolution of dinosaurs to birds is not something that happened overnight. It was a long, slow transitional process, but as soon as the first birds appeared, a burst of evolution followed. They had a very successful new body plan and type of ecology which led to an evolutionary explosion. So next time when you’re at a quiz and they ask what is most closely related to a pigeon: a T. rex or a monkey, you know the answer.
Reference: Singer E., ‘How dinosaurs shrank and became birds’, Quanta magazine, June 2 2015