The relationship between birds and dinosaurs

Birds are a group of vertebrate animals that are supported by their hind limbs and whose fore limbs are adapted to flight (although some species have lost the ability to fly, and others such as penguins have adapted their limbs to swimming)  

Thanks to the evidence found in the fossil record, we now know that birds evolved from a group of reptiles called archosaurs, and that they are related to the extinct dinosaurs . In this article we will explain what the process of evolution of birds consisted of and what characteristics they shared with dinosaurs.

Background to the evolutionary study of birds

The idea that birds were related to dinosaurs arose in the 19th century, shortly after the publication of Charles Darwin’s book The Origin of Species. At that time the first fossils of  Archeopteryx were found , an extinct genus of primitive birds that shared characteristics with modern birds and other dinosaur fossils. 

One of the most unique characteristics of these animals is that their bodies were covered by feathers, as can be seen in the fossils found. On the other hand, their forelimbs were crowned with claws and they had a long bony tail, similar to reptiles.

At first it was believed that this was the missing link that alone explained the evolutionary process of birds , but as new fossils of feathered dinosaurs appeared, the evolutionary relevance of  Archeopteryx declined. On the other hand, these fossils gave important support to Darwin’s theory of evolution.

Shared characteristics between birds and dinosaurs

One of the distinctive characteristics of birds is the presence of feathers on the body . The feathers are composed of keratin and are made up of a central rod, called the rachis, and some filaments that come out from the central part in a symmetrical way, called the feathery barbs. The base of the feather is broadened and hollow, it is called a calamus, and between the base and the feathery barbs there may be another type of filament called feathery barbs, with a cottony appearance.

Appearance of the ability to fly

These types of feathers have been found in various species of dinosaurs , both flying and terrestrial. It is believed that their original function was the regulation of body temperature, since they have an insulating function, and that the ability of flight appeared later.

There are two theories in this regard, one according to which the first birds would have taken off by taking a run from the ground, and another that maintains that they were arboreal species and that they used them to glide among the trees, subsequently acquiring the ability to fly.

However, in the fossil record some species of dinosaurs have appeared that could have lost the ability to fly once acquired , and that would have preserved the feathers in a vestigial way. Therefore, it is not clear that the presence of feathers is indicative of a unidirectional evolutionary process between reptiles and birds.

On the other hand, bones are a clear indicator of biological adaptation to various functions. In the case of birds, many of their bones are hollow to have less weight and facilitate flight. Some structures such as the clavicles or the keel, an extension of the sternum where the muscles related to flight are anchored, are good examples of this.

Modern X-ray analysis techniques also make it possible to understand the structure and arrangement of internal organs in fossils. Thanks to this we know that some theropod dinosaurs had air sacs similar to those of birds . These are hollow spaces within the body that contain air, which is renewed by breathing. These types of structures have been found in the lumbar and cervical vertebrae of various species of dinosaurs. 

Its function could be to improve respiratory capacity , favoring a high metabolic rate, or to allow body cooling through breathing , to control the excess thermal insulation produced by the feathers. It has also been suggested that these structures could have arisen in response to a lower level of oxygen in the Atmosphere during the Triassic period.

Another characteristic shared by birds and dinosaurs is the presence of stones in the gizzard. The gizzard is a part of the stomach present in some groups of vertebrates whose function is to grind food before digesting it. Some animals, such as birds, ingest stones to facilitate this process. When these stones are found inside a fossil, they are called gastroliths . 

Curiously, the only existing reptiles that possess this characteristic are crocodiles, a group related to birds and which, together with them, are the only archosaurs that have survived to this day. It would, therefore, be the closest living relatives to the dinosaurs.