Are Birds the Closest Living Relative to Dinosaurs?

The classification of living and extinct animals in itself aims to place the biodiversity around us in a comprehensible framework: we humans like to see things organised. In doing so, biologists and palaeontologists always try to truthfully capture the evolutionary interrelations of lifeforms and update this framework when new information becomes available. This was traditionally achieved by comparing as many morphological traits of known and newly described animals as possible under the assumption that anatomical similarity always reflects underlying evolutionary relationships. Although this works reasonably well, some quite similar animals may look alike because of comparable ecological roles rather than because of close family relationships. Because of this so-called convergent evolution, sharks and dolphins superficially resemble one another, although cartilaginous fishes and secondary aquatic mammals are only remotely related. On the other hand, two very differently looking animals may actually be closely related or even represent the same species. The males and females of some bird species look completely different, and the lion and jaguar are actually sister species within the same genus, underlining their close evolutionary relationship despite their quite different appearances. Biologists now better understand exactly which morphological traits more reliably reflect heritage, and can even draw from comparison of DNA and protein sequences to fingerprint species and calculate when two lineages separated from each other.

Crocodiles superficially resemble lizards such as monitor lizards and iguanas, but actually belong to a different group altogether. Their lung structure, for example, much more closely resembles that of birds than those of their cold-blooded lookalikes. A detailed comparison of their DNA with those of other animals confirms that there is indeed more than meets the eye: crocodiles are actually more closely related to birds than to lizards. Crocodiles and birds are the only living representatives of a larger group named Archosauria, or “ruling lizards”, which also includes the extinct pterosaurs and dinosaurs. Birds actually still have scales that recall their “reptilian” ancestry, although they are now confined to the lower legs and feet.

The wrist of raptorial dinosaurs contains a halve-moon-shaped bone that allowed their hands hand to move sideways rather than engage in the “human-like” up-and-down movement that raptors often erroneously feature in movies. This exact movement is what allows birds to fold their wings over the back when they are not used. The wrist of a bird is made up of bones that have fused to strengthen the wing for flight, but this original halve-moon-shaped bone can still be clearly recognised as part of one of their wrist bones. Furthermore, birds often have small humps on one of their arm bones that evolved to attach the long feathers. Very comparable humps have been identified on corresponding arm bones of some extinct dinosaurs, but are absent in crocodiles. These two features have been demonstrated to truthfully reflect a closer relationships between dinosaurs and birds than between dinosaurs and crocodiles.

The final piece of the puzzle again requires understanding of the way biologists classify different lifeforms and, more specifically, how we delimit groups. Progressive insight has revealed that many groups described quite some time ago actually represent “non-natural” groups. This means that such groups contain animals that may not be closely related after all (such as “pachyderms”, now known to be convergent in their large stature and thick skin rather than close relatives) or groups that were supposed to encompass an evolutionary lineage but were later found to exclude certain members (mammals, for example, evolved from reptiles but are themselves not considered reptiles). This latter scenario also applies to birds. Although it gradually became clear that birds evolved from small feathered dinosaurs, they nevertheless initially remained excluded from dinosaurs. However, because we now increasingly rely on definitions of evolutionary groups as “the last common ancestor of species X and species Y, and all of its descendants”, all species that originated in one specific group, such as birds within dinosaurs, will have to be considered members of that group. This means that birds are therefore not only the closest living relatives to dinosaurs but are, by definition, dinosaurs themselves!