When asking any paleontologist closely involved in the field of phylogeny, they may not hesitate to say that birds are dinosaurs. Entering into a more confusing debate, it will surely also clarify that birds are “avian dinosaurs” and, in turn, reptiles .
Despite getting lost in semantic nuances and over-complicating things with these types of issues, terminological and phylogenetic reflections in the world of evolution are necessary: Rome was not made in a day, and human knowledge requires an exhaustive analysis and the testing of theories and multiple points of view to cement its foundations.
Therefore, in this space we are going to focus on the “scientific consensus” based on the matter that concerns us today, but we must be clear that none of the above is an immovable iron dogma. For example, some specialists continue to argue that birds evolved from crocodilomorphs, a theory that is difficult to justify, but one that exists.
It is because of this disparity of opinions that nothing is entirely real or absolutely false. We are operating in highly speculative terrain, which in my personal opinion is quite stimulating. In the world of science, black and white colors are obsolete: the answer is usually found in a gray scale. Well, are birds dinosaurs or not? Do not despair, because here we expose the possible answers.
Are birds dinosaurs? A phylogenetic conglomerate
Phylogeny is a term that refers to a scientific discipline in charge of studying the kinship relationships between species or taxa in general . To reconstruct the evolutionary history of a taxonomic group, it is not enough to look at the external characteristics of the animal and place the families as far or as close as we want from the ancestor: strict genetic analyzes and mathematical criteria must be followed.
Certain computer programs based on the application of algorithms build the phylogenetic trees with the genetic evidence provided from the different organisms. It is important to note that this reconstruction is based on the famous Ockham razor or parsimony principle. That is, the most likely answer lies in the tree that is built from the fewest possible changes .
For example, when faced with a winged animal analyzed today, two different assumptions could be made:
- That present wings in their ancestral condition, lose them at some point in their evolutionary history and the current descendants recover them again.
- That it did not present wings in its ancestral condition and then appear derived from structures of its primitive ancestor.
We embrace the assumption that evolution occurs in the simplest way , and therefore, unless reliable paleontological evidence is provided, the second option at first glance will always seem more viable.
The complex phylogeny of birds
In addressing the evolutionary history of birds, we have to start by setting the record straight: the current scientific consensus is that their evolutionary divergence most likely began in the Triassic period from a unique clade of theropod dinosaurs, the coelurosaurs. But what do current birds share with these primitive beings?
1. The feather debate
At this point it is essential to introduce perhaps the most key piece of this puzzle: the Archeopteryx fossil found in 1861 . This evolutionary imprint shows us a clearly transitional individual between dinosaur and modern bird: the relationship is undeniable.
From this point on, feathers have been discovered in many other taxonomic groups of dinosaurs. Until relatively recently, this morphological trait had been associated only with theropods (as we have already said, those considered ancestors of modern birds), but a series of fossils dating back 160 million years found in Siberia suggest something different. , as evidence of feathered ornithischian dinosaurs has been found.
According to scientists interviewed by the National Geographic source, “this probably means that the common ancestor of all dinosaurs had feathers.” Following the previously exposed principle of parsimony, it is more coherent to think that the ancestral group had feathers and that some members lost them, than the record of a featherless-feathered-featherless story again (two evolutionary steps with respect to three) .
Still, there is no shortage of detractors who oppose this idea. A small scientific minority argue that the presence of certain “proto-feathers” in fossilized imprints would correspond to the degradation of collagen fibers, and that the species that unquestionably possess feathers, such as oviraptorosaurs and dromaeosaurs are not dinosaurs, but true birds not related to them. This argument is dismantled considerably when some studies have shown the presence of supporting color melanin in these structures, something that is expected from a feather but not from a collagen formation.
The relationships between the skeletal structures of birds and dinosaurs is reliable, and it is an essential reality to take into account when establishing relationships between birds and dinosaurs. More than 100 avian anatomical features have been detected in theropod fossils , among which are similarities in these structures:
- Low density hollow bones.
- Pubis relocated, from an anterior position to a more posterior one.
- Five or more vertebrae incorporated in the sacrum.
- Flexible wrists with lunate carpus.
- Elongated upper extremities.
- Presence of furcula.
The number of general similarities reaches more than 20. Even so, the presence of furcula, the result of fusing the two clavicles, is especially striking, since it is only present in birds and theropod dinosaurs . As has been demonstrated today, this strengthening structure of the rib cage appeared before the flight. Along with the presence of the feathers, the furcula is one of the characteristics that have allowed the most direct correlation between birds and theropods.
3. Lack of DNA
We can continue naming similarities between theropods and birds endlessly: egg laying, pulmonary air sacs, four-chamber hearts, the use of gastroliths (structures that facilitate digestion in the animal’s stomach … etc). Still, an essential clarification is necessary. For now, no reliable record has been obtained of the extraction of DNA from fossil samples .
This greatly complicates the task of reconstructing phylogenetic trees, since any geneticist will affirm that genetic tests should always be the first of the bases when it comes to reconstructing the evolutionary history of any living being. Even so, there is other reliable evidence that is not based only on external skeletal and morphological characters.
For example, traces of tissue have been obtained inside the bones of Tyrannosaurus rex , and after rehydration and analysis, seven different types of collagen were discovered . When compared with those of various birds (especially chicken), their morphological similarity yielded even more evidence of the correlation between theropods and modern birds.
Even so, despite all these morphological and structural similarities, we still lack the definitive key: a concordant DNA that silences all the voices against the facts presented here. Despite this, something is clear from the scientific community, and that is that there is no more accepted theory of the origin of birds today that is not linked to theropods .
We can answer that yes, that birds are dinosaurs and leave behind all the nuances exposed so far. This statement errs on oversimplifying things, but the supposed “debate” between the relationship of theropods and birds is something that is already considered settled in the scientific community. Despite the fact that there are detractors to this idea, the same propositions made by them violate dogmas as accepted in the world of phylogeny today as the principle of parsimony.
Most likely, the birds are direct descendants of the coelurosaurus theropods , since there is no other theory that explains their emergence with fewer evolutionary steps. As simple as that.
Therefore, the current consensus is that birds are a group of maniraptor theropod dinosaurs that originated during the Mesozoic.