Are birds dinosaurs? The keys to their classification as animals

Can birds be considered dinosaurs that survived extinction? Let's see.

When you ask any paleontologist closely linked to the field of phylogeny, he or she may not hesitate to affirm that birds are dinosaurs. Entering into a more confusing debate, he or she will probably also qualify that birds are that birds are "avian dinosaurs" and, in turn, reptiles..

Despite getting lost in semantic nuances and over-complicating things with this kind of topics, 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" on the subject that concerns us today, but we must be clear that none of the above is an unshakable iron dogma. For example, some specialists continue to argue that birds evolved from crocodilomorphs, a theory that is difficult to justify, but which exists.

It is because of this disparity of opinions that nothing is entirely true or absolutely false. We move in a highly speculative terrain, which in my personal opinion, is quite stimulating. In the world of science, the colors black and white are in disuse: the answer is usually found in a scale of grays. Well, are birds dinosaurs or not? Do not despair, because here are the possible answers.

Are birds dinosaurs? A phylogenetic conglomerate

Phylogeny is a term that refers to a scientific discipline in charge of the study of 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 to the ancestor: we must follow strict genetic analyses and apply mathematical criteria.

Certain computer programs based on the application of algorithms construct phylogenetic trees with the genetic evidence provided for the different organisms. It is important to note that this reconstruction is based on the famous Ockham's razor or principle of parsimony. That is, the most probable answer lies in the tree that is constructed from the smallest possible number of changes..

For example, in the case of a winged animal analyzed today, two different assumptions could be made:

• That it presents wings in its ancestral condition, loses them at some point in its evolutionary history and the current descendants recover them again.
• That it did not present wings in its ancestral condition and then appeared derived from structures of its primitive ancestor.

We accept the assumption that evolution happens in the simplest wayTherefore, unless strong paleontological evidence is provided, the second option will always seem more viable at first sight.

The complex phylogeny of birds

In addressing the evolutionary history of birds, we must begin 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 today's birds share with these primitive creatures?

1. The feather debate

At this point it is essential to introduce perhaps the most key piece of this puzzle: the fossil of the Archaeopteryx 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 mentioned above, the considered ancestors of modern birds), but a series of 160-million-year-old fossils found in Siberia suggest otherwise, as evidence of feathered ornithischian dinosaurs has been found.

According to scientists interviewed by National Geographic, "this probably means that the common ancestor of all dinosaurs had feathers". Following the principle of parsimony previously stated, 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 history again (two evolutionary steps with respect to three). and that some members lost them, than the record of a featherless-featherless-featherless history again (two evolutionary steps compared to three).

Still, there is no shortage of detractors who oppose this idea. A small scientific minority argues that the presence of certain "protofeathers" in fossilized imprints would correspond to the degradation of collagen fibers, and that species that unquestionably possess feathers, such as oviraptorosaurs and dromaeosaurs are not dinosaurs, but true birds unrelated to them. This argument is considerably dismantled when some studies have shown the presence of support-colored melanin in these structures, something expected of a feather but not of a collagen formation.

2. Skeleton

The relationship between the skeletal structures of birds and dinosaurs is irrefutable, 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, including similarities between birds and dinosaurs.among which we find similarities in these structures:

• Neck.
• Hollow bones of low density.
• Pubis relocated, from an anterior to a more posterior position.
• Five or more vertebrae incorporated into the sacrum.
• Flexible wrists with semilunar carpus.
• Elongated upper extremities.
• Scapula.
• Presence of furcula.

The number of general similarities reaches more than 20, 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 to date, this strengthening structure of the thoracic cage appeared before flight. Together with the presence of feathers, the furcula is one of the characteristics that have allowed a direct correlation between birds and theropods.

3. The lack of DNA

We could go on naming similarities between theropods and birds endlessly: egg-laying, pulmonary air sacs, four-chambered hearts, the use of gastroliths (structures that facilitate digestion in the animal's stomach...etc). However, an essential clarification is necessary. For the time being, no reliable record of DNA extraction from fossil samples has been obtained..

This greatly complicates the task of reconstructing phylogenetic trees, since any geneticist will affirm that genetic evidence must always be the first of the bases when reconstructing the evolutionary history of any living being. Even so, there are other reliable evidences that are not based only on external skeletal and morphological characters.

For example, traces of tissue have been obtained inside the bones of Tyrannosaurus rex. Tyrannosaurus rexand, after rehydration and analysis, seven different types of collagen were discovered.. By comparing them with those of various birds (especially the chicken), their morphological similarity yielded even more evidence of the correlation between theropods and modern birds.

Yet, despite all these morphological and structural similarities, we still lack the definitive key: a concordant DNA that would silence all voices against the facts presented here. Despite this, one thing 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. there is no more accepted theory of the origin of birds today that is not linked to theropods..

Summary

We can answer yes, that birds are dinosaurs and leave behind all the nuances exposed so far. This statement is oversimplifying things, but the supposed "debate" between the relationship between theropods and birds is something that is already considered settled in the scientific community. Although there are detractors to this idea, the very propositions made by them violate dogmas as accepted in the world of phylogeny today as the principle of parsimony.

It is most likely that birds are direct descendants of coelurosaurian theropods, since there is no other theory that birds are direct descendants of coelurosaurs.There is no other theory that explains their emergence with a smaller number of evolutionary steps. It is as simple as that.

Por lo tanto, el consenso actual es que las aves son un grupo de dinosaurios terópodos maniraptores que se originaron durante el Mesozoico.

Referencias bibliográficas:

• Are birds really dinosaurs? Universidad de Berkeley. Recogido a 9 de septiembre en https://ucmp.berkeley.edu/diapsids/avians.html#:~:text=Ask%20your%20average%20paleontologist%20who,birds%20are%20technically%20considered%20reptiles.
• Feduccia, A. (2002). Birds are dinosaurs: simple answer to a complex problem. The Auk, 119(4), 1187-1201.
• Feduccia, A. (2012). Riddle of the Feathered Dragons: Hidden Birds of China.
• Rezende, E. L., Bacigalupe, L. D., Nespolo, R. F., & Bozinovic, F. (2020). Shrinking dinosaurs and the evolution of endothermy in birds. Science advances, 6(1), eaaw4486.
• Schweitzer, M.H., Wittmeyer, J.L. y Horner, J.R. (2005). Soft-Tissue Vessels and Cellular Preservation in Tyrannosaurus rex. Science 307:1952–1955
• Wellnhofer, P. (1988). "Ein neuer Exemplar von Archaeopteryx". Archaeopteryx. 6: 1–30.
• Xu, X; Norell, MA; Kuang, X; Wang, X; Zhao, Q; Jia, C (Octubre de 2004). «Basal tyrannosauroids from China and evidence for protofeathers in tyrannosauroids». Nature 431 (7009): 680-4.