For a few decades, it has been claimed that “ birds are descended from dinosaurs ”. But it does not penetrate society and generates difficulties of understanding. To what extent is it true that birds are descended from dinosaurs? And how did we come to that conclusion? The first bird that we know of lived in the Jurassic period, 150 million years ago, is Archeopteryx.
The foundation of a hypothesis
Since the discovery of the first almost complete fossil of Archeopteryx in 1861, the so-called ‘London specimen’, and later, in the 1870s, the discovery of the best preserved fossil of this animal, the so-called ‘ Berlin specimen ‘, it is clear that birds and reptiles were closely related. Something that Charles Darwin already hinted at in his fourth edition of The Origin of Species .
“Until now it was held that the entire class of Aves came into existence during the Eocene period; but we now know, on the authority of Professor Owen, that a bird did indeed live during the deposition of the upper greensandstone; and still more recently this strange bird, Archeopteryx , with a long lizard-like tail, and bearing a pair of feathers at each joint and its wings decorated with two free claws, has been discovered in the Solnhofen oolitic slates. Hardly any recent discovery shows more strongly than this how little we still know of the ancient inhabitants of the world.”
Already Thomas Henry Huxley , nicknamed ‘Darwin’s bulldog’, after analyzing the Berlin Archeopteryx , ventured that birds must have descended from dinosaurs. However, it was an idea opposed by most contemporary scientists, led by Professor Richard Owen , who named the group ‘Dinosauria’ some time before.
Since then, many hypotheses have come and gone on the subject: Seeley , who said that it was an evolutionary convergence and that they really had nothing to do with it, Heilmann and his thecodont hypothesis, Walker and the crocodile ancestor hypothesis. ..
But John Ostrom ‘s work with Deinonychus in the 1960s and 1970s, Gauthier ‘s application of cladistics to dinosaurs, and the discoveries of feathered dinosaurs in recent decades have ended up showing who was right in this debate. Something that Huxley’s brilliant mind was able to glimpse from analyzing a single copy. And it is that, indeed, birds descend from a very specialized group of Jurassic dinosaurs, which have Archeopteryx as their main representative.
Are they descended from dinosaurs or are they dinosaurs?
Just as the expression “ birds descend from dinosaurs ” is beginning to become popular, the same has not happened with another statement, very similar, but which incorporates a more categorical nuance: “ birds are dinosaurs ”.
If we follow the rules of classification based on phylogeny, that is actually the correct expression. Simplifying, it is like saying that “dogs are descended from mammals” or “dogs are mammals”.
When something is said to descend from something else, it might seem that the descendant is said to have ceased to be that which was its ancestor. A person is descended from his mother, but he is not his mother, he is a different person. By saying that “birds are descended from dinosaurs”, it is implicitly implied that this small group of dinosaurs, represented by Archeopteryx , at some point in the last 150 million years, ceased to be a dinosaur and became a bird.
But evolution doesn’t work that way . When populations, when species evolve, they change and new species are formed. Given enough time, new groups will form, new lineages that will in turn diverge like the branches of a tree. Some groups will become extinct and others will continue to diversify. But, and this is important, a new species, a new group, never ceases to belong to the group to which its ancestors belonged. This is what is called monophyletism.
If the descendants of a species that belonged to a certain group are excluded from that group, it falls into paraphyleticism , an incorrect way of proceeding in evolutionary terms —for example, if dogs were excluded from the category “mammals”— . In the same way, if a group of species is formed without including its ancestors, another error called polyphyleticism is committed – for example, if it were affirmed that dogs and humans belong to the same group, excluding the rest of primates, canids, and a good number of groups of mammals more—.
In terms of phylogeny, only those monophyletic groups are valid, that is, those that include the most recent common ancestor and all its descendants.
If we take the phylogenetic definition of dinosaur as any animal that descends from the last common ancestor between Tyrannosaurus , Triceratops and Diplodocus —three animals always classified as dinosaurs—, we find that Archeopteryx is included in that group, and with it, all birds.
Excluding birds from the clade Dinosauria implies that the clade is paraphyletic, and therefore incorrect. The only monophyletic clade that falls under the definition of ‘dinosaur’ includes birds, as a sine qua non .
Therefore, it is not that birds are descended from dinosaurs; is that they are, in fact, the only group of dinosaurs still in existence . Every time you see a fluttering hummingbird sucking a flower, a sparrow on a bar terrace looking for crumbs from a sandwich, a seagull soaring over the beach or a terrifying cassowary, you are seeing living dinosaurs.
Because birds are dinosaurs .
References:
Gascó Luna, F. 2021. Eso no estaba en mi libro de Historia de los Dinosaurios. Guadalmazán
Hecht, M. K. et al. 1982. The paleobiology and phylogenetic position of Archaeopteryx. Geobios, 15, 141-149. DOI: 10.1016/S0016-6995(82)80108-3
Mayr, G. et al. 2005. A Well-Preserved Archaeopteryx Specimen with Theropod Features. Science, 310(5753), 1483-1486. DOI: 10.1126/science.1120331
Ostrom, J. H. 1976. Archaeopteryx and the origin of birds. Biological Journal of the Linnean Society, 8(2), 91-182. DOI: 10.1111/j.1095-8312.1976.tb00244.x
Switek, B. 2010. Thomas Henry Huxley and the reptile to bird transition. Geological Society, London, Special Publications, 343(1), 251-263. DOI: 10.1144/SP343.15
Wellnhofer, P. 2010. A short history of research on Archaeopteryx and its relationship with dinosaurs. Geological Society, London, Special Publications, 343(1), 237-250. DOI: 10.1144/SP343.14
