If birds are dinosaurs, then why aren't all vertebrates fishes? Are they only dinosaurs when speaking cladistic, or also Linnaean?

 I understand that, with clades, birds are dinosaurs, but when considering clades, birds are fishes, cnidarians, et cetera. This distinction is next to useless for the general purposes of language, where calling everything a bacteria would be meaningless. Thus, for the purposes of casual language, are birds really dinosaurs?

Another question of Quora, if you read these entries you already know that I considered the site interesting before they chose quantity (traffic) instead of quality. They also have a politic of top writers (those who attract more readings) that has made havoc with the level of answers related to animals and ethology.

However, I once wrote on Quora this text. Enjoy...

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Birds are dinosaurs for both systems of classification.

Birds are dinosaurs from both viewpoints, although it carries some problems for Linnean classification (1). And don't missunderstand me, I do really like Linnean classification and prefer that it be the first one to be taught to children because it is a great description of the world as we see it, such as Cladistics can't be by definition, because they have to include features of extinct organisms. Here in Spain children are introduced to classification overly early, in the first grades of Primary, and how could they understand the links between reptiles and birds that Cladistics bring when they are under 10 years?.

Coming back to dinosaurs, it is a group that was not defined on the range of characters we can observe on a living animal that include external features such as appearance, skin traits (having fur, feather, glands, layers...), metabolism traits (such as weter they could maintain a level of internal temperature independently of enviromental conditions or not, chambers in the heart, etc), skeletal morphology and others. It wasn't possible because the description was made from the fosil remains that only depicted skeletal features (2), hence all the group was typed from some few relevant skeletal traits. There is not way of comparing how they descripted mammals, birds and reptiles with the way they did with dinosaurs.

Source of the picture: aurornis Xui: A New Flying Dinosaur that Lived About 150 Million Years Ago has been Discovered in China

What happened is that accordingly with that key skeletal features, birds must be included into the dinosaur group (from a Linnean viewpoint) or are in the dinosaur clade (from a Cladistic one). Birds fulfill all the "must to" to be named dinosaurs by their own and not as derived from dinosaurs. Of course that there is a lot of polemics sorrounding this point, for some anatomist birds display very original adaptations to flight that would tell them apart from more "primitive" dinosaurs (3). Some even maintain that they could derive from an earliest ancestor that was only related to dinosaurs, but was not a dinosaur (4). I think that the scientits that defend those viewpoints have failed to demonstrate that birds lack the skeletal characters that make them dinosaurs, so far at least; in fact the critics I link bellow are debunked. Probably we will see more of this polemic in the following years, and this is the kind of discussions I like, because in spite that they can be really boring for taxonomists, for general public show some really interesting features of the animal studied. And also, it is like a real scientific soap opera on the internet.

But for the purpose of casual language, birds can be called birds, and we can forget for a while that they are dinosaurs.

Finally, in our dayly life, as far as we are not interested in the evolutionary history of some organism or some feature in some organism, I think it is more useful, and amazing, talking about them in the way we see and experience our present world, and here is where Linnean classification makes the best role. Do you see those beautiful long legged horses as fishes?. Indeed, nobody does. The insistence of calling birds dinosaurs come from the fact that the polemics between supporters and the remaining detractors of birds as dinos still continues, nothing is more noisy than a battle field.

__ Links and an introduction to some problems and controversies derived from dinosaurs classification. I only recomend point 1 and 4 to dino freaks.

(1) Well, this point is annoyingly long, I agree. But as I have seen so many people confused with the idea that dinosaurs are birds, I like to explain why they feel so stranged with it.

The problems that Linnean classification must face due to that birds are actually dinosaurs derive from that dinosaurs include animals that highly resemble reptiles and others that, well, are birds.

For the lay person the problem can be post as "So when we think about the term dinosaur, what are we thinking about a bird or a reptile, or both?". In fact the matter reminds me the Necker cube , an optical illusion in what we can see the same drawing of a cube looking up, down or both alternately.

Regarding dinosaurs, there is not such dilema, as I have told in the text, dinosaurs were defined on different basis than any living animal Class (reptiles and birds are Classes accordingly to Linnean classification), so there is not contradiction in that dino groups animals from both Classes. It is a little uncomfortable for Linnean supporters, but not a nuclear misile that will destroy the concept.

Cladistics supporters, on the other hand, have more strong feelings about the Linnean Class reptilia (i.e reptiles). Some schools of thought would like to redefine it in a way that includes birds. But they also have to face their own problems, because it is not clear how to group the different extant types of reptiles accordingly to evolutionary characters. For the moment reptiles are a nightmare for evolutionary biologists and cladistics zealots, good for the old brave scaled animals. Who wants that their children are told that birds an reptiles are the same at the tender age of seven?. I don't.

In fact dinosaurs are not the only group that defy the traditional Linnean grouping, just the most popular. If Synapsid would have been so know as dinos, we would have assisted to a long and vively discussion about wether they are retiles or mammals or what. Finally synapsid were split from reptiles.

I have brought about the example of synapsid to talk about the complexity of classifying extinct animals that depict characters that make them close to two different Linnean Classes, species or whatever. In the end, making classifications is drawing borders that looked from an aerial perspective seem to be very clear, but the closer you get to these borders, the more artifitial and debatable they appear. This is the lesson that we, lay people, can draw from the problems with classifying dinosaurs.

(2) What is the scientific diagnosis of what is a dinosaur, and what is just another archosaur? Several skeletal characteristics are currently used as diagnostic dinosaurian features. You may also view a large-screen picture of a dinosaur skeleton for a lesson in anatomy : Morphology of the Dinosauria

(3) Are birds dinosaurs? New evidence muddies the picture (bear in mind that the objections exposed in this article are nowadays debunked).

Origin of birds

(4) Longisquama (debunked. But I really find Longisquasma amazing, in spite that I only see the head half of an animal skeleton over some feathered thing).