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Ultermarto

Are birds dinosaurs? Should we classify them as dinosaurs?

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Birds are descended from dinosaurs, that much is pretty clear, even to a layman like me. I can hardly look at a pigeon walking around on a street without seeing the theropod resemblance. I'm completely willing to accept that at some point (late Triassic, Wikipedia tells me), the most early things we can think of as birds (perhaps archaeopteryx) branched off from other bird-like theropods. What confuses me is when a scientist claims that we should actually classify birds as modern dinosaurs, that is, their class would surely be reptilia rather than aves, and their order surely that of the dinosaurs (though there doesn't seem to be an agreed order). This system of classification frustrates the neat-freak in me. It seems that, under these rules, any species is included in its ancestral groups as well as its own. In that case, why am I not, alongside being a hominid, an ape, a primate, and a mammal, also reptile, an amphibian, a fish, a polyp, a protozoan? I was taught biology such that all species could be given eight taxonomic specifications, and as I understood it, a new class, such as mammals, could pop out of a single species; the very last of the reptilian mammals before we draw the line between reptiles and mammals. This system makes way for future classifications. The system proposed by the idea that birds should be considered dinosaurs will lead to the long-term piling up of taxa, or an infinite line of new sub-special groups being made up for the sake of new classifications.

 

But of course, as I understand it, the birds = dinosaurs thing is put in place due to birds being more closely related to dinosaurs. But I'm still not sure what this means. When is a thing more closely related to another thing? When it branches of later in time? When there is less morphological difference than otherwise? Someone please help.

Edited by Ultermarto

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Gnathostome phylogenies tend to be a mess of paraphyletic groupings. It's a known problem that has been discussed in the literature (though I'm not at all current in that area). There will always be all sorts of nested classes/orders/families because biology is messy. Evolution doesn't care about strict delineation

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Birds are dinosaurs much the same way that bats are mammals...

Edited by Moontanman

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And most ancient fish aren't..................fish.

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In that case, why am I not, alongside being a hominid, an ape, a primate, and a mammal, also reptile, an amphibian, a fish, a polyp, a protozoan? I was taught biology such that all species could be given eight taxonomic specifications, and as I understood it, a new class, such as mammals, could pop out of a single species...

 

It depends on what kind of taxonomic tree you're looking at. Phylogenetic trees are most common and rely on genetic relationships and allow us to infer at what point in time species diverged from each other, and other broader clades (taxonomic groupings). Phylogenetic trees do not imply that specific clades are directly descended from broader clades. For example, chimps + humans form a clade, and chimps + humans + gorillas form a clade as well (but not chimps + gorillas). This doesn't necessarily mean that humans + chimps descended from gorillas. Rather, it implies that humans + chimps shared a common ancestor with gorillas; this common ancestor may not have been a gorilla, nor a human/chimp resemblance (let's call him pre-human/chimp gorilla). This pre-human/chimp gorilla had some children that were reproductively isolated from other ancestral gorillas, and these pre-human/chimp gorilla children continued to live on and eventually gave way to the common ancestors of humans + chimps, which then branched off into humans and chimps; the ancestral gorillas continued on evolving into the gorillas we see today. Thus, it would be more on-point to say that birds descended from dinosaurs, not that birds are dinosaurs.

 

You can extend this concept to broader clades. We aren't polyps, for two reasons 1) the most common ancestor between humans and polyps may not have been a polyp (it was some pre-chordate/cnidarian ancestor) and 2) polyps have gone through many years of evolution since we diverged, again so that we would not fit the current definition of a polyp.

 

To answer your second question (should we classify them as dinosaurs?) I would not be able to say yes or no, because I am not a specialist in bird-dinosaur comparative anatomy & physiology/embryology/genetics.

Edited by ahyaa

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