Traditionally, birds are classified as a class of dinosaurs and the term non-avian dinosaurs is introduced to refer to the rest.

As recent discoveries suggest that some dinosaurs may have been warm-blooded and that some dinosaurs had feathers, the distinction between birds and dinosaurs seems blurred.

I have often heard statements like "Birds are the only dinosaurs that survived the Cretaceous–Paleogene extinction event.". With recent discoveries, isn't this how the clade of birds is defined rather than a fact about the extinction?

// not sure if this question belongs in biology, sorry

  • $\begingroup$ Related: If dinosaurs could have feathers, would they still be reptiles? $\endgroup$ – Remi.b Mar 7 '17 at 16:00
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    $\begingroup$ The question relies entirely on a definition. There is no right or wrong when dealing with definitions. Anyone is free to define birds and dinosaurs as (s)he likes. The question of weather the definition is accurate or very arbitrary with loads of limit cases would be of interest though. Of course a lot of dinosaurs lineages went extinct. One lineage did not, they became bird. $\endgroup$ – Remi.b Mar 7 '17 at 16:03
  • $\begingroup$ Related: Why do the ancestors of birds still exist? $\endgroup$ – Remi.b Mar 7 '17 at 16:08
  • $\begingroup$ Dr. Thomas Holtz wrote a full chapter on it in "Dinosaurs: The most complete and up-to-date encyclopedia", Random House, 2007. $\endgroup$ – Triceratops Aug 14 '18 at 14:26

The placement of the bird clade within the dinosaur clade is based on much more than just "having feathers and being warm-blooded"; paleontologists have discovered many, many ancient bird and theropod fossils, and as a result their understanding of what the theropod clade was like overall and how the bird clade developed within it is precise enough that discovering another group of dinosaurs was warm-blooded or even had feathers won't result in the conclusion "wow that dinosaur was closer to birds than we thought", but "wow feathers were a much more common feature among dinosaurs than we thought".

If you look at the Wikipedia page you can see there are many "bird-related" clade names that cover different levels of similarity to modern birds. From "Neornithes", which is defined as the clade containing all modern birds, to "Paraves", defined as the group containing all organisms closer to modern birds than to Oviraptors, which are all within Maniraptorans, which is itself pretty deeply nested within the Theropod clade...

So basically what I am trying to say is, our understanding of where birds fit within Dinosauria is precise enough at this point that no discovery about a single trait is going to "blur the distinction between bird and dinosaur"; it might change our idea of dinosaurs in general to be more bird-like, meaning that traits we thought were bird innovations might have been standard-issue in dinosaurs for a long time before, but it will not necessarily change our whole understanding of how birds and dinosaurs are related.

As for this :

With recent discoveries, isn't this the definition of what a bird is rather than a fact about the extinction?

It isn't; the paleontological definition of what a bird is is not at this point an ad-hoc definition related to "whatever survived the K/Pg extinction". Even Neornithes, the crown group of all living birds, originates to before the K/Pg extinction. For example, the splits between ostriches and other birds and within that latter group, of ducks and geese from other birds are thought to have happened during the Cretaceous. And there are some researchers who argue that a few fossils indicate that some non-avian dinosaurs survived the extinction; the consensus view is that those fossils are dated wrong, but that this position exists shows that "non-avian dinosaurs went extinct at the K/Pg boundary" is a fact-based question, not a definitional one.

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  • $\begingroup$ Also note that several major groups of "birds", like the enantiornithines and hesperornithiforms, didn't survive the K-T extinction. $\endgroup$ – jamesqf Mar 8 '17 at 19:23
  • $\begingroup$ Right; these aren't in the crown group of modern birds, and enantiornithines in particular are way upstream of modern birds, but they're definitely animals that if you saw them you would think they were birds (in fact the first fossils of these animals were misidentified as belonging to modern bird groups). They are one reason why "bird" can refer to many different paleontological groups; the crown group of modern birds, a higher-order group like Avialae to include those archaic forms, etc. Which further illustrates that the definition of "bird" doesn't hinge on feathers or the K/Pg boundary. $\endgroup$ – Oosaka Mar 8 '17 at 21:11
  • $\begingroup$ Yes. I think the problem is mostly linguistic, in the distinction between "are" and "evolved from". I mean we all evolved from fish, but if someone said you are a fish (or expected you to breathe under water :-)), you'd look at them somewhat askance, no? $\endgroup$ – jamesqf Mar 9 '17 at 23:57
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    $\begingroup$ @jamesqf not quite; there is a linguistic issue but also a substantive one. People would look at you askance if you said you were a fish, but nobody would disagree if you said you were a mammal. The relationship between evolution and the nested hierarchy that life by and large falls into is that the nested hierarchy is the result of evolution - groups diversify, spawning subgroups etc, so all descendants of one group are still more like each other than they are like descendants of other groups, meaning that "is descended from" and "belongs to this group (based on overall similarity)" coincide. $\endgroup$ – Oosaka Mar 10 '17 at 7:26
  • $\begingroup$ Exactly my point. Saying that humans are mammals conveys one set of information; saying that humans are synapsids conveys a different (and generally not very useful) set, which is less confusingly conveyed by saying that humans are mammals, and mammals evolved from synapsids. Likewise saying that birds are dinosaurs conveys different information from saying that birds evolved from dinosaurs. (And at least two very different groups of dinosaurs, and other creatures like pterosaurs & dimetrodon that people think are dinosaurs.) $\endgroup$ – jamesqf Mar 11 '17 at 5:39

One modern definition of birds, due to Gauthier, is the smallest clade containing all extant dinosaurs. However, this is not equivalent to the dinosaurs that survived the Cretaceous-Paleogene event, as these are not a clade. The bird clade dates back about 150 million years, and the Cretaceous-Paleogene event was also a mass extinction for birds, although after that they became much more diverse.

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