Most recent common ancestor of all living birds

So birds evolved from dinosaurs. And I’ve seen many news articles over the last few years about various discoveries of dinosaurs with feathers, and some that may have even had wings. Did modern birds arise from a single one of these species, or multiple dinosaur species? Did the most recent common ancestor (MRCA) of all birds have a beak? Did it have teeth? Could it fly?

Birds didn’t just evolve from dinosaurs, they are dinosaurs. That isn’t just a semantic quibble. Modern day birds like chickens are more closely related to T-Rex than T-Rex is related to crocodiles or even other dinosaurs like the Stegosaurus. The list goes on but the dinosaur grouping has had lots of members over hundreds of millions of years and modern day birds are closer to the middle than the fringes of it.

That said, I am not an expert on the part of your question about modern birds evolving from another single dinosaur species or not and then branching off from there. Everything I have read leads me to believe this is still a hot-topic in avian evolutionary biology.

The most recent common ancestor of all birds had teeth, but the most recent common ancestor of all living birds didn’t. We know this because no living birds have teeth.

That doesn’t logically have to be true, does it? Isn’t it possible (though perhaps less likely) that descendants of the MRCA might have lost their teeth in separate lineages?

Sure, though by the same logic humans are fish (which, if I understand correctly, is true in cladistics). But we don’t generally refer to people or mammals as a subgrouping of fish, so I think it’s also reasonable to use the words ‘bird’ and ‘dinosaur’ as they are commonly used.

All modern birds are descended from a common ancestor that could fly, and lacked teeth in its beak. Only one non-flying feathered dinosaur gave rise to modern birds (although we have not really identified which one it was).

Interestingly, Archaeopteryx, often cited as the ancestral bird, may not be on the direct line to modern birds. It was just another one of several feathered flying dinosaurs.

No, it is not. Modern birds are firmly in the middle of the evolutionary pack when it comes to describing ‘dinosaurs’ that everyone tends to agree on in common nomenclature. They aren’t some modern offshoot of them. They are in fact dinosaurs and more closely related to species that anyone one would agree is a dinosaur than other ancient dinosaur species are to each other.

Contrary to popular belief, dinosaurs never died out due to catastrophe. They simply got smaller and much better at maintaining their juvenile characteristics to help them survive in a new environment. Those are the birds that you see all around you today.

Whilst it certainly is more plausible and likely that the MRCA did have teeth than it did not fly or did not have a beak, it’s still much more likely the MRCA of living birds had no teeth than not.

The reason that having teeth is much more likely than being flightless or lacking a beak is that flight and beaks were very old evolutionary innovations in the ancestors if birds by the time of the MRCA and for multiple reasons it very implausible that it lacked either of these features, whereas the loss of teeth happened very close to the time of the MRCA. There are fossils which look very similar to modern birds and must’ve been fairly close relatives of the MRCA that have teeth.

The reason to believe the MRCA didn’t have teeth, is that the loss of teeth was actually a fairly major innovation in the bird lineage and required specialized adaptions in the beak. Indeed it is very plausible that the toothless birds were able to survive and radiate out to become the modern birds, whilst their toothed relatives died out because of the advantages conferred by this adaption.

Why would losing teeth be an advantage?

Because the comet killed off all of the dino-dentists?

Seriously, though, teeth rot and/or fall-out, limiting the ability to eat. A beak seems a simpler solution. Although why more creatures don’t have beaks instead of teeth speaks against that idea.

Or for whatever reason, the toothless bird was actually better able to survive the comet strike.

I’m guessing it was mostly a reduction in weight. That seems to be the major idea behind a lot of bird adaptations.

And in fact, maybe we can’t identify which one it was, because we happen not to have ever found a fossil of an ancestral bird species. It’s misleading to speak of gaps in the fossil record: Really, it’s almost all gap.

Apparently, the pathway to tooth growth is not all that deeply buried in the evolutionary history of birds: Hens’ Teeth Not So Rare After All

No it doesn’t (the graped part) - evolution doesn’t work like that. Selection can only occur from things that are there to be selected. If other creatures never developed a mutation for a proto-beak, then it couldn’t be selected, and it doesn’t matter if it would have been better than what they had.

Fine – they’re dinosaurs. They’re also birds, so I don’t think I’m confusing anyone when I say “birds evolved from dinosaurs”.

And, by the way, humans are more closely related to certain kinds of fish (like lobe-finned fish, if I understand it correctly), then those certain kinds of fish are to other kinds of fish (like sharks). So humans as a sub-category of fish, cladistically speaking, is just as accurate as birds as a sub-category of dinosaurs.

One thing often overlooked (or just assumed to be general knowledge by the more informed, but it isn’t) - birds, as in fairly recognizably modern birds, were around before the other dinosaurs died out. That is to say, the process wasn’t “Most dinosaurs die out->a few then evolve birds” , it was “Most dinosaurs die out->only the birds survive.” The split between the ratites & tinamousand all other birds being reasonably certain to have occurred before the K-Pg extinction event.

Damn, there’s a part of me that still just automatically wants to type “K-T extinction”

“evolve into birds”, not “evolve birds”

Teeth are heavy. A horny beak can provide similar food-gathering and processing capability at a considerable saving in weight. Also, moving much of the food-processing to a muscular gizzard puts more weight near the center of gravity, instead of in the head where it would be more difficult to balance while maneuvering in flight.

You understand correctly. But not just sharks-- all ray finned fish as well.

When you consider the rareness of fossils, the odds are stacked against it being in the direct line of descent. But, of course, every paleontologist wants to be the guy (or gal) who discovers the so-called missing link, and not just some ancestor to a dead end branch on the evolutionary tree.