Evolution of the beak

[Seijun]

To me at least, a bird's beak seems very different from the toothy snouts of even the most bird-like of theropod dinosaurs. At what point did the ancestors of modern bird's start to loose their teeth and evolve true keratin-covered beaks? Have we discovered any species that transitional between the two?

Along a similar line, have we discovered any transitional species between tailed theropods and tail-less birds?

[Metallisuchus]

It's funny but I just read an article on Hesperornis regalis in Prehistoric Times #101.

[ZoPteryx]

Yes, it is rather odd that the deinonychosaus were closest to birds, and yet had toothy snouts.  However, this is probably due to their predatory lifestyles.  Plenty of early birds had both beaks and teeth (enantiornithes, Hesperornis (a more modern bird)) and of course plenty of dinos (oviraptors, therizinosaurs, many ornithsochians).  To me, this means beaks can evolve pretty much at random amongst the archosaurs, but the line of beaks leading to birds probably started back in the early species of the maniraptors and was subseguently lost, retained, or re-evolved amongst later dino and bird lines.

Toothlessness seems to be even more random. Example: Confuciusornis was a very primitive bird, but lacked teeth, while Hesperornis was a relatively advanced bird and had teeth.  I think it's just by pure coincidence that all modern birds lack teeth, stemming from the fact that their ancestor(s) were the only birds to survive the K/T event.

As for tails, I'm not sure.  Many bird-like dinos had a pygostyle, the short fused vertebrae at the end of the tail that anchor larger feathers.  All avian-dinos have this feature too, but the short fan tail as we know it today (just the stumpy pygostyle visible) differed from one group to another.  In most enantiornithes, a pygostyle was present, but the feathers attached to it had no airodynamic function (unlike in modern birds).  So I'd suspect that the transitional fossil (or closest thing to it) would be a Epidexipteryx, which had a relatively short tail with a pygostyle, but still had a typical dino bodyplan and yet is grouped in the Paraves (group that includes modern birds).

[Horridus]

A birdy beak is lighter than a set of toothy jaws. Therefore, it's advantageous for a flying animal to have a beak. It's no coincidence that the most advanced pterosaurs had toothless beaks (it's convergent evolution!).

[Dinoguy2]

Yes, it is rather odd that the deinonychosaus were closest to birds, and yet had toothy snouts.  However, this is probably due to their predatory lifestyles.  Plenty of early birds had both beaks and teeth (enantiornithes, Hesperornis (a more modern bird))

For the record, almost all enantiornithians known lacked any beaks as far as we know. Only one evolved a beak (Gobipteryx) probably independently.

Beaks first show up in euornithians like Yanornis, which was mostly toothly but had beaked jaw tips lacking teeth. This is the same situation as Hesperornis and Ichthyornis. So it looks like this was the general condition for a long time--beaks at the tip, teeth in back. In the ancestors of modern birds, the back teeth were finally lost, leaving the beak to take over the whole jaw.

Now, another question is when lips were lost. Most theropods probably had lips--did sat, Hesperornis have lips covering its teeth but not its beak? Or were the toothy sections already lipless?

Along a similar line, have we discovered any transitional species between tailed theropods and tail-less birds?

Nope. We go from long tails like Jeholornis to no tails like Confuciusornis with really no in-between species yet. Zhongornis might be one, but it's also suggested to be just a baby Confuciusornis (indicating that tails shortened as they grew, kinda like frogs).

[ZoPteryx]

Yes, it is rather odd that the deinonychosaus were closest to birds, and yet had toothy snouts.  However, this is probably due to their predatory lifestyles.  Plenty of early birds had both beaks and teeth (enantiornithes, Hesperornis (a more modern bird))

For the record, almost all enantiornithians known lacked any beaks as far as we know. Only one evolved a beak (Gobipteryx) probably independently.

Oops, good call your right.   For some reason I was thinking of Confuciusornis when I wrote that, despite the fact that it didn't have teeth and wasn't an enantiornithe.

[Sharptooth]

Now, another question is when lips were lost. Most theropods probably had lips--did sat, Hesperornis have lips covering its teeth but not its beak? Or were the toothy sections already lipless?

Maybe Hesperornis was lipless like a croc, afterall it seems to me that lips were first lost in the archosaurs more adapted to a fish diet (terrestrial "crocs" like Postosuchus, aetosaurs and such were more on the lips side, judging by their skulls and teeth arrangements, but i could be wrong)... And what about spinosaurids? Do you think they had lips like other theropods or maybe they were too croc-like in that regard?

[Metallisuchus]

Now, another question is when lips were lost. Most theropods probably had lips--did sat, Hesperornis have lips covering its teeth but not its beak? Or were the toothy sections already lipless?

Maybe Hesperornis was lipless like a croc, afterall it seems to me that lips were first lost in the archosaurs more adapted to a fish diet (terrestrial "crocs" like Postosuchus, aetosaurs and such were more on the lips side, judging by their skulls and teeth arrangements, but i could be wrong)... And what about spinosaurids? Do you think they had lips like other theropods or maybe they were too croc-like in that regard?

As far as I know, all you have to do is look at the skull and if there are little holes where the lips should be, then they were able to move their lips a bit. I know a lot of theropods had these holes, but I don't think Spinosaurus did.

[Gryphoceratops]

I don't think dinosaurs with "lips" would be able to move them much with regards to snarling and such like some mammals do today since there aren't any modern reptiles with "lips" like squamates that can move them. 

[Metallisuchus]

I don't think dinosaurs with "lips" would be able to move them much with regards to snarling and such like some mammals do today since there aren't any modern reptiles with "lips" like squamates that can move them.

It was just a reference to something I read a long time ago. I don't think they'd necessarily be able to move them the way a giraffe - for instance - would be able to either, but I would imagine they would have a little more control over them then most reptiles would.