[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Making Lip of It



To be fair, I wouldn't say there's absolutely no clear transition. In
fact the transition seems to have maybe been similar to the one in
birds. You have taxa like dsungaripterids with teeth restricted to the
back of the jaws, and toothless, presumably beaked, jaw tips. From
there it's a short step to losing the teeth altogether, because you
already have a beak up front ready to take their place. Even
fully-toothed pterosaurs like _Pterodactylus_ and _Rhamphorhynchus_
appear to have had small keratin beaks situated in between the
anterior-most pair of teeth (based on soft tissue impressions).
Compare with the evolution of beaked birds: you have fully toothed
jaws basally, then forms like hesperornithines and ichthyornithines
with teeth restricted to the posterior and with beaked, toothless
tips, and then you have modern toothless, beaked birds.

Not that I think it's all that likely for pterosaurs to have had lips,
I just haven't seen anybody present any evidence either way.

Matt

On Fri, Sep 23, 2011 at 11:05 AM, Sim Koning <simkoning@msn.com> wrote:
>
>      I think I'm comfortable saying that most (if not all) pterosaurus almost 
> certainly did not have lips. A great many of them had very long interlocking 
> teeth similar to those of a gharial. It seems that their length and/or angle 
> of projection would definitely preclude the lips we see in lizards today. 
> Then of course we have the more derived forms with no teeth at all. It seems 
> improbable to me that they would go from a 'gharial-toothed' form with big 
> fleshy lips to a bird like beak with no clear transition. I can think of a 
> few more reasons, but for me at least, these two are enough.
>
> Sim
>
>
> ----------------------------------------
>> Date: Fri, 23 Sep 2011 08:28:14 -0400
>> Subject: Re: Making Lip of It
>> From: martyniuk@gmail.com
>> To: simkoning@msn.com
>> CC: dinosaur@usc.edu
>>
>> "Pterosaurs seem to have been a largely toothed but lipless group with
>> some non-fishing inland forms"
>>
>> Again, is there any actual evidence for this assertion? AFAIK the
>> evidence is just as equivocal for non-beaked pterosaurs as it is for
>> non-beaked dinosaurs.
>>
>> Matt
>>
>> On Thu, Sep 22, 2011 at 10:23 PM, Sim Koning <simkoning@msn.com> wrote:
>> >
>> >
>> >
>> >
>> >      To reiterate my point about pterosaurs: there were insectivorous and 
>> > toothed inland species (some Istiodactylids) that probably weren't making 
>> > a regular habit of dunking their heads into water. Even the species that 
>> > were fishing would have been doing so in saltwater, something that tends 
>> > to rapidly desiccate tissues. Plus I doubt they spent their *entire* day 
>> > soaking their heads in sea water, as I'm sure they would have needed to 
>> > roost at some point. Kaprosuchus was most likely a terrestrial croc with 
>> > what was apparently a largely terrestrial ancestry, and it almost 
>> > certainly didn't have lips. Then there are the numerous mammalian examples 
>> > with exposed teeth despite the fact that they are not easily replaced. I 
>> > really don't think tooth dessication was that much of a problem at all for 
>> > most animals with exposed teeth. Nor is a dry mouth for that matter: 
>> > parrots have very dry mouths as they spend most of the time with their 
>> > mouths partly open, and they are *dinosaurs*. In fact, if a parrot has a 
>> > wet tongue it is usually a sign of illness. I really see no barrier to a 
>> > dinosaur evolving a largely dry mouth, or exposed teeth, or both.
>> >
>> >
>> >
>> >
>> >       What can phylogenetic bracketing tell us? If we look at terrestrial 
>> > crocodylopmorphs such as Sebecus and Kaprosuchus, we see jaw morphology 
>> > with interlocking teeth that would not have left much room for lizard like 
>> > lips. Pterosaurs seem to have been a largely toothed but lipless group 
>> > with some non-fishing inland forms. modern birds have beaks as did many 
>> > dinosaurs, many of which seem to have evolved independently. Kaprosuchus 
>> > seems to have had a keratinous 'bumper' of some sort on the tip of the 
>> > snout. Perhaps this is how the typical archosaurian beak started out? I 
>> > can see the advantage in having a hard covering on the parts of the the 
>> > mouth that were most prone to abrasion and injury. All this suggests to me 
>> > that 'liplessness' was the basal condition for most of Archosauria. 
>> > Perhaps a lack of fleshy lips allowed for the rather large sharp teeth we 
>> > see in so many predatory archosaurs. Lizard like lips may very well have 
>> > been more trouble than they were worth for a great many archosaurs. This 
>> > is something I have yet to see addressed. The assumption often seems to be 
>> > that lips would have been beneficial for predatory archosaurs, and so no 
>> > good reason can be found for why they would be lacking. If you look at the 
>> > open mouth of a lizard such as Komodo dragon, you will notice that their 
>> > teeth are not even visible despite their large size. If a dinosaur such as 
>> > Deinonychus had lips of this sort, it would have looked toothless when it 
>> > opened its mouth. The reality is that the sharp teeth of Varanids actually 
>> > tend to lacerate the tissue surrounding them. I suspect that this would 
>> > have been a problem for many theropods, not an advantage.
>> >
>> >
>> >      I suspect the typical pattern that independently lead to beaked jaws 
>> > in so many archosaur lineages may have been something like this:
>> >
>> > 1. Lizard lips with interlocking teeth > 2. Lizard lips covering jaws with 
>> > an overbite > 3. Reduced lips with an overbite > 4. No lips with exposed 
>> > teeth (most of Archosauria) > 5. Exposed teeth with a hard scaly covering 
>> > on the skull > 6. Teeth with a rostral 'proto-beak' > 7. A reduction in 
>> > size and/or number of teeth > 8. A toothless beak.
>> >
>> >
>> > Simeon Koning
>> >
>> >
>> >
>> >
>> >
>
>