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Our teeth are prone to fractures when dry due to their crystalline internal structure. Because of this, a majority of terrestrial animals with a similar tooth makeup have lips to keep the teeth moist.

What composites for a tooth could be made to circumvent this issue? Ideally, this structure wouldn't be too metabolically expensive, and horns, claws and potentially fur could be made with some of the materials.

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    $\begingroup$ whatever tusks are made out of? $\endgroup$ – ratchet freak Oct 31 '18 at 16:55
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    $\begingroup$ I can't speak for anyone else, but I use Jurgens Teeth Moisturizer myself. I feel bad because of how they test it on emus, but nothing quite gets my teeth quite so soft and pliant as their product. Other teeth moisturizers don't compare. $\endgroup$ – John O Oct 31 '18 at 17:16
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    $\begingroup$ What kind of animal is that? Herbivore, carnivore or omnivore? Please, specify a little bit more about him. $\endgroup$ – Faed Oct 31 '18 at 20:26
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    $\begingroup$ @Faed This one specifically is a herbivore (non-obligate), and eats like how an iguana eats. However, there are relatives to this animal that are carnivorous. They grind up their food in a crop-like organ. $\endgroup$ – Tardigreat Oct 31 '18 at 20:30
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    $\begingroup$ Saber-toothed animals managed just fine. $\endgroup$ – Roger Oct 31 '18 at 21:09
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This is not as big a problem as you think, you really don't have to do anything teeth tend to only crack if the inner parts of the tooth is exposed. Cut ivory has a much higher risk of shrinkage because the cut expose the interior dentine of the tooth, which A dries much faster and more easily and B. shrinks a lot more when it does. Note most of a tusk (just like most teeth) is dentine with the thick wall of enamel on the outside. Enamel is much less prone to shrinkage, and it is mostly water tight.

When ivory is turned into crafts it is cut exposing the dentine which dries out, in preserved skulls instead the dentine is exposed by the lack of gums. This is why elephant tusk don't crack in the animal, moisture is resupplied from the inside slowly but fast enough to replace the very small amount lost through the enamel. Broken or heavily worn tusks have a much higher risk if cracking for the same reason crafted ivory does the dentine is exposed to the air and can dry out.

Of course this is only a problem if you hang on to the teeth for a long time.

Everything that is not a mammal Just keeps growing new teeth so it's not even an issue. Limited numbers of teeth is a mammal only thing, the tooth is only kept for a short while before it falls out and is replaced. This is how Crocodiles and some dinosaurs dealt with their teeth being exposed.

Tusk cross section, it is a dentine core with an enamel exterior, the tip is solid enamel for strength.

enter image description here

This image shows how croc teeth grow in and are replaced

enter image description here

https://www.sciencedaily.com/releases/2013/05/130514101457.htm

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Teeth can regenerate. Even humans have two sets of teeth: temporary and permanent. Having a constant regeneration cycle will allow for normal teeth to survive without lips to moisturize.

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    $\begingroup$ So basically be like sharks? IE: OP shouldn't make the teeth different, but just let them break and keep growing new ones all the time? $\endgroup$ – Loduwijk Oct 31 '18 at 17:45
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    $\begingroup$ Yes exactly. Same teeth, but constant growing of new ones. $\endgroup$ – keiv.fly Oct 31 '18 at 17:46
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    $\begingroup$ Note mammals are the only group of animals with limited sets of teeth, everything else just keeps replacing them, that's why crocodiles who have exposed teeth don't have to worry about it. $\endgroup$ – John Oct 31 '18 at 23:46
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    $\begingroup$ @Penguino If the hypothesis is that lips are to keep teeth moisturised, then the fact sharks live in water might be a confounding factor. :) $\endgroup$ – Graham Nov 1 '18 at 0:46
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    $\begingroup$ @Graham - but what about flying sharks..imdb.com/title/tt3977848 ...how do they avoid their teeth fracturing/ $\endgroup$ – Penguino Nov 1 '18 at 2:39
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But tusk bearing animals have them for a pretty long time, elephants have them from 1 year old until they die at around 60.

The tusk's ivory is maintained by having blood circulation throughout internals of the tusk. The only real metabolic cost is the maintenance of the tissue and water evaporation.

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  • $\begingroup$ this is not the only reason, tusks don't crack $\endgroup$ – John Nov 1 '18 at 0:18
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You can have normal teeth, just cover it with something other than lips. I'll propose something that will look aesthetically like bare tooth exposure, but I will cheat a little bit, so be free to down vote if you must.

Have them be covered on thin layers of keratin cells, just like the epidermis. Underneath the epidermis, allow a system like the lymphatics that will keep the whole tooth system hydrated. The moisturizer should be the same chemicals as saliva.

Now the problem, every time your creature chews something, this layer will be destroyed. As it gets shredded it'll release more saliva on the teeth, as the saliva vessels will be open, contributing to a better digestion and swallowing, but more importantly, a continuous stream of moisture.

If you have an herbivore, the rate which the layers must be repaired needs to be faster, as they're always chewing. That will not be too energy consuming since there's not much surface to cover. If you have a carnivore, they might get days without eating and the regeneration could be slower. You'll have to calibrate that system. Also, iguanas don't chew that much, they basically bite and swallow, keeping the layers more or less intact.

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As comments and another answer have put it, do tusks.

Tusks

Or use cheratin like a Rhino's horn or bird beaks, or chitin like spider fangs.

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In order to properly answer this question, you have to first consider the process(es) put into place to allow this.

It would seem evident that it's simply much easier and more efficient to moisturize the teeth with the lips, and retain that moisture. Considering that teeth are a fundamental part of ones health. The energy cycle would need to implement this change in dynamic if other energy was being used to reconstruct, or to protect the teeth.

In the instance of the lips, the saliva is a somewhat double-medium and the lips offer multiple functions.

So, without the lips - you actually have more issues as well that the lips help simplify. Such as disease, breathing in of air from a polluted climate, and simply not being able to really control what goes inside of your mouth.

So really, while this is the problem you're trying to solve - this problem is relative within the nature of the true solution that lips provide.

In short, it may be simplest to simply remove the teeth altogether and grind your food in a morter in order to consume it.

Or,

If you could somehow integrate iron production within the body, and produce a form of titanium/steel - that would be the ultimate solution probably.

But really, this question is much bigger than simply caring for the teeth - because the lips offer many health benefits and evolution always finds the best fit.

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Our jaws and teeth are only one solution to the problem of growing articulated cutting and grinding surfaces for biting and chewing.

Birds approach the maceration problem by swallowing gravel; this gravel fulfils the same function as molars, grinding swallowed food to pulp. It has the advantages of very low metabolic cost and easy replacement. The biting function of is fulfilled in raptors by a shark hooked beak. Bird beaks are keratinous like fingernails, bear, dog and cat claws, rhino horns and hooves. They are not as hard as enamelled teeth but they grow continuously and can be very sharp (cat claws).

An interesting question: What is the advantage conferred by very hard teeth over a softer but continuously renewed surface? Imagine a mouth full of short broad sharply tapered cat claws.

Nature tries everything, and viable solutions rarely disappear altogether. The beak, for example, also appears in squid and octopi. That teeth as we know them are so dominant implies significant advantage, but this is not a certainty; it may have been carried along with a general body plan that was highly successful.

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Typical enamel is tough and long lasting. Use it. You can always add a conveyor type system like sharks to replace them throughout the creatures lifespan. Great news for the creature. Bad news for creature dentists.

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  • $\begingroup$ I know it was a joke, but probably dentistry as we know it would never even form. I imagine the only procedure for any tooth issue will be to pull it out to promote the growth of its replacement. $\endgroup$ – Trotski94 Nov 1 '18 at 13:01

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