|Smilodon populator. used w/permission credit deviantartist Dontknowwhattodraw94 (Robin Liesens)|
|Another possibility, more classically felid. credit deviantartist Unloborgis. used w/permission (c) Sergio Perez|
All the while I knew that if I shook the tree hard enough something might just fall out... which it did and it splattered everywhere.
Time to crack this thing wide open...
And crack is the word of choice here because as traditionally depicted cracking is what sabertooth canines would be doing a lot of. Not just because of being exposed to blows from prey/competitors. Not just because of grit working away at the denticles. But because of chemistry.
Special kudos to Brad McFeeters for turning me onto the abstract of an as yet to be published work on the relationship between enamel health, saliva, and closed oral lips. Something any good dentist could tell you. Indeed it was a man who said he teaches students taking the dental admission test that presaged this very fact to me in a comment from my last post:
Please excuse the "appeal to authority" but it is none other than the American Dental Association that refers to saliva as "the bloodstream of the mouth" :
Intuitively this makes complete sense. Ever fall asleep with a stuffy nose and then wake up with a mouth that feels like a crypt? The bad breath and general oral malaise of just spending one night with your mouth exposed and dried out is but a preview of the dental horrors that would befall a sabertooth predator spending a lifetime with its physiologically expensive canines unsheathed.
Here is an excellent, easy to follow, and well referenced summary of the benefits of saliva by the European Food Information Council: Saliva - more than just water in your mouth
Of special relevance is the chemistry of saliva which is crucial in maintaining the mineral balance of tooth enamel and dentine and preventing the loss of calcium and phosphorous. I am not going to pretend to be a chemist or that I can explain it any better than they do so let me just outsource their summary below:
Essentially not having saliva - which is buffered with loads of calcium and phosphorous - would open up the crystal structure of hydroxyapatite (the building block of dentine and enamel) to chemical attack. The attacking molecule that would strip calcium and phosphorous out of the crystal structure is... water "in water the crystal would steadily lose ions form the surface and shrink". Best keep your cutlery sheathed and lubricated with saliva otherwise you are looking at some brittle canines because water is pretty much everywhere!!
Time To Stomp the "Tusk" Argument to Smithereens
Clearly I did not state my case strongly enough in the previous post that the last thing you want to compare sabertooth canines to is tusks which differ in just about every fundamental metric from sabertooth canines. Never the less you still have people chiming in "but what about fanged deer"? Please don't insult these proud and magnificent predators by comparing them to..... deer ( I jest, just a little). To be fair musk deer have continual growth of their canines and as I will explain shortly tusked deer will most likely share histological features of other tusked animals. Let me reiterate; tusks are sexo-social symbols and hence need to be displayed while sabertooth canines are not; tusks are used for not only combat but coarse gouging of sediment, bark, roots, and other gritty abrasives - sabertooth canines do none of these activities, in fact to the contrary, the fine serrations on many sabertooth canines would be damaged through excess grit; tusks grow constantly, sabertooth canines do not. These are all patently good arguments that are also essentially adaptationist in their approach. However paleontology and evolutionary theory have fallen out of love with adaptationism to some degree dismissing such arguments as "just so stories". I blame Stephen Jay Gould for this pervasive sentiment. I don't mean to kick a man when is down (literally six feet under in this case) but Gould was a little less than precise in giving us good examples of evolutionary spandrels after all. But I digress.
This just means I got to look a little bit harder and deeper than "just so stories" to persuade people that tusks are the last thing that you want to compare to sabertooth canines. Luckily enough for us there is quite a bit of information out there on tusks because ivory - the cultural trade and carvings of large animal teeth - has been a pretty important thing in human economics that people have looked at. When we investigate the histology of tusks (i.e. large exposed teeth) there are some interesting surprises...
|credit USFW services|
Notice that in the schematic above there is just a bit of enamel at the tip but the bulk of the external layer is actually cementum, followed by dentine. Now in the hierarchy of hardness scale enamel is hardest, then dentine, and finally cementum. However given that enamel occurs only at the tip where it is actually worn away during the life of the tooth it is a bit of a paradox. Why would tusks - which are subject to all sorts of rough and tumble activity - only have such a paltry layer of the hardest structure which is enamel? Not only that - it is counterintuitively cementum that covers the bulk of the exterior of tusks which is the softest of the three materials? It's a bit of a paradox that tusks disinvest in enamel but invest heavily in the softer tissues of dentine and cementum (also btw which is why ivory can be carved). The reasons I would suggest are manifold:
1) Enamel once it is formed can not keep growing, only maintained. Dentine on the other hand can be grown continuously via odontoblastic cells in the pulp cavity - which is why tusks keep growing. Cementum too can repair itself and grow continuously via cementoblasts.
2) Enamel is the hardest structure of the three but also the most expensive physiologically to grow, form, and maintain. Minerals are very precious things after all and investing so much mineral wealth in a structure that will be winnowed away is maladaptive.
3) Because the enamel in tusks is not constantly bathed in a buffering solution of saliva that maintains calcium and phosphorous balance of the crystal structure it is prone to not only physical attack but chemical. The investment of enamel in the tip of the tusk is a throw away investment. Over time it will be lost. From USFW on elephants/mammoth tusks: "Enamel is only present in the tusk tip in young animals. It is soon worn off and not replaced."
4) I will suggest that cementum and to a lesser extent dentine - having less of a solid crystalline structure than enamel and a higher percentage of other protein tissue that form it (e.g. collagen) - are less subject to chemical attack than enamel in the exposed state and offer an ideal compromise: just enough hardness to get the job done without the excess chemical attack that enamel suffers. Again, both dentine and cementum can grow constantly while enamel can not.
If this is a robust and true pattern we should see it again and again in animals that leave their teeth (i.e. tusks) exposed to the air. Through convergence this prediction should be met and these animals will disinvest in enamel on the exterior layer of the tooth and have relatively higher investment of the less mineralized and softer dentine and cementum.
And gee willikers look what we have here:
Walrus (Odobenus rosmarus)
From the USFWS: "The tip of a walrus tusk has an enamel coating which is worn away during the animal's youth."
|cross section walrus tusk. C=cementum, PR=primary dentine, SD=secondary dentine|
Also of note cementum is softer than dentine. This follows the somewhat counterintuitive pattern in having the least mineralized (softest) part of the tooth exposed to the environment as to not become demineralized. The cementum literally shielding the dentine from exposure.
Sperm and Killer Whales (Physeter catodon & Orcinus orca)
From the USFWS: "Both species display conically shaped teeth with a small amount of enamel at the tip. The rest of the tooth is covered by cementum."
Narwhal (Monodon monoceros)
From USFWS: The narwhal horn (actually a modified upper incisor): "Enamel may be present at the tip of the tusk."
From the USFWS: "a longitudinal enamel band with approximately one-half to two-thirds coverage mark the tusk's surface in the raw unpolished state."
From the USFWS: "A broad longitudinal band of enamel covers approximately two-thirds of the surface area of the tooth... The surface which is not coated with enamel displays a very thin layer of cementum... Hippo incisor crowns are covered with enamel."
|credti Quartl CC3.0 Huge Lips to Cover Huge Tips|
"Ok then, what about crocs? They leave their teeth exposed to the elements - including water - which can itself strip away calcium and phosphorous?"
All is not as it seems as well with our smiling crocodilian friends. Their sinister smile betrays a sneaky way around the problem of exposed teeth. First of all - analogous to ever growing tusks of mammals - crocodilian teeth have the ability to get replaced. Second of all crocodilian teeth show a histology more congruent with tusked mammals than the enamel rich dentition of mammalian carnivorans or predatory theropods. Their teeth actually act more like miniature tusks in that they too share the pattern of a thrifty use of enamel on the exterior.
A recent study of this very issue (Enax, 2013) provides some telling information:
Some things attract my attention here.
"Virtual sections through the tooth and scanning electron micrographs showed that the enamel layer is comparably thin."
To beat that drum again that is exactly the pattern in tusked animals with exposed teeth and opposite the pattern in animals with closed lips and sealed mouths that allow teeth to bathe in saliva and remain enamel enriched.
"The crystallites in the enamel are oriented perpendicularly to the tooth surface."
Very interesting here in that the crystal structure - being perpendicular to the tooth surface - is oriented in a manner that minimizes exposure to the outside environment. Could a very logical (and adaptationist) hypothesis be that this orientation is not haphazard but in fact that it has selective benefit in minimizing surface area exposure to chemical attack from the environment?
From the discussion section and conclusion:
The authors explain the thin layer of enamel in crocodiles which contrasts to the thick layer of enamel in mammals due to "the enamel of crocodile teeth is very thin.. because crocodiles do not use their teeth for cutting and chewing." I don't really understand why people say crocodile teeth do not cut, I mean tell that to anyone or anything that actually who has got bit by a croc. Those teeth do cut. Point taken they are >mainly< graspers but they can inflict some pretty nasty damage as well.
Never the less it is readily apparent that the authors failed to compare croc teeth to exposed mammalian teeth (i.e. tusks) which they do share a convergence of composition, microstructure, and hardness. By the way also interesting that croc teeth compare in hardness with human teeth given the high disparity in bite strength. Probably helps in preventing brittle deformation in croc teeth but still strange to think about.
I would venture to say that in the worn teeth of old crocs all of the enamel has been worn off through either mechanical or chemical wear and you are in fact looking at the dentine after the thin enamel tip has been worn away.
|any enamel left in those tooth crowns? I doubt it. American crocodile. credit Daderot public domain|
Shark teeth... well I don't even want to confuse the situation further.
The main point is that croc teeth and mammalian exposed teeth (i.e. tusks) show congruence in composition and structure that suggests a possible selective advantage in being thrifty with the amount of enamel coating the tooth. Tusk bearing animals that maintain an oral seal via lips such as hippo can coat their teeth with relatively more enamel due to the salivary benefits to enamel health.
So the million dollar question is: if sabertooth predators indeed left their canines exposed to the environment then they should show histological tooth characteristics similar to crocodilians and exposed tooth bearing (i.e. tusked) mammals such as minimal enamel coating of the tip and potentially a layer of cementum covering the coat of most of the tooth. If they were just good ol' mammals that kept their lips shut and their teeth sheathed and bathed in saliva then we should see bountiful amounts of enamel especially on the exterior of the tooth.
So which is it?
Unfortunately I can not find adequate and thorough histological work on the microstructure of all sabertooth predators. But I can find such work on the creme de la creme, el utlimo hombre of sabertooth predators Smilodon fatalis. If this predator keeps the pattern of thick enamel forming the exterior of the working tooth and not just the tip it squarely lines up with animals that cover their enamel rich teeth in big luscious lips and bathes them in a protective broth of saliva. And if this penultimate of sabertooth predators can cover its extreme canines with lips there is all the reason in the world to expect all the other sabertooth predators to fall in line.
Answer: Smilodon fatalis is a good ol' mammal that maintains lots of enamel... ergo it had big ol' lips draping over said enamel.
*Update thylacosmilids had more of a tusk like growth pattern with only a shallow enamel layer
From the paper Cementum on Smilodon sabers (Riviere & Wheeler, 2005) these depictions clearly label the cementoenamel juction. Everything on the exterior of the tooth towards the tip from this junction is enamel. Smilodon is not like a tusked mammal or crocodile but congruent with every other enamel rich mammal that sheathes its teeth in lips and saliva.
Interestingly the Riviere & Wheeler paper came to the conclusion that gingiva - in other words gum tissue - covered the root of the tooth all the way to the cementoenamel junction.
From (Riviere & Wheeler, 2005):
This interior soft tissue protection would inhibit infection, alveolar bone resorption, periodontal disease, additional tactile capability, and tooth stability. In short very consistent with the soft tissue benefits of large lips and a sealed oral cavity for tactile ability, protection, lubrication, and enamel health. It all meshes together to create a very gummy, lippy, and infinitely more adaptive vision of these predators than is classically portrayed.
|used w/permission credit LWALTERS. deviantart page LWPaleoArt|
Here is a more classically felid look that could work too.
This is for the haters - if your mind changes as the facts change this need not apply to you, move along and let it not apply to you - but if you attacked me with name calling or just dismissed my arguments as "ludicrous" then yeah, this is aimed directly at you. Can you feel that?
Call me childish, but hey, talk to me after you take a chance on something and take the shots I do. Gonna rub it in their face just a little bit.
And finally back to the yet-as-unpublished work by Robert R. Reisz that Brad McFeeters brought to my attention after the last post was published. Presented at the 2016 Canadian Society of Vertebrate Paleontology. Unpublished stuff is always "iffy" but hey if CNN is gonna cover it and the press is all over it then fair game for me I say. So far the press has put out the stock line you see in all theropod related news and somehow T. rex has to be the star of the show... all the while (predictably) ignoring the more obvious implications for sabertooth predators that this study also hints at.
I am very bolstered by the observations by Reisz and Larson on the histology of tusked mammals differing from theropods which converge with my argument regarding tusked mammals falling down as an analog to sabertoothed predators. However I would certainly encourage Reisz and Larson to look more closely at their statement "we propose that this requirement of hydration is not possible to maintain if the tooth is exposed permanently". As I discussed earlier saliva's benefit has much to do with maintaining chemical equilibrium of enamel while water that is not buffered with calcium and phosphorous can strip it away. Water has some pretty unique chemical properties (dipole moment, net negative charge) that can overtime strip enamel of positively charged calcium and phosphorous. Not saying that hydration does not play a role in all of this - it likely does - but chemical equilibria is another important aspect of enamel health. Again, remember hippos were the only "tusked" animals that have a lots of enamel on the exterior of the tusk but only because they keep them bathed in saliva.
To drive the point home one last time:
Facial tissue completely or mostly sheathing the upper canines as is corroborated by ALL extant terrestrial mammalian carnivorans and is the best null hypothesis; exposed and constantly growing tusked mammals use their non-serrated tusks for coarse hacking, chopping, digging, combat and display and are an inferior analog to sabertooth canines and need not be considered as they fail in comparison along nearly every metric; exposed toothed mammals (e.g. tusked) and exposed toothed crocodilians have differing tooth histology from sabertoothed predators*; unlike smilodon these animals have minimal enamel laid down at the tip of the tooth often worn away during the animal's lifetime; exposed toothed animals have an exterior tooth largely covered in cementum or dentine, both of which can grow constantly unlike enamel; saliva plays a crucial role in the dynamic chemical health of enamel; saliva provides a buffering role in hydroxyapatite crystal as saliva is enriched with calcium and phosphorous preventing demineralization of enamel; this salivary coating is only maintained in closed mouth (e.g. lipped) animals; the clouded leopard which has long canines comparable to many sabertoothed predators covers its canines completely; all five radiations of sabertooth predators display osteological evidence of protective sheathing on the lingual inferior aspect of the canine via a mandibular flange - a logical evolutionary inference is that protection for the superior labial aspect of the canine was also selected for in the presence of a large fleshy upper lip; the presence of this large fleshy upper lip is corroborated osteologically by the relatively large infraorbital foramen found in all sabertooth predators; this large infraorbital foramen supplies the blood and nerve supply to an extremely large and sensitive "nerve pad"; the extremely innervated nerve pad provides tactile support to make precise and crucial placement of canine entry for bite as well as early warning for violent torsional twisting of prey that could damage/snap canines; such tactile support would be diminished or non-existent in sabertooths depicted with modest sized upper lip region as this area would be scrunched away from the bite area when the mouth is opened and it would be the vulnerable canines that would "feel out" where to bite; extensive gingiva possibly reaching up to the cementoenamel juction protected, sheathed, and provided tactile support to the canines; large lips, gingiva, and supporting nerve pad evolved in lock step with increasingly large canines and forequarter strength for maximum safety and efficiency in these highly precise yet vulnerable predators.
Your puny lipped sabertooth kitty is not only smashed... it is curb stomped!!
Destroy all those lines of evidence systematically and I will disavow this hypothesis. Come at me bro.
No we don't need a sabertooth mummy, we already have more than enough to sensibly conclude LARGE LIPS FOR THE WIN!!
*Update thylacosmilids had more of a tusk like growth pattern with only a shallow enamel layer
Special thanks to the work of Jaime Headden and of course this post has the spirit of "All Yesterdays" (Naish, Witton, Kosemen) smeared all over it. Whether or not these researchers agree with my conclusions this work would not have been compiled without their existing works, thoughts, and efforts.
Also special thanks to the deviantartists who took a chance on depicting big lipped sabertoothed cats. I fully believe that your risk taking will end up on the right side of history.
P.S. This was just the warm up. I'm coming for ya' next lizard lipped theropods...
Ivory Identification Guide. USFWS. https://www.fws.gov/lab/ivory_defined.php Natural Ivory. https://www.fws.gov/lab/ivory_natural.php
Saliva - More Than Just Water in Your Mouth. EUFIC. http://www.eufic.org/article/en/artid/Saliva-more-than-just-water-in-your-mouth/
Enax, J., Fabrittus, H.O., Rack, A., Prymak, O., Raabe, D., Epple, M., 2013. Characterization of crocodile teeth: correlation of composition, microstructure, and hardness. Journal of Structural Biology 184 (2013) 155-163 link
Reisz, RR, Larson D. (2016) Dental anatomy and skull length to tooth size rations support the hypothesis that theropod dinosaurs had lips. unpublished abstract 2016 Canadian Society of Vertebrate Paleontology
Riviere, HL, Wheeler, HT. 2005. Cementum on Smilodon sabers. The Anatomical Record. 7 June
2005. 285A 634 - 642. link