Tyrannosaurus rex - as both a scientific and cultural phenomena - is imbued with both values as goes it's appearance. And humans - as culturally adapted critters - try as we might can not decouple the two as stringently as we might hope in reconstructing this beast.
This post is going to be necessarily both a cultural and scientific deconstruction of Tyrannosaurus rex facial appearance focusing on two main aspects. As such we must wear both a cultural and scientific hat here in an attempt to de-clutter and re-imagine a creature undeniably real but also imbued to an obscene degree by cultural baggage, gender notions, and aesthetics. This treatment will also hold implications for many other extinct theropods to greater or lesser degrees. Where you fall down in the end as goes appearance - will be a blend of science, aesthetics, and culture - which is as how it has always been with these guys (i.e. extinct animal restoration).
The beginning (and really the end as well we shall see) of T. rex facial reconstruction must really start at ground zero - the skull. Just take a second to breath it all in... it really is structurally and aesthetically a work of art and powerfully built symmetry.
|AMNH 5027, A.E. Anderson Public Domain
Now as appealing as this bony synthesis of power and grace is we must concede that the bony appearance of an animal's skeletons is often times not a direct reflection of an animal's outer appearance. In T. rex the skull's beautiful, symmetric, and aesthetically pleasing appearance may have a lot more to do with being influenced by a uniquely and profoundly strong set of muscles than it being a true reading of it's life appearance. T. rex has evolved to deliver crushing bites and sustain stresses in multiple force vectors. It is built to give and take a licking and keep on ticking.
But just take a second to bathe in the below photos...
|Red Tegu photos credit from Helen Zhu.
As the above youtuber puts it these prominent muscles in the tegu form some very voluptuous "neck boobs" on the side of the jaw. Crocodiles also feature said neck boobs that form the dominant jaw closing group of muscles in their particular jaw apparatus (they don't put a lot of muscles on the top of the head for concealment reasons).
Now to what extent and how much T. rex, and other tyrant lizards - or even theropods in general had prominent pterygoideus muscles is equivocal and loaded with uncertainty as is the muscular reconstruction of any extinct animal. But as one of the largest and in fact the strongest biting terrestrial vertebrate T. rex likely had some rather prominent "neck boobs", at least more so than is almost always depicted.
Here are some pics on how the pterygoideus muscles may have attached in T. rex and Majungasaurus:
|Pterygoideus group in purple (from Bates & Falkingham, 2012)
|Pterygoideus muscles = mPTv from C.M Holliday 2009
|credit Ira Block/ model Brain Cooley
One avenue of looking into how much muscle T. rex was packing there is to compare the size of this muscle in juvenile versus adult crocodilians. As is seen in these dissections of immature crocs the size of the pterygoideus is fairly modest in relation to the animal's whole head. This changes with adulthood when we the grotesquely flared pterygoideus of large crocs.
If we peruse this nice color schematic of the jaw closing muscles of the American alligator (Alligator mississippiensis) it is obviously that it is a youngster (big eyes) and it does not have quite the degree of pterygoideus flare or "neck boob" as the boomer sized adults have. From Holliday et al 2013 Plos One A 3D Interactive Model and Atlas of the Jaw Closing Musculature of Alligator mississippiensis)
|credit Holliday et al 2013 pterygoideus in orange/red
|Note prominent jaw muscle "neck boobs" CC2.0 credit fvanrenterghem
Following from this maybe... just maybe... T. rex had to grow some quite impressive jaw closing pterygoideus muscles to do it's job amply - after all it was certainly no light weight.
Again my main point in all of this is not to convince you one way or the other if T. rex had red tegu sized neck boobs or something more moderate. Chances are it had something prominent there and given it's size and proclivity for a particularly strong, tenacious, and evenly distributed bite the pterygoideus would and should be apparent in life.
But do restorations give this muscle enough flare, weight, and voluptuous girth? I would say no and the reason I think it is underrepresented I suggest is cultural.
We have all been smitten, bedazzled, and suffered a bit of a man-crush on T. rex's hypermasculine and all too handsome jawline.
You heard me right, the jaw line of T. rex features a jaw line that any A-list Hollywood actor would kill for. The human male jawline is a perhaps seldom mentioned but very distinctive and sought after trait in male sex symbols and is also ubiquitous across... hypermasculine male superheroes.
|credit Elaine Thompson AP
And I did not even mention the strong chin of T. rex which adds a whole other layer to the hypermasculine attributes we are drawn to in the T. rex skull.
|John Gurches now iconic T. rex. Note prominent chin and jawline well displayed. Archetypal hero stance
But what if the strong jawline of T. rex - seemingly chiseled out of tooth, scale, and hypermasculine hero worship - was cloaked by layers of feather, flaps of skin, or as suggested earlier in the post by particularly voluptuous "neck boobs" and jowels giving the animal a decidedly softer, rounder and perhaps feminized appearance? Things could change a lot as goes appearance. The skull of T. rex is optimized for muscular performance, not to appeal to the cultural and sexual biases of extant hominins.
Astute readers should note that this cultural and anatomical analysis of T. rex appearance has implications for how other theropods are restored to greater or lesser extents it is just easier to make the point with big ol' sexy rexy.
For the next part of my analysis I want to talk about a soft tissue that regular readers of the dino-blogosphere should be well aware of and which is quite possibly one of the most contentious issues in theropod soft tissue restoration: lips. Yep, I am going into lips.
Instead of making a long and lengthy review of this issue I am going to outsource some of the background as the issue has of course been gone over extensively by Jaime Headden - Making Lip of It, Support For a Lipless, Cheekless Dinosaur World, Cheeky Commentary on Ornithischians & others
To summarize my views going in and add some arguments that probably need reiteration:
Theropods most likely had lips. I also think that these lips (contra most depictions including my own) would have mostly or even completely covered the teeth. While the notion of a croc like skin sheathing the head of theropods has been argued, probably most vociferously by Tracy Ford, what I find lacking is that fully exposed teeth and oral cavity would put a lot of burden on the animal in terms of water loss via exhalation. Here it is worth noting that terrestrial predators keep a pretty tight seal on the mouth. Aquatic animals, not so much - sharks and crocs come to mind but there are many more examples of exposed snaggle toothed aquatic animals. There definitely seems to be a bit of bias for aquatic animals having more exposed oral regions than terrestrial.
One of the critiques for lower lips in theropods is that the preserved skulls for many theropods suggest that the upper teeth would penetrate into the lower lips and gums of said animals cutting such features to shreds. Check these pics out. However one of the best rebuttals to this argument was done by Tyler Keillor in the chapter Jane in the Flesh: The State of Life-Reconstruction in Paleoart from the excellent book Tyrannosaurid Paleobiology. Let me quote Keillor directly as I don't know if this argument gets enough exposure:
|What's my name? extreme jaw closure credit James St. John CC2.0
On Jaw Closure (pp 160-161):
"...fossilized theropod skulls have been found with the jaws tightly closed. While some artists have used this as the living animal's closed-mouth pose, I offer another interpretation. The jaws that are tightly clenched may show a postmortem deformation, akin to the "death curve" seen in the axial columns of many vertebrates under certain conditions. As tissues desiccated and shrank in the dead animal, the massive jaw closing muscles may have shortened and pulled the jaw tightly closed, more so than it would have been in life. Punctures in the palate of Sue occurred after death, when the jaw's dentary teeth were closed further than they had been in life (Brochu 2003). In skulls that are preserved right-side up and resting on their jaws, overlying sediment compaction after burial could further crush the jaws closed in dorsoventral compression (Bakker et al. 1988)"
I would actually go a little further than Bakker and suggest that irregardless of whether the skull is preserved right side up compression from multiple angles could compress the jaw shut. Why? Because the earth shifts and moves all the time not just from top to bottom.
Furthermore Keillor draws attention to a rather simple and seldom mentioned critique of the "extreme close mouthed" interpretation which is bone on bone contact with no room for soft tissue. When a skull was put in this pose (extreme close mouthed) Keillor noted "contact between the quadrate and angular, the jugal and extopterygoid come close to touching the surangular, and the dentary teeth contact the palatal bones and medial maxillae (pp 162)".
Keillor also decides that in the situation of tyrannosaur lips he must look beyond the extant phylogenetic bracket of modern birds and crocs due to their unique specializations (beaked and aquatic) not offering the ideal evolutionary context for a toothed terrestrial predator. And here I agree the EPB feels a little lacking. Keillor expands the bracket to consider large terrestrial predatory lizards i.e. the komodo dragon as the ideal model for looking at the type of "lippy" tissue present and the ideal neutral position of jaw closure. Essentially this lizard lip type arrangement allows a tight seal when closed - to enhance "sniffing" presumably - as well as protect the oral cavity and teeth from dehydration and abrasion. Yeah it is kind of a "just so" argument but keeping your cutlery sheathed and moist probably has some fairly obvious benefits to it in drying terrestrial environs full of abrasives.
Probably one of the best representative views of the lizard lips hypothesis is that done by the promising game Saurian. Picture below borrowed from said development team:
|credit Saurian development team
That the upper teeth went below and lateral to the dentary is corroborated by the path the nutrient foramen follow on the dentary.
Below one of my favorite skeletal mounts of T. rex because it eschews the overdone Rex vs Triceratops battle. You can see quite clearly how the nutrient foramen on the upper jaw come right to the edge of the alveolar margin on the upper jaw (maxillae). But if you trace the path of the nutrient foramen on the bottom jaw (dentary) you will notice that while the nutrient foramen come up against the edge of the teeth in the anterior and posterior they take a noticeable dip towards the middle of the jaw - which corresponds directly with where the longest teeth from the upper jaw would be presumably in closure.
|Houston Museum of Natural History. credit Daderot CC. Edmontonia & Wyrex
|note how nutrient foramen take a dip along bottom margin of jaw corresponding to longest upper teeth
Not so fast Mr. Lizard Lips...
To prime you for my argument please take a good long hard look at another readily available wiki pic of ol' sexy rexy that is taken from the front. If you look into its orbits and squint just a bit you will see it....
|credit ScottRobertAnselmo CC3.0 "Sue"
|Cave Bear. wiki commons
To clarify I should stipulate that I am not suggesting we consider muscular mammal style lips in T. rex and other theropods but basically loose and hangy jowls of flesh (think condor cheeks but not continuous across mouth).
"Too far outside the phylogenetic bracket"
- Well when we infer lizard lips we are already outside the phylogenetic bracket... so.... and it is a bit telling that many seem to have no problem inferring fleshy mammalian (albeit non-muscular) cheeks in ornithischians but how do dare you consider fleshy, hangy lips in a theropod!!
Or my favorite: "I find them unlikely"
- Like that is all you have to say on bulldog style lips to not consider them? No support at all for not considering, just saying "unlikely"...
I suspect that there is more to it than this and people don't want to consider bulldog style lips because well... aesthetically and culturally it is not what we might like or even feel comfortable with on theropods and especially ol' sexy rexy. Because as I have been saying all along these are as much cultural creations as they are objective scientific animals. Both academics and fans of theropods and T. rex have ignored the idea of hangy, floppy lips in these animals because... for the most part they just seem to laugh and ridicule the idea away.
So as opposed to trying to decipher the type and extent of "lip" in these animals via skeletal traces which I think is a proposal loaded with potential pitfalls - for example how do you account for the amount of stem cells that might just grow loads of skin and tissue (?) - I am going to take another line of inference. I will be exploring lizard lips versus jowl style hanging bulldog lips in terms of relative adaptive benefit. This is admittedly not going to seal the deal either way because there is some subjectivity involved as well as the fact that animals don't always evolve into "optimal" or "perfect" organisms. However I do think comparing the relative benefits of either style of lip in lieu of really being able to eliminate either possibility is a valid form of inquiry. Again it might not seal the deal but it may open up some minds to possibilities...
The Smell Situation
There is one important distinction between lizards and theropods/mammals. Lizards primarily scent the world via their Jacobson's organ - the vomeronasal organ. In all lizards and snakes this organ is present and in reptiles that scent their prey the distinctive fork tongue is the tool that is used to gather sensory input from the environment and put into contact with the vomeronasal organ. Lizards and snakes - because of this system of sensory input don't need loose lips to let information from the environment into their oral cavity because their forked tongue does all the work for them. For lizards and snakes having loose lips to let chemical cues into the oral region is redundant and an unnecessary potential source of water loss. It is true that certain mammals have this organ but in general it is substantially reduced compared to reptiles. I can find no reference to this organ in birds and crocs and it is likely absent or extremely reduced in dinosaurs.
However, as anyone who has had or watched a baby explore its environment via its mouth can attest, having a relatively open and exploratory labial region may carry significant adaptive advantage. Animals can taste things and loose lips that can be pressed into various substrates can help pick up and adhere odors that can then be picked up by the nose or tasted. Having loose jowl type lips in this scenario of helping to scent or taste things would hold substantial adaptive advantage over the tight sealing lizard lips associated with reptiles that explore the world via their fork tongue.
The bloodhound dog breed - a specialized scenting breed of dog - has low hanging ears that trail along the ground, loads of wrinkles on the face, dewlaps, and very prominent jowls. All of these features have been suggested to stir up, trap, store, and distribute sensory cues for the nose.
|Bloodhound. credit Superfantastic CC2.0
|Hellhound Rex by Duane Nash
Tactile Input, Prey Handling & Delicate Nipping
Another benefit of having more open "bulldog" style lips versus closed lizard type lips is the potentially higher amount of sensory data collection points available to "feel out" things. This ability comes in handy in terms of how to react and fine tune grip/bite strength/position when engaged with struggling prey. Having lips that can sense and anticipate muscular twitches of struggling prey is a great advantage to have as it allows the predator to fine tune its own biting and avoid suffering undue injury. Lizard lips, exposing much less sensory surface area than bulldog type lips, offer less data collection points to make these rapid adjustments. Again, adaptive advantage goes to the bulldog lips.
I know that there is something about cats I should be saying here I read somewhere. That when biting struggling prey the sensitive lips and whiskers are able to detect and collect information on the prey. Ok found something I can give to you from The Other Saber-tooths: Scimitar tooth Cats of the Western Hemisphere:
"large scale movement of the prey relative to the predator can be constrained by powerful forearms, but fine scale adjustments in upper canine placement require tactile input from whiskers, lips, and nerves in the periodontal ligament and pulp cavity."
Granted T. rex was probably not making the precise bite adjustments of a felid but still could be of use.
The "incisiform" front dentition which allows delicate scraping of meat off of bones is also potentially aided by exposed and extensive lippage. Having a bit of a blind spot here and lacking sensory tactile front paws T. rex could feel out where the trace bits of meat are on a bone and better position its incisors for delicate nipping. Thin and not too supple lizard lips would be less efficient in these regards. Or whiskers...
Take home point: precise biting/nipping with heterodont dentition is associated with loose lips in extant animals and not lizard lips.
Although humans have bastardized this traditionally violent signal of aggression into something called a smile the ability to bare your teeth in a visible threat display is a potent and well understood universal across the animal kingdom. The lizard lip model might allow the teeth to show a little bit and maybe not at all in some theropods but in the bulldog lip model the upper teeth are potentially totally obscured ( I doubt that they had facial muscles to enact a sneer) but the bottom teeth and gums would be totally exposed when the theropod opened its mouth just slightly, creating a startling and disturbing visage of exposed gums and teeth. Once again, adaptive advantage goes to the bulldog lips.
Now you would thing that the tight fitting lizard lips would hold a substantial advantage over the more loose and draping habit of bulldog lips there might be more than meets the eye. Mucous could be particularly viscous inhibiting loss. The gap between the lower two mandibles forms a natural trough so that saliva would not be spilling out at a high rate anyways. And camels. Yes camels. If there was ever any animal that should be outfitted for water conversation among mammalia it is camels. Yet camels are noted for having particularly loose and jowly lips. And also, not coincidentally I suspect, camels are noted for a particularly good sense of smell. So whatever evolutionary disadvantages are incumbent upon having a loose set of lips in the dry desert they are not profound enough for camels to evolve a tight set of lips - or at least tighter fitting than the pics below attest.
Tight Seal For Sniffing
Sort of a toss up. Reptiles with tight fitting lizard lips should get a pretty tight seal that will enhance sniffing. You would think that the more loose and drapey lips of mammal sniffers would be inferior but it does not seem to stymy them much as they seem to be doing just fine in terms of olfactory prowess. I don't see why such an arrangement in theropods would not work as well for sniffing.
Probably a mention of an extensive secondary palate in T. rex is worth mentioning here...
On a related note I did some research viewing of how bears - the penultimate scenters - actually do their sniffing. What I found was surprising and interesting and I don't know if it has been explored further. What I noticed is that sometimes when a bear is intently sniffing it will repeatedly open and close its mouth. Is it trying to taste the air? Or is it trying to suck air in closer to the nose by opening and closing it's mouth creating a vacuum. Check it out:
Check out the same sniff then open and close mouth behavior in this video as well:
And this brown bear does it too:
Anyways if the bears in these videos are trying to draw air into and closer to the nose I can easily imagine a T. rex doing a similar sort of behavior. Or maybe this behavior has something to do with a (reduced) Jacobson's organ?
Another interesting bear fact I learned in making this is that the lips of bears - apart from all other carnivorans - do not attach directly to the gums. Presumably this is to allow fine tune manipulation of small objects - such as plucking a single berry off a bush. Interesting to think how omnivory and the quest for high quality food items might engender "prehensile" appendages (I am looking at you ankylosaur tongues).
There is no terrestrial predatory tetrapod alive today that shares the suite of characteristics that T. rex has and which also has tightly sealing lizard lips. Asides from the vagaries of what can be gleaned from being one bracket closer on the subjective extant phylogenetic bracket in choosing the Komodo dragon as the best analog I would suggest that we look at the complete adaptive package of T. rex /tyrannosaurids and find the best and most comparable extant analog. Tyrannosaurus rex and other tyrannosaurids line up more closely with mammalian predators in terms of adaptive features including; a dominant nasal olfactory apparatus; heterodont dentition with precise "nipping" incisiform like front teeth; forward facing eyes; high metabolism; and an extensive secondary palate. I suggest that we have been more than a little hoodwinked by a serrated toothed, poisonous, scaly faced, fork tongued, and lizard lipped trickster in the Komodo dragon that fails compared to mammalian carnivorans in terms of nearly every metric listed when comparing tyrannosaurid facial anatomy, especially in terms of extraoral tissue (i.e. "lips").
Let the saliva spray!!
|HorridRexDarkerSepia by Duane Nash
|credit Carli Davidson
As I alluded to earlier in the post although I concentrated on T. rex this inquiry into lips may have implications for many theropods to varying degrees. Certainly I would hedge my bets towards more of the jowly type of lips in olfactory dominant tyrannosaurids and dromaeosaurids. There is room for nuance in many of the other lineages of theropods though; with variation ranging from more of the lizard type arrangement in some theropods up to this more jowly visage with all the various factors I mentioned in this post coming in to play: how important is scent? tactile prey handling or precision biting important? threat display important?
I also did not mention feathers in this post as this issue is taken up very well in this youtube video:
Feathers can be on top of skin, scales can be on top of skin but feathers and scales can not exist on top of one another.
And I am under no illusion that this post will overnight cause a whole scale reevaluation and overhaul in how we depict theropods. I fully expect most to adhere to the lizard's lip paradigm as that is what classic phylogenetic bracketing dictates. But in the end I find the lizard lips adaptation adaptively inferior to the bulldog lip adaptation. Ironically I have old images that still show lizard style lipped theropods which I still plan on using too... funny thing is I really wanted to concentrate on theropods other than T. rex but this is where my questions took me!!
"A Long habit of not thinking a thing wrong, gives it a superficial appearance of being right, and raises at first a formidable outcry in defense of custom". Thomas Paine
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