Tyrant Tantrums Part II: Skin'n'Feathers'n'Scales

When I first made my "Pachyderm Rex" drawing, I was quite proud of it as I looked into the creature's facial covering. However, later on, it seemed that it wasn't up to date with the current info. Several changes were needed to make it up-to-date, and those changes will be addressed in this post, as well as something I have been thinking about for quite some time.

So, without further ado, let us once again take a look at the appearance of the Tyrant Lizard King.

The first thing I want to get started one is, of course, those maxilla dents seen in many a rex.



Those depressions in the maxilla are quite odd, and I was puzzled as to what they were. I originally thought they were naked skin that had thickened due to their sorta rough texture. For a decent amount of time, I've been trying to find out as to how those dents may have been, and I think I have found the answer, but that will be addressed in a later post.

For now, I will focus on a part of the skull that has been intriguing me for quite some time. We are talking about the massively wide, back end of rex's skull, especially on the jugal.

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That big ol' boxy backside of rex gives this creature its signature bite and glare, but I bet none of you have noticed the big protrusion on the jugal.

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That giant protrusion isn't a sign of taphonomy, other adult tyrants have this, including Stan, the AMNH specimen, Samson, Wankel, Peck's Rex, etc. Basically all adult rex specimens have this protrusion. The wide and thick jugal suggests the skin there was rough and tough, but what animal alive today has a similar structure with a similar texture? Of all things, a stickin' suid.

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Looking at this skull, you would think that it would have the typical hog head, but with rougher skin. That is where things get weird my friends.

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Yep. That skull belongs to a male Giant Forest Hog. The large protrusions support large structures of thickened skin that comes in handy when defending against predators and their own kind. With this thick skin protrusion coming out of the sides of pig heads, it would be interesting to see tyrannosaurids restored with this large facial protrusions coming from this part of the skull.

But even T. rex's jugal bulge has competition. Among tyrannosauridae, it's rivaled by its cousin Tarbosaurus

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While the largest individuals are almost half the mass of the largest rexes, this carnivore has some pretty big jugal bones once it hits adulthood. It rivals the rugosity and size of some of the biggest rex "jugal protrusions". Such massive jaws would deliver powerful bites, and thick skin around the jugal area could have been useful in deflecting these bites. The thick skinned facial covering could have also been a display feature, but in the way of recognizing who's above you or below you rather than displaying for the opposite sex, since it seem all adult rexes have this "jugal protrusion".

Now, we get to the part of rex's integument that everyone seems to be arguing about. Yep, I'm stepping right into the hornet's nest in this case: The case of feathered tyrannosaurids. This topic will last for the remainder of the post, and we will be taking a look at all the evidence in this debate. I admit, there is probably gonna be some parts that you may disagree with me on, but just bear with me until the end. Of course, before we start, I would like to link Mark Witton's post on the subject, and I agree with many of the ideas and claims he puts up.

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So, the whole argument started with the description of Bell et al.'s paper on the scale impressions of tyrannosaurids. First of all, I want to say that its a pleasure to finally have these impressions published and described. Up until now, all we had to go by are rumors, but now we have some good ol' hard science.

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Now, onto the paper itself. The study describes that the scales come from many regions of the animal, and so suggest that tyrannosaurids were mostly scaly, integumentary wise. They then suggest several hypotheses as to why tyrannosaurids would loose their feathers and replace them with scales, such as warmer climates, higher rates of activity, or because they were already retaining heat as they got larger and don't need their feathers. A lot of people seemed to have gotten upset at this study since it suggests tyrannosaurids were completely scaly. A lot of people seemed to have made a lot of defenses for a heavily feathered rex. Many people seem to argue that they are caused by taphonomy and not even scales at all. Others suggest that it doesn't change a thing, or that the scale patches are extremely tiny and don't eliminate a mainly fuzzy pelt. Well, I've been digging deep into the whole issue of feathers, scales and naked skin, and I have come to my own conclusions as to the extent of feathering on rex and its cousins, why they lost feathers, and when they lost them, and perhaps you might be a bit surprised.

First off, we will be talking about potential feather distribution on rex and its cousin, Tarbosaurus. Let's start of with the tail of the animal. For a long time, it was thought that the scales were located at the base of the tail, suggesting that the scales were on the underside of the animal's body. Many people still think the same and show this in diagrams of the skin impressions preserved on rex.

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However, according to the paper, they found at least 10 patches of scales on three caudal vertebra. The scale patches were all over the vertebra, with some on top of he vertebra's "spines", suggesting that the entire tail was scaly, instead of just the base of it. So extensive feathering on the tail seems unlikely.

Now, for the hips. At least one scale impression is known from the hip region of the animal. The placement of the scales on the hip exactly is on the backend of the ilium. This suggest bare legs and hips, something that its Early Cretaceous cousins lacked. As to why they would evolve naked legs, I will come back to.

Next, there are the neck impressions. This is where things get complicated, since the paper doesn't really specify where the scales are placed on the neck. They could be on the ventral, lateral, or dorsal part of the neck. We don't exactly know, so for now, its down to some speculation. You could put feathers on the dorsal and lateral parts of the neck if they are ventral, but its just as possible if they are from the dorsal part of the neck.

So with these impressions in mind, we know that the tail, hips and legs, and some or all of the neck, would be mostly devoid of feathers. Based off of close relatives, like Albertosaurus and Daspletosaurus, the belly would have also been quite scaly. That leaves the back, arms, and flanks to be potential feathered in the giant tyrannosaurines at least. These parts of the body don't have any feathers preserved, but it is plausible they were covered in feathers. Of course, how dense the feathering should be depends. However, its probably not as dense as some renditions show it to be.

Now, a lot of people bring up the whole "but feathers help cool animals down". Well kinda true to an extent. Feathers help block out solar radiation and heat. However, they are very good at keeping heat in as well. Modern ostriches suffer the problem. They live in very hot environments, and while their feathers are good at keeping out the heat from the sun, their feathers trap a lot of their internal heat, and this could lead to overheating if their temperature gets to a high enough point. That is why they have evolved bare necks, flanks, and legs, which help shed heat. They also flap their wings to release any excess heat. Now, you might be saying this.

"But muh emus"

Yeah, emus have been shown striding out into the sun when kangaroos are lounging out in the shade. Well, I should mention that emus are considerably smaller than ostriches. Male ostriches are a a third taller than male emus, and more than 3.5 times heavier. This would probably explain the difference in feather amount in the two. Even then, emus do have a somewhat bald head, with several feathers just poking out, so they too are dumping out heat, albeit on a smaller amount of their body.

The fact that many modern birds are already having trouble with heat at their size, one needs to wonder what happens to a theropod that crosses all these limits by multiple magnitudes. While rex didn't live in an environment as hot as emus or ostriches, it did live in a warm climate, with temperatures averaging at around 20 degrees celsius in the upper layer of Hell Creek. Being so huge, and perhaps having a high metabolism, rex was probably in danger of overheating if the feather covering was too dense, like say that of an emu or an ostrich. Now, I know what you're gonna say. You are gonna bring up Andrea Cau's post on how a 6 tonne rex has a metabolic rate similar to a 1 tonne mammal and a surface area MUCH greater than an elephant. There is also a paper suggesting that rex had a metabolism similar to a 800 kg carnivorous mammal.

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First thing I want to address is that Cau uses a 6 tonne rex for his model. While that's not wrong, it doesn't explain the surface area of a 7, 8, or even 9 tonne rex. Luckily estimated the surface area of a rex in that size range. It suggests a total surface area for Sue to be perhaps around 70 square meters or more, much larger than an elephant's surface area (though African Elephants can get heavier, and of course, they are quite wrinkly). The long tail and tapering body do disperse heat much better than an elephant's body, which increases the threshold for loosing fuzz (though not completely eliminating it) Second thing I want to mention is that Cau uses only elephants. Elephants have a really low surface area:volume ratio, which explains why they aren't built to dissipate heat. Cau doesn't seem to mention any other giant herbivores, like rhinos or hippos or giraffes in his analysis. It would be interesting to see how these other giant mammals stack in for comparison against rex. It possible that the giraffe may be closest in terms of surface area, though more research is needed. The third thing I want to mention is that measuring the metabolism of an extinct dinosaur is hard, and any assumptions should be taken with a grain of salt before jumping onto them. While it seems obvious rex was probably somewhat warm-blooded, exactly how much so is still uncertain, so yeah, any metabolic studies should probably be heavily scrutinized before people jump on the bandwagon. Lastly, Cau says "1 tonne mammal". I don't know about you, but I find that quite vague. There are many 1 tonne herbivorous mammals, all of which range hugely from almost totally naked to furry.

"But muh air sacs" 

Yes, air sacs do help in cooling the body of birds. It would probably give T. rex a higher threshold for loosing feathers, but it doesn't eliminate the problem. T. rex is just as pneumatized as many modern birds, perhaps less so. Given this, the air sacs were probably proportional to its size as modern bird air sacs, which, while helpful in dumping heat, isn't as efficient as air sacs on a dinosaurs of smaller size.

So, while air sacs and feathers help in thermoregulation, they can only go so far, and the surface area of rex probably decreased as it aged, suggesting they became less capable of dumping heat more efficiently. There is the fact that mammals have an insulating layer of brown fat, which archosaurs don't have. This, along with the air sacs, would probably give a larger amount of freedom to theropod dinosaurs when it comes to loosing filaments. However, this doesn't eliminate the problem. With all of this in mind, the image of a rex with a dense amount of feathers in places we haven't found scales in yet is somewhat doubtful. Large tyrannosaurids like T. rex, Tarbosaurus, and Daspletosaurus may have had a "light" covering of feathers, perhaps akin to a buffalo or a sumatran rhino, though we have no sure way of knowing. So extensive, but not densely packed, feathers is the most likely option for tyrannosaurines, and of course, integuments can mix. Sparse feathers could have grown between the scales of rex, perhaps on the tail, hips, and neck, so even extensively scaly parts of the creature could have harbored some feathers in life.

Now, while we have a rough idea as to the integument of tyrannosaurines, one must wonder why would tyrannosaurs want to loose feathers? What is the gain of loosing an integumentary feature? Well, as noted above, it has been said that the loss of filaments may be either because of gigantism, temperature, more active lifestyles, replacing them for more colorful scales as display structures, or simply because they didn't need them. Perhaps the most likely of these hypothesis, in my view, is the more active lifestyle. Tyrannosaurids seemed to have lived in more open environments than their ancestors and were dealing with bigger prey. To chase their prey, they had to run after it, and running can increase an animal's body temperature, which is quite bad for an animal covered in dense feathers across most of its body. Yutyrannus, seems to have been the exception, but it lived in more wooded and colder environments than its later cousins, and was smaller in size, which would explain the huge amount of feathers. However, if you were to get rid of your super dense coat in favor of a less dense one, and perhaps even better, remove feathers from some areas completely. We see this modern birds like ostriches, and even in rex's scale impressions themselves. The scales indicate that the entire tail, hips, and legs were pretty much devoid of any extensive coat of feathers. These parts of the body can be used as heat dumps, releasing any excess body heat from the animal when chasing after prey or tackling it to the ground. These intensive bouts with prey may explain the rugged lifestyle of rex and perhaps some of its own bodily physique (more on that in another post). In fact, a few stray filaments sprinkled across the tail and hips in small amounts may help the process even more, since it turns out some stray filaments can draw heat away from the body better. This is seen in modern elephants. 

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Tyrannosaurids were also getting up close and personal with prey. This explains the fact that many seem to have injuries associated with taking on very large prey. Such high-energy bouts would easily produce a lot of excess heat in the animal, and with that in mind, large patches of naked skin or feathers would make sense.

So tyrannosaurids may have lost most of their feathering potentially because they were more "active" and were in more energy-fueled bouts with their prey, but when did tyrannosauroidea started loosing their feathers? The paper above states they were switching to a more scaly integument at around the Aptian and Albian stages of the Cretaceous. However, I wouldn't see many of the tyrannosauroids living at that time loosing their feathers for scales gradually. Xiongguanlong is a tyrannosauroid with a long snout, but wasn't really living in an environment that would suggest loosing your feathers (forested with rivers), and at around the size of an ostrich, it probably didn't have a dissimilar feather covering than most tyrannosauroids. Santanaraptor lived around this time, but the known specimens are juveniles, so we can't really tell how big the adults were. They too weren't living in an extreme climate, forested environment with mild temperatures, nor did they reach huge sizes. Perhaps tyrannosauroids started switching feathers for scales later in their history. Perhaps a good candidate for when tyrannosauroids started loosing feathers is Alectrosaurus.
 
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Alectrosaurus was a tyrannosauroid living in an tough environment. It was found in place that would have been an open desert. Temperatures would have been quite high. This perhaps, suggests tyrannosauroids lost their feathers as they moved in hotter environments. Another thing to point out about Alectrosaurus is that it has quite long legs for its body size. In one study about theropod speeds, Alectrosaurus' came out as one of the faster theropods, only behind several smaller coelurosaurs and juveniles of T. rex and Gorgosaurus. This most likely suggests that it was built for running, which puts even greater pressure on dumping heat. Perhaps Alectrosaurus was less feathered (or at least less densely feathered) than even the depiction above, though that is up to speculation.

So with all this stated, from size, to air sacs, to the evolution of feather loss, where does that leave the giant tyrannosaurids of the Campanian to Maastrictian? Were they scaly, or where they covered in feathers. Well, while a scaly rex is plausible, we would probably expect some filaments on rex, as they ancestral to coelurosauria. Even then though, a densely rex might not work. Despite the thermoregulation of feathers, air sacs, and the lack of brown fat, its clear that these aren't enough to eliminate the problem of loosing feathers, as we already know tyrannosaurids lost most of their feathers on the tail, hips, leg, and neck regions. So, while a feathered rex is definitely possible, the feathers might have only been limited to the back, flanks, and arms (with maybe some stray filaments on the tail and leg), and the feathers would probably be less dense than most depictions show them as. With the skin impressions of rex and its close cousins, the cooling systems of birds and mammals, and the environments that tyrannosaurids had to endure, it seems likely that tyrannosaurids seem to have swapped feathers for scales for the majority (but perhaps not totally) of their bodily integument....

Or did they?

Now, I've been calling those scales on tyrannosaurids scales for the entirety of this post, but I was calling them the wrong thing this entire time. For all of those who are versed in some field of bird integument, you know what I'm getting at. For those of you who don't, well, let's just say tyrannosaurids never lost their feathers. No, they were just reshaping them. Yep, that's right, T. rex "scales" are in fact modified feathers.

You may know these "feather scales" by their more common name, reticula. Reticula are found in all modern birds, usually on the feet. Genetic analysis shows that these scales were indeed derived from feathers (unlike the foot scutes, which aren't derived from feathers). This suggests birds with reticula, and anything that does have reticula, descended from an animal that originally had feathered portions of that body. This suggests birds, and in fact perhaps all dinosaurs, evolved from an ancestor covered in feathers. Now, with this in mind, it is possible to turn feathers into reticula, it is seen in many modern birds today. Many baby birds start out with fluffy feet, but as they get older, their foot feathers start to shrink and harden, until its just reticula on the feet. Sometimes this can be taken to the extreme, as seen in wood storks. While the adults don't have reticula on their head, their skin keratinizes, until its basically kind of like really big reticula on the neck. This could apply to tyrannosaurids. In fact, one might say the juveniles are mostly feathered, and adult mostly scaly. Yeah that's right. Feathered Juvie/Scaly Adult isn't looking to implausible now seeing as how feathers can turn into reticula in several growth stages. Perhaps rex switched from fuzzy to scaly as it grew, with hatchling being almost entirely feathered, and old individuals potentially have very little fluff. This would make sense based off of the available data, but I have perhaps another scenario for adult plumage in rex.

Everyone knows of famous examples of feathers turning into reticula, but did you know that reticula can turn back into feathers? Its true, though it take a bit more time. All you need to do is look at some modern breeds of pigeons and chicken. Their ancestors had reticula covered scales, but over years of selective breeding, these pigeons and chickens have become FABULOUS!

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Sure, they may have been selectively bred, but they are proof that reticula can turn back into feathers. Now, for all of you who are all thinking, "SELECTIVE BREEDING DOESN'T COUNT!!!!!", then I'll give you some natural examples. Some owls start off with scaly feet, but as they grow, feathers start appearing on the feet, sometimes in small quantities, but other times in very large ones, like the case with Great Horned Owls. Another natural example would perhaps be the Ptarmigan. In the summer, their feet are mostly scaly with barely any feathers on them, but as winter nears, they go from scaly feet to FLAFFY FEET!

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If owls and ptarmigans weren't enough for you, then how about some extinct dinosaurs. Anchiornis is said to have fluffy feet and legs, but some specimens have reticula going up to the thigh. The variation between specimens is quite confusing, with some specimens having fully fluffy legs, but others having partly scaly feet and ankles, and still others having some scales on the thigh. This is some odd integument stuff going on, especially when you account they are all adults, and not different growth stages. 

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There is also the baby enantiornithine preserved entirely in amber (totally cooler discovery than those rex scale impressions). The specimen preserved scales on its feet, which are typical, but also scales on its fingers and even under some of its body feathers (the feathers aren't growing from the reticula, but from around and between them). Yet another example is Scansoriopteryx, a maniraptor closesly related to Epidexepteryx. Fossils of this creature have been preserved with feathers on the top of its tail, but the bottom part was covered in reticulam quite odd for a maniraptoran. This may suggest a reticula covered belly as well, something no maniraptoran has preserved thus far (at least to my knowledge). The fact that there is so much more variation in integumentary traits between even individuals of the same species among extinct dinosaurs than in living ones is something to ponder about.

In fact, such variability among extinct begs the question: Why are extinct dinosaurs much more fluid when it comes to integument variability. Well perhaps the question we should be asking is why are modern dinosaurs so much more restricted to their integument then their extinct forbearers? Well, this is quite a question, but there is an answer for this. Many modern birds fly with feathers, and because of this, they need to keep any other kind of integument limited to one spot, like say wrinkled skin to the head or reticula to the lower legs. Many flying birds are also small, and even the largest aren't really "huge" in terms of mass, the largest barely crossing 20 kg, so they need feathers to keep them warm and support their fast metabolism. If they didn't have as much filamentous insulation, then they wouldn't be able to keep themselves warm. 

However, most extinct dinosaurs were limited to the ground, and even many arboreal ones couldn't really fly or even glide (like Anchiornis and Scansoriopteryx). Most flightless dinosaurs are usually larger than many, if not all flying birds. Some of the large ones probably had slower metabolisms and perhaps had lower body temperatures to keep them overheating too much. They also aren't constrained to the demands of flight, in which small changes in integument can end badly for a flying bird, so they can experiment with much wider "designs" of integument. They could expand their naked skin and scales to other places, like the tail, upper legs, neck, and even some of the body. In fact, we see this many modern ground birds. Ostriches have evolved bare flanks, bottoms, necks, and legs, something their flying ancestors couldn't afford. 

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Cassowaries evolved from flighthed ancestors, but now that they are grounded, they can experiment with all new kinds of integument. They have completely bare heads with large waddles, and have evolved a giant head casque on their skull. Their aerodynamic body feathers of their ancestors have become simple filamets covering the entire animal's body, and their wings now sport only the thickened and enlongated shafts of their ancestor's flight feathers.

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Perhaps tyrannosaurid integument wasn't set in stone. Perhaps they too, like some extinct dinosaurs, had fluidity within their integument. They could switch their covering from season to season perhaps. In colder months, feathers grown between the scales and keep the animal warm, but in warmer months, many feathers are shed or turn back into reticula. Perhaps populations differed in terms of feathering and scalation. T. rex remains are known from the Sonoran Desert to Canada, a huge range that covers thousands of miles. The ones far south could have been barely feathered (but might still have had some feathers. Remember, elephant hair), as they lived in a hot and open environment, but the ones up in Canada could have had a thicker covering of feathers to block out the cold and keep their internal temperature at a constant temperature. Such variation is seen today. Modern wolves in India have a thin coat that is brown in color to camouflage with the environment, but the wolves in Alaska and Northern Canada have much thicker coats, and range in color from black, gray, and even white. Even with all this variation in coat length and thickness, these wolves are all the same species, so it wouldn't be surprising if tyrannosaurids were the same. There is even the possibility T. rex changed its integument over time. Lower Hell Creek (around 68 million years ago) had a average temperature of 7-14 degrees celsius. That is quite cold, and in fact, it may have snowed sometimes, though not often. However, Upper Hell Creek (around 66 million years ago) was much warmer, with annual temperatures around 20-23 degrees celsius, and could have reached max temperatures of perhaps nearly 40 degrees celsius from time to time. Perhaps rex started converting feathers to reticula as the environment got hotter over time, making the rexes of the latter part of HC less feathered than ones from older times. With all these factors, from population, environmental pressures, seasons, and the fluidity of extinct dinosaur integument, the feathering of rex seems to be even more confusing then we thought.

The flexibility of extinct dinosaur integument extends far beyond T. rex's plumage. In fact, it could have applied for a whole array of dinosaurs. The other giant coelurosaurs, like Therizinosaurus and Deinocheirus, may have been more different than what we originally thought. Therizinosaurus may have been even more prone to feather loss than rex, because it was a herbivore, and herbivore guts ferment a lot, creating huge amount of heat. Sure it was a maniraptor, but even fabulously feathered lineages can loose some of their plumage, after all, ostriches did, and they aren't even close to theri in size, nor have a huge fermenting gut. Deinocheirus may have been even prone to feather loss since it seems to have some relations with the water. While there is supposed evidence of a pygostyle, not every animal with some fused tail vertebra has a tail fan of feathers. For example Beipiaosaurus has a pygostyle but no prominent tail feathers, and Caudipteryx has a prominent tail fan, but no pygostyle, so it seems you don't need fused verts to have a tail fan, not to mention ornithomimosaurs don't seem to have tail "fans" to begin with, they're more like feather bushes. While none of these animals would have completely lost their filaments, they may have been perhaps "less fluffy" then we depict them as. 

The paper by Bell et al. has re-sparked the debate about whether rex had feathers or not, but it seems perhaps, it was much more complicated than just "Feathers or No Feathers". With the flexibility of bird feet, and the much larger amount of fluidity of extinct dinosaurian integument, the feather coverings of some large coelurosaurs seems to be a lot more complicated than we thought. Individual variation, seasonal events, changing climates, and different populations all existed in the time of rex and still do today. With such huge and different factors at play, its hard to know what exactly the integument of a large-bodied animal would be. In the world of theropods, skin coverings aren't set in stone, they aren't something you are stuck with for your whole life. Feathers can become reticula, but reticula can sometimes go back to being feathers. Feathers could sprout between scales, but fall of the next season. Two T. rex individuals of the same species who, skeletal wise, look pretty similar may look very different externally, with one being almost bald and the other being quite feathery (but again, probably not as feathery as you think). The fact that dinosaurs could change their integument over their lifetime is quite astounding. Most reptiles are set with what they are born with. Their scales stay and just grow with them, and are stuck with them till the day they die. Avian skin is more dynamic. A baby bird could be born with fluffy feet, only to find out they become scaly as they grow. Scales can creep up onto parts of an animal that are normally feathered for a special time of the year, but then creep back down, and feathers take their place once again. Such variation is something we have to keep in mind. Despite this post being titled towards T. rex and its fellow tyrannosaurids, it has a deeper meaning. A meaning to all of Theropoda, or perhaps even all of Dinosauria: That avian integument isn't the solid. With that being said, rex is just a small player in a whole world of fascinating creatures, and with the potential of changing integument throughout its lifetime or population, perhaps it appeals to all three sides of the argument. One for scales, one for feathers, and one for naked skin (granted, that side isn't very large). We have data that suggests rex and other dinosaurs could probably change their appearance with the seasons, through growth, and perhaps overtime to accommodate for new changes in the environment. Perhaps, like what Mark Witton said, "Everyone wins".

And I'm just gonna throw this out to you guys for some food for thought: Seeing as how Anchiornis shows evidence of potential seasonal change with its integument, perhaps some of the large dromaeosaurs were perhaps, scalier than we thought.

Let that sink in to your mind....


UPDATE: Edited some parts of my post, including the one about metabolism and surface area of rex and why flightless theropods had more flexibility with their skin than one that can fly (or at least glide). Thanks to and for informing me.