Evolution and Trends in Tyrannosauroid Integument and Skull/Limb Increase/Reduction

It's been a long while! An entire year since my last post! Well, this is good, because I have really learned my shit. God, I was such a stupid little twat back then (probably will say this about me now next year). So, I'm back with a very big blogpost. So, let's begin!

In paleontology, nothing is ever certain. We might assume something based off similar remains or what not, but they can still surprise us. And this is what people keep forgetting about when they talk about tyrannosaurid integument, thanks to that paper by Bell et al. which suggests tyrannosaurids might've been fully scaly. The thing is, people were too quick to jump the gun on this. Nothing really new came out this paper. We've known about the Wyrex skin impressions for ages now, though I am glad it is published finally. The authors of the paper even admit in their own paper that scaly tyrannosaurids isn't the only option, suggesting they may also have had reduced coats of feathers. But the media ignored that, because sensational headlines misinforms people...

But, really, what is going on here? We have two sides here, well, three. One is people who are firm and think that the paper is correct. Then the second, which are people who are firm and think tyrannosaurids had feathers. The third is more neutral, not really having an opinion on it. Though I have my own hypotheses about this, I think we must consider that one paper is not conclusive. And that there is far too little data. Approximately three known patches of skin are preserved from HMNS 2006.1743.01 (“Wyrex”. The first comes from the neck, though where on the neck is not known. This measures 15-20 centimetres and shows small scaly skin, but the scales do not overlap. This is actually fairly typical of dinosaurs, with the exception of the large scales on Triceratops. Anyways, the second patch of skin is located on the ilium, and is a measly 1.5-2 cm. The third patch of skin is located on the proximal end of the tail, and measures less than 20 centimetres.

Compared to an animal that can measure 12 metres and weigh 6-8 tonnes, these impressions are tiny, so much so, that you probably wouldn't have seen them in life unless you are really close. Anyways, these skin impressions don't give us much in the way of figuring out what the whole animal’s integument is. Now, I am going to talk about a hypothesis of mine, which is not in any way peer review and should not be treated as fact, nor the most plausible explanation.

Head First

This recent paper in a way goes in line with a paper published in May by Thomas Carr and colleagues. Thomas Carr is well known as the world's authority on tyrannosaurids, and is currently in the process of writing a 1,000 page monograph on BMRP 2002.4.1 (“Jane”). He described a new species of tyrannosaurid from the Two Medicine Formation, Daspletosaurus horneri, and also wrote that D. horneri might have been a descendant of D. torosus, and if so, shows evidence of fossil anagenesis. However, while these points were the main focus of the paper, the media and everyone focused on a small, rather insignificant part of it; the facial and oral integument.

When you look at tyrannosaurid skulls, you notice this rough, almost sculpted texture to the skull, especially on the nasal ridge, around the orbital and lacrimal portions of the skull, as well as on the jugal. Carr compared the D. horneri skull to that of Alligator mississippiensis, and severals ducks, as well as ostriches. From this, he concluded that D. horneri and other tyrannosaurids had ISOs (integumentary sense organs), and crocodylian style facial integument, leaving no room for lips. There a few problems with this; first off, the comparison of modern animal skulls is very weak. Crocodylians and avians are highly derived members of their group and do not correlate with most of their relatives. Second off is the lack of comparisons from other reptiles, such as lizards and turtles, and even mammals. While phylogenetically not close, they do bring more options to the table.

So in short, do tyrannosaurids have lips, do they have ISOs, and what was their cranial integument like? There is no reason to think tyrannosaurids lost lips, for their teeth do not show any signs against them having lips. In addition to that, tyrannosaurids actually can’t close their jaws all the way. This again suggests room for lips. Thus, with this and the rather sketchy results of Carr et al.’s paper, I say we should stay with lips for now. But what about the ISOs?

ISOs are small sense organs used in a variety of things, such as touch, detecting vibrations, and determining temperature. These are most will known in crocodylians, who use their ISOs to detect pressure changes of vibrations of possible prey. Although the most noticeable pits form a line running across the lower maxilla, they actually cover a lot more, with large clusters of pits on the premaxilla as well, and more pits covering the edges of the dentary too. These pits are pretty similar to the ISO pits seen in Alligator skulls. So a sensory function for these seem plausible for tyrannosaurids. Furthermore, the recent cranial scan of the carcharodontosaurid Neovenator salerii, published by Darren Naish and colleagues, also shows evidence of some form of sensory organs, possibly ISOs. Thus, sensitive snouts seem to be a trait in at least avetheropods.

Now, for the final bit, what covered the face? Looking at the rugose textures of the skull it appears there is something going on. It is important to note that some mammals have similar skull texture but as a result of blood vessels and not a rugose surface. Carr described the facial integument of tyrannosaurids like those of crocodylians, calling them “large, flat scales”. Crocodylians do not have flat scales on their face, instead having it covered in keratin, which creates a menagerie of cracks on the skull, especially as they age. Funnily enough, we see this same pattern in tyrannosaurids. Younger individuals having a less rugose texture, but larger ones having them, possibly correlating with a change in hunting methods and bite strength and behaviour.

Crocodylians produce some of the most powerful bites of any extant animal. Like tyrannosaurids, their teeth are conical and well suited to crush things. In addition, both groups share a secondary palate, which also helps with the skull resisting the force of their tremendous bites. Finally, the keratin covered faces of crocs do help further with handling stress from their bites. I hypothesize that tyrannosaurids do indeed have a crocodile like facial integument, and for the same purposes crocs do.

Bust of the head of Nanuqsaurus houglandi, showing off the crocodilian like face, face biting wounds, and ISOs are visible along the premaxilla and maxilla.

Another possibility in these animals is some keratinous facial displays. The nasal ridge on tyrannosaurids somewhat resembles the snout ridge seen on pterosaurs such as Darwinopterus, which we know supports a large crest. In tyrannosaurids, the nasal ridge could have supported a large nasal crest, similar to what is seen in their relatives such as Guanlong and Proceratosaurus. In addition, the lacrimal horns in some tyrannosaurids creates a small projecting area, which could've also been extended into longer horns for display. They might also have grown these structures seasonally, as in some birds such as pelicans, in which some species grow a nasal crest which falls off once the breeding season is over.

Another very peculiar topic of interest are the maxilla. As discussed in a video by WitmerLab, some adult Tyrannosaurus skulls have these strange, leaf shaped indentations to the maxilla, anywhere from 5-6. It's thought these indentations have something to do with facial integument, possibly even display. Some ideas as to what these are include a placeholder for larger scales on the face, or raised ridges of keratin on the face. The idea that it might have something to do with display might have some merit, as not all Tyrannosaurus adult skulls have this, and especially not juveniles. Either way, it is a very interesting feature that is currently being researched.

So, in short, due to similarities with crocodylians and lizards, I think that tyrannosaurid skulls were covered in crackled keratin, and thus suggests no feathers on this part of the animals. Indeed, if they had a rough keratin covered skull, it would also prove useful to protect themselves in intraspecific face biting competitions that we know tyranosaurids did. This also might suggest tyrannosaurids were more like crocs in terms of hunting behaviour; clamping their powerful jaws shut on their prey, possibly somewhere vulnerable such as the neck, and crush it to death.

Going Down the Tree

As I’ve thought more and more about this, I’ve begun to consider something that neither points to feather or scales in tyrannosaurids, but instead, to something more variable. The tiny, non-overlapping body scales of many dinosaurs greatly resemble the “scales” on the feet and legs of birds. However, these “scales” are in fact reticulae, feather buds which were halted during development and came out as something similar to “scales”. Thus, the scales on dinosaurs might be reticulae instead… again, we need genetics for this, and sadly, you don't get DNA from fossils. As I go down the family tree of tyrannosaurids, I will be using any known skin impressions, as well as the size, ecology, and environment of these animals, to try and build a hypothesis for the scaliness or lack thereof for the tyrant reptiles. It's not as simple as “Tyrannosaurus has feathers and thus you must show all tyrannosaurids with feathers”. You have to find a logical reason for them not to have feathers or for them to. Not included in this list is Bistahiversor, which instead might be a derived non-tyrannosaurid tyrannosauroid.

Albertosaurus/Gorgosaurus

I shall start with the first subfamily of tyrannosaurids, the albertosaurines, which include Albertosaurus sarcophagus and Albertosaurus/Gorgosaurus libratus. Unlike most other tyrannosaurids, albertosaurines have long legs and a slightly more gracile build, which might suggest they went after faster moving prey, or relied more on endurance than ambush hunting. These animals at adult size are about 9-10 metres and weigh 2-3 tonnes. Gorgosaurus (Albertosaurus) libratus is primarily found in the Dinosaur Park Formation, a fossil formation covering several million years in the Campanian epoch, in Alberta, Canada. The environment was a coastal woodland floodplain, with climate similar to that of southernmost Britain, but at a higher latitude, and thus seasonal changes in daytime/nighttime hours. It is a warm temperature place, 6-12.3°C in mean annual temperature, with cool dry seasons and warm wet seasons, which sometimes resulted in flash floods.

Now, we do indeed have a skin impression from A. libratus, which help us get a start. The impression is very small, 2x2cm, and comes from a ventrolateral part of the tail. The scales are either polygonal or subcircular, but these only occur as faint colour variations in the tail, so the exact nature is rather hard to tell. These skin impressions show that albertosaurines do seem to have variability in scale shape and size across different parts of their body, but sadly don't give a clear idea of a predominantly scaly animal. Looking at these skin impressions, as well as their size, and environment, I’m inclined to believe Gorgosaurus either had a very small (sparse?) coat of feathers on the body, or a predominantly scaly body.

Now comes Albertosaurus sarcophagus, a species known from the earliest Maastrichtian mainly known from the Horseshoe Canyon Formation. This formation was a coastal forested floodplain, which was pretty cold, with a mean annual temperature of 10°C. It should also be noted that this is the same MAT as the Yixian Formation (which has Yutyrannus). In fact, Horseshoe Canyon would've probably had areas even colder than Yixian due to the influence of the Western Interior Seaway.

The two known skin impressions come from an A. sarcophagus specimen named RMPT 1994.186.001. In this, the first patch of scales comes from the side of the gastral ribs and an unidentified long bone (possibly a hindlimb element) and measures 10x8cm, and extend further down and onto the ventral side of the gastralia. These scales are pebbly and subcircular, but turn into larger somewhat hexagonal scales. Strangely, two feature scales are known from this impression, measuring 7mm long and 2.5mm tall, similar to what is found on the abelisaurid Carnotaurus sasteri. The second path of skin, which is 7x12cm, comes from an unknown part of the body, and seem to show uniform diamond shaped scales.

Firstly, we know from this that the scales on Albertosaurus changed depending on which part of the body they are from. Second, this pretty cold climate (probably accompanied by snow) suggests to me Albertosaurus sarcophagus was more extensively feathered than Gorgosaurus. I’d say some pretty extensive feathering on the neck, body, forelimbs, and possibly even tail and down the legs, to keep it warm. After all, despite being the same length as A. libratus, it weighs much less, only about 1.5 tonnes. Thus, while A. libratus might be more scaly, A. sarcophagus seems to be a lot more fuzzy. This alone shows the variability of integument between members of the same genus.

Alioramini

The alioramini tyrannosauroids are some of the most peculiar animals of the group, and have a somewhat unstable phylogeny, some analyses putting them outside of tyrannosauridae, although the more recent studies have found them to be tyrannosaurine tyrannosaurids. In my opinion the naming of these animals is a bit problematic since they are based on juvenile specimens, but I’ll let that be since these are clearly distinct from the contemporary Tarbosaurus. Typically divided into three species and two genera, I propose one genera and two species of alioramini. The differences of Alioramus remotus and Alioramus altai in my opinion could just be ontogenetic change, which makes since given A. remotus is a full metre to metre and a half longer than A. altai. The only known adult alioramini is Alioramus (Qianzhousaurus) sinensis, from China.

Alioramini are only known from Mongolis and China during the Maastrichtian epoch. A. remotus and A. altai are known from the Nemegt Formation. It was a very wet place, with heavy rainfall supporting large vegetation filled floodplains, resembling a lot like the Okavango Wetlands in Africa today. It is positioned at 43°S latitude, and thus would've had at least seasonal differences in temperature; perhaps 12-16°C in winter months, and 20-23°C in summer months, thus creating a rather warm environment.

Alioramus have very long legs, and an overall very gracile body, much more so than Albertosaurus, and thus weighed much less. The adult specimen of Alioramus sinensis measures 9-10 metres in length, and probably weighs 1-1.5 tonnes. Alioramini can also be noted to be distinct due to their long, narrow, shallow snouts, much more so than any tyrannosaurid. Thus, I suggest it was hunting fast running, but small animals. From the temperature estimates and body size, I think Alioramus probably could've had feathers, having lots of bare areas like ostriches, as a means to cool off. So… I’d say a coat of feathers covering the arms, front portion of the body, and the neck sounds about right for these mysterious, enigmatic tyrannosaurs.

Daspletosaurus

Daspletosaurus, a genus the tyrannosaurinae subfamily, which includes the rest of the tyrannosaurid family. Despite being younger than the oldest known tyrannosaurine, Daspletosaurus is among the most, if not, the most basal member of the subfamily. Of course this depends on your phylogeny, as some studies have found it as a very derived tyrannosaurine. In any case, there are two known species of Daspletosaurus; Daspletosaurus torosus, the older species from areas such as the Oldman Formation. The second species is the recently described Daspletosaurus horneri from the Two Medicine Formation, which has been hypothesized by Carr to be an example of anagensis; D. horneri being a descendant of D. torosus.

Since D. torosus is the oldest Daspletosaurus species, we will start with it first. D. torosus is known from the southern Alberta, Oldman Formation, which was an inland floodplain, drier than the nearby Dinosaur Park Formation. Due to being more inland Oldman was probably warmer than Dinosaur Park. There is also evidence of seasonal tropical storms in the area, causing mass death sites of Centrosaurus apertus. Daspletosaurus is a large animal, 8-9 metres long and weighing 3-4 tonnes, being more robust than Albertosaurus. So what does this mean? Well, due to the larger size, as well as warmer stratigraphic unit, I’m more convinced of a completely scaly species than a feathered one.

The second species, Daspletosaurus horneri, comes from a completely different formation; the Two Medicine Formation from Montana, USA. Its environment therefore is much different from the Oldman, instead being a semi-arid mountainous area. It is warm, the mean annual temperature about 20°C with a temperature range of 8°C depending on the season. Dry seasons in Two Medicine were more arid, and several fossil sites there show evidence of drought related deaths. This is usual for Two Medicine as it might've had rainshadows. Sadly, no skin impressions of D. horneri are known, but given the semiarid warm nature of its habitat and that it was the same size as D. torosus, I believe it was firmly a scaly animal.

Teratophoneus

Restoration of an adult Teratophoneus curriei. Note the very deep snout.

These next two tyrannosaurines both come from Utah, and have some significant implications on tyrannosaurid evolution. The first of course, is Teratophoneus curriei, a tyrannosaurine known from the Kaiparowits Formation of the middle Campanian epoch, first originally thought to be an albertosaurine. Known from a fragmentary adult specimen and a more complete subadult specimen, it is probably the largest predator in its area, measuring 8-9 metres as an adult. The most distinctive feature of Teratophoneus is its short, deep snout, which might've allowed for a stronger bite force.

The Kaiparowits Formation, while undersampled, is one of the best fossil sites in the world, preserving many vertebrate fossils and floral fossils. During its existence, it was bordering the Western Interior Seaway, and a very wet, humid area, filled with an abundance of plant life. As most of the late Cretaceous ecosystems go, angiosperms were the most common flora, followed by ferns, and conifers. This was deposited at a time when the Western Interior sealevels were higher, and therefore created a very wet area filled with swamps, rivers, and lakes, possibly even creating a dense jungle. Temperatures were high, about 19-23.6°C on average.

So, Teratophoneus lived in a very warm tropical area at the time. Thus, I am inclined to believe that Teratophoneus was scaly than having any sort of fuzz. Kaiparowits seems to be very wet, humid, claustrophobic, and warm, so the need for feathers decreases in this species. Obviously I could be wrong (as with all of these), but this is what makes the most sense to me at this moment. It is important to note however that among forest dwelling mammals, there seems to be an increase in fuzz, as opposed to their open world relatives. This is seen among Indian elephants, as well as rhinos, and buffalo.

Lythronax

The next Utahn tyrannosaurid is the oldest known one, Lythronax argestes. At 80 million years, it is a full 3 million years older than the next oldest tyrannosaurid species. However, it is already pretty derived, recorved as pretty close to the Tyrannosaurus clade. Thus, while Lythronax seems to have been the oldest known tyrannosaurid, the origins of tyrannosaurids seems to go back even further. Due to the presence of it and Teratophoneus in southern Laramidia, the tyrant reptiles may have originated from this area, although I personally think a northern Laramidia origin to be more likely, due to the presence of albertosaurines (which had to have split off from tyrannosaurines already) and the basalmost tyrannosaurines (Daspletosaurus), all of which first appeared 3 million years after Lythronax, despite being more basal. Alternatively, Lythronax and Teratophoneus might be an example of a southern radiation of tyrannosaurids, whilst Daspletosaurus and co. are an example of a separate northern radiation.

Lythronax is slightly smaller than Teratophoneus, about 7.3 metres long and weighing approximately 2.5 tonnes. It also differs in lacking the short deep snout of Teratophoneus, suggesting that was a trait which converged along with later genera like Tyrannosaurus. It is also distinctive for having fairly long teeth, although whether it is an artifact of preservation or not I do not know. The skull is already pretty broad, and the bauplan of forward facing eyes is driven further back to Lythronax, and not to as previously thought, unique to Tyrannosaurus.

Lythronax is known from a single adult specimen from the Wahweap Formation, which precedes the Kaiparowits Formation. Not much is known about Wahweap, although it was deposited at a time when the Western Interior Sea was less expanded, and thus created a sort of arid to semiarid environment, with at least several rivers present within. Plant life and temperature estimates are lacking, although I’d assume a temperature similar to that of Kaiparowits. Thus, it is a bit hard for me to determine what to think of for Lythronax. Maybe predominantly scaly, with maybe hints of fuzz or no fuzz at all? Hard to say with such lacking knowledge of the Wahweap area.

Nanuqsaurus

The smallest tyrannosaurid known, as well as the most northerly species known, occurring in the Prince Creek Formation at the north pole, in Alaska. It is sadly known from very little, including a partial tip of the dentary, a partial skull roof, and a part of the antorbital fenestrae. From this, it is deduced that it was closely related to Tyrannosaurus, and measured only 6 metres and weighed less than a tonne. The small size might be a factor of the arctic conditions, and also due to the fact that Alaska was an island separated from the rest of North America at the time.

Prince Creek was a rainforest located within the Arctic Circle, filled with swamps, and many different flora including angiosperms, horsetails, ferns, and conifers. The annual mean temperature is estimated to be about 2.5-5°C, with a comfortable summer estimate of 14.5°C and a winter estimate of -2°C, with occasional cold snaps of -10°C. The formation would've been supplied almost year round by rainfall. During winter, snow would definitely have fallen, but it would've mainly been wet snow, creating a wet, slushy environment in the long dark.

What does this mean for Nanuqsaurus? With skin impressions absent we must solely rely on the environment and the skeletal features. It is the coldest place we’ve ever found a tyrannosaurid in, and Nanuqsaurus is very small, probably not weighing more than 700 kilograms. It may have had some help from its size but it's still not enough. Honestly I have to go with feathers for Nanuqsaurus. Sure, it can be supplied by fat instead, but it wouldn't last long. Feathers are by far the best insulation in this climate, especially for a small bodied animal. I believe therefore, that Nanuqsaurus had a thick coat, covering the neck and most (if not, all) of the torso, as well as the arms and maybe part of the legs too. I just do not see it surviving that well with no feathers, it's not logical.

Tyrannosaurus

Restoration of a scaly T. rex as per Bell et al. 2017. Credit to Mark Witton who illustrated this for his own blogpost.

And finally, to cap off the tyrannosaurid list, we come to the most famous of all tyrannosaurids; Tyrannosaurus. Here we will discuss both Tarbosaurus (Tyrannosaurus) bataar and Tyrannosaurus rex, both of which are known from several skin impressions. These species are the largest tyrannosaurids without any doubt, and size is an important factor to play into this. To start off, let's first start off with Tyrannosaurus bataar. T. bataar is known from Mongolia and China, specifically well known from the Nemegt Formation, the same fossil unit from which Alioramus and kin are from. Tarbosaurus differs from its sister taxa by a smaller body, as well as a thinner skull which lacks binocular vision, and seems to resemble more that of carcharodontosaurs than anything tyrannosaurine.

Fortunately we have a few Tarbosaurus skin impressions to start us off with. The first is sadly no longer available to study due to fossil poachers destroying it, but it showed a sort of gular pouch on T. bataar, which was covered in the typical tiny non-overlapping scales of dinosaurs. The second impression is more promising, coming off the trackways of Tyrannosaurus (Tarbosaurus) bataar. They come from the foot pad behind the toes, and greatly resemble what was found with the first impressions. The third is more promising, coming somewhere from the abdominal region (perhaps the belly) of the animal.

Tyrannosaurus bataar is on average around 10-11 metres long, although the largest specimens measure up to 12 metres. However, due to the more gracile nature of T. bataar, it weighed approximately 4-5+ tonnes, already larger than any tyrannosaurid discussed thus far. Of course we mustn't forget the rule that larger animals create more body heat just by being larger and thus need less outside influences to regulate their body temperature, so I’m firmly on the side of an entirely scaly T. bataar. Indeterminate tyrannosaurid remains from the Kundar Basin of Russia have been suggested to be T. bataar, but this is not confirmed as of yet.

Now for the last tyrannosaurid, the infamous and universally known Tyrannosaurus rex from North America. It is by far the bulkiest tyrannosaurid, with a very broad body and tail, as well as a very broad head which allowed for a tremendous bite force. Most individuals don't reach much more than 11-11.5 metres and 6-7 tonnes, except for a few specimens including the most famous of all, FMNH PR 2081, commonly known as “Sue” (though the sex of the specimen is not yet determined). While not the oldest specimen (“Tristan” and “Scotty” might be older by 5-8 years but remain unpublished), it is the largest at 12.3 metres and 8.6 tonnes in weight.

As we’ve discussed in the intro, three skin impressions are known from T. rex, from the neck, ilium, and tail. We also must take into account its environment, which changed quite a bit over time. The formation with the most Tyrannosaurus fossils is the Hell Creek Formation which covers the Dakotas and Montana. At the early and middle parts of the formation, the annual mean temperature was 7-11°C, which is pretty cool, possibly accompanied by frosts and even the occasional freak snowfalls. So maybe just a bit of fuzz? Not sure, it’s so large that I might not even see the need for any feathers. At the end of Hell Creek, when the flood basalt Deccan Traps were erupting in the-then island India, global temperatures rose, and Hell Creek had an annual average temperature of 20°C. At this point I clearly see no reason for feathers of any sort. As it turns out, despite what many people say, size indeed does matter.

But Hell Creek isn't the only site of which Tyrannosaurus is known from. They are found as far north as the Frenchman Formation, in southern Saskatchewan. During the mid Maastrichtian, temperatures here were unprecedentedly cold, with a MAT of 3-5°C, which strongly suggest temperatures below zero in winter months, and guarantees snow. Here, I am more convinced of at least partial feathering covering, if not, more extensive feather covering, for the T. rex populations living this far north. If this is true, there might've actually been not just difference in integument covering in the same genus, but perhaps even in the same species and in different populations to be more specific. This is a very interesting topic of discussion, and also a very interesting hypothesis to put forward maybe a little later...

So, perhaps we should maybe tone down what we say about tyrannosaurid integument? While the “scaly side” might be a nuisance, so are the “feathery side”. I think we have overpromoted the possibility of feathers on tyrannosaurids that we begin to misinform people and treating it as fact, when, I have just shown, does have some reasonable doubts. But does this mean Saurian’s T. rex is wrong? Not at all. It is just one of many hypotheses on what Tyrannosaurus looked like. But this topic is nowhere near complete. There is still, SOOO much more to discuss.

Restoration of FMNH PR 2081 with all its injuries. This amount of feathering for T.rex is still valid and some northern populations (like Frenchman) might've had such feathering to keep warm.

Ontogeny and Seasonal Changes, and How it Affects the Integument Debate

As I’ve just discussed up there, there are many hypotheses on tyrannosaurid integument and life appearance. But I didn't include absolutely everything there, because there are things that still make a feathered Tyrannosaurus possible… The first one is seasonal changes. Some of these tyrant reptiles live in northern latitude areas which experience periods of darkness and low temperatures. Indeed, perhaps these animals even had differing amounts of feathers based on the seasons. Just as an example, ptarmigans show this, albeit on a much smaller scale. They have scaly feet during the spring and summer months, but as autumn and winter approach, they begin to grow feathers between their scales, and come winter, their feet are fully floofed up. I can see this perhaps in Nanuqsaurus, and perhaps even in albertosaurines, where such an adaptation can be advantageous.

An example of ontogenetic change of integument, here with an adult and juvenile Daspletosaurus horneri.

Another thing to consider is ontogenetic change of feathering. Juvenile tyrannosaurids tend to be less robust and thus need to rely more on outside forces to help them regulate their internal temperature. And, feathers would do very good at it. This is even seen in some birds. When ostriches are born, they have a thick coating of feathers covering their body, as well as thighs, and neck. However, as they grow larger, they reduce the feathering on their neck and leave their thighs (and sometimes even their backends) featherless as a means to control body temp and not overheat. Now imagine this, but on a much grander scale. We have a hatchling tyrannosaurid less than a metre and only 1-2 kilograms, and then imagine a 12 metre 7-8 tonne adult. I'm sure if tyrannosaurids displayed this ability, we would see a change akin to ostriches but much, much more severe.

So, to end this section of the post, I think most tyrannosaurids were predominantly, if not, completely scaly with a few exceptions, but retained a feathery coat in their early days to help control their little bodies. But, as I’ve said, this could easily be falsified; nature is full of surprises. I just personally think this idea is more plausible as it neither precludes, nor includes the need or lack thereof for feathers and scales in the tyrant reptiles and is still open to interpretation.

Tyrannosauroid Integument Evolution

We’ve focused on tyrannosaurids so far, but I think it's time we take a look at the other tyrannosauroids. Because my hypothesis only brings up more questions, such as “why did they lose their feathers” and “when in tyrannosauroid evolution did they lose them?” This is indeed a relevant and fascinating topic of discussion, because this has implications beyond tyrannosaurids, and possibly to the rest of dinosauria as a whole.

This of course all depends on which hypotheses of dinosaur integument you believe. One is that the fuzzy structures seen in pterosaurs, ornithischians, and coelurosaurs are all homologous. The second is that the fuzzy structures in coelurosaurs and ornithischians are homologous, but are not so with pterosaur fuzz (pycnofibres). The third and final hypothesis is that the fuzz on ornithischians, pterosaurs, and coelurosaurs are all convergent and only theropods/coelurosaurs have these structures. While the lack of data here is apparent and a solid answer cannot be said, I think that fuzz was at least ancestral to dinosaurs, if not, ornithodirans as a whole. Regardless, this will be the hypothesis I will follow for this next section, and tackle the earlier members of the tyrannosauroid lineage.

The Chinese Tyrannosauroids and Convergent Gigantism

To begin, we must go back to the most basal group of tyrannosauroids; the proceratosaurids. They first appeared during the middle Jurassic, and lived across Europe, Asia, and North America. They were small animals, only about 3-4 metres in length, and these Jurassic genera, specifically Proceratosaurus and Guanlong, had large, osteological crests on their head which were pneumatic and probably used for display. While we don't have integument impressions of Jurassic proceratosaurids, they probably most certainly had feathers, due to smaller size. Indeed, the early Cretaceous Dilong had a coat of protofeathers covering their body, and this is what was the ancestral condition for tyrannosauroids.

Perhaps the most fantastic and beautiful of proceratosaurids was Yutyrannus huali. This is one of the largest known proceratosaurids, known from three well preserved specimens all at different stages of ontogeny, and most importantly; the largest tyrannosauroid currently known which possesses feathers. Due to preservation it is difficult to ascertain what types of feathers they had, though something resembling downy fuzz is a safe bet. The feathers were extremely extensive on Yutyrannus, going across the entirety of the tail and body, and even down to the metatarsals. Matt Martyniuk once said that he spotted at least one scale on the underside of Yutyrannus’s tail, but I have never since been able to confirm it. Still, this is far from a big stretch, as Juravenator, Compsognathus, and even maniraptorans like Scansoriopteryx have scaly ventrals on the caudal.

But Yutyrannus was very integral to the tyrannosaurid integument debate. When it was first published, it was the first time people have found more solid evidence of feathered tyrannosaurids, since Yutyrannus is of similar size of most tyrannosaurids. But people jumped the gun too quickly. Integument, even within the same dinosaur family, is far from uniform. Centrosaurus and Triceratops for instance have very different scales, one with small scales surrounding feature scales, while the second having enormous, thick scales. However, it is still interesting to find out why Yutyrannus still kept its feathers despite being that large.

Restoration of Yutyrannus huali in the snow. Credit to Mark Witton. To see the full painting go this his patreon

First, like we did with the tyrannosaurids, we must look at Yutyrannus and its environment. For starters, the adult specimen is indeed enormous, 9 metres long, comparable to say the albertosaurines in length. But that's just in length. If you look at the rest of Yutyrannus’s body it becomes clear that it is still far smaller than any similar length tyrannosaurids, weighing only 1.5 tonnes. The build of Yutyrannus is more gracile. Hell, when Yutyrannus was first described, Darren Naish and a few other people pointed out how carcharodontosaurid it was like, and even considered it to be a carcharodontosaurid. However, all recent studies have firmly placed it into tyrannosauroidea. Still, it goes to show that Yutyrannus was more lightweight than the later tyrannosaurids.

The environment too is rather grim; estimates for the mean annual temperature of Yixian is about 6-14°C, suggesting a pretty cold climate, typical of the early Cretaceous, and especially for the latitude of Yixian. Furthermore, Yixian at the time was at a higher elevation, thus further reasoning why it was cold. The environment itself was pretty forested, and was home to volcanoes which did erupt on several occasions, and helped preserve the beautiful specimens we find there today. Winter seasons are pretty cold, definitely frosted and probably snowed as well. This further explains the need for such a fuzzy body on Yutyrannus.

In short, the smaller size of Yutyrannus, plus the environment, might explain the need for feathers. However, I have come to conclude that the true reason Yutyrannus was extensively feathered is because it simply could. In addition to being part of a tyrannosauroid lineage which is known to be extensively feathered, it might have affected what types of integuments are possible on tyrannosaurids, but Yutyrannus is still not really the best comparison, due to a colder environment, it's position within the tyrannosauroid tree, and it's deceiving small size.

The Appalachian Tyrannosauroid Complex

Skeletal of Appalachiosaurus montgomeriensis, showing the known elements. From Carr and Schwimmer, 2010.

Laramidia is the go to place to study not just tyrannosauroids, but also North American dinosaurs in general. But that is just one half of the North American continent. To the east, right up to the Atlantic coast, is the mysterious subcontinent of Appalachia. Here, dinosaur fossils are few and far between, and it really frustrates me, because from what we have Appalachia seems to have been a very interesting place in the late Cretaceous, and I wonder how different it was to Laramidia… This lack of fossils is due to two factors: one is that Appalachia is mainly mountainous today, and due to tectonic forces, fossils don't tend to survive well when mountains form. The second is, that during the Pleistocene ice ages, the glaciers extended down into Appalachia, and each time they retreated, they also took with them rocks and sediment, as well as fossils, and stole them. So, in short, Appalachia is a shithole for fossils.

But that doesn't mean everything in Appalachia is shitty. A few (and I mean very few) fossil animals have actually been pretty well preserved. And one is a tyrannosauroid; Appalachiosaurus montogmeriensis, the most complete theropod from Appalachia. It is known from a single subadult specimen about 7 metres and 600 kilograms or so, with a good portion of the skull, a few dorsal and caudal vertebrae, fragments of the pubis and ischium, and an almost complete set of hindlimbs, and possibly a humerus as well. Part of why it is so well preserved is for the fact that it was washed out into the Western Interior Seaway.

Due to not being full sized, a fully grown Appalachiosaurus may have been around 9-10 metres in length, and in the range of 1-2 tonnes. This also makes it the largest theropod we have yet recovered from Appalachia. Since the actual environment where Appalachiosaurus resided in is unknown, we must rely on Campanian climate models to get data for the temperature there. It is estimated that at the middle Campanian (where Appalachiosaurus is from), the southerneastern United States was in the range of 23-24°C, indicating a pretty warm environment.

Two things I noticed; it's very warm, and the tyrannosauroid is pretty large, yet another example of gigantism converging in another tyrannosauroid lineage. So… maybe more scaly than most tyrannosauroids? Hard to tell if it was completely scaly, but at the very least the available data suggests to me reduced feathering on the body of the animal. Sill, it is important to remember that Appalachiosaurus was very gracile, possibly more so than any other tyrannosaurid, so it might've still had more body feathering than I hypothesized above.

Skeletal reconstruction of Dryptosaurus, by GetAwayTrike.

The second (less well known) Appalachian tyrannosauroid comes from the north, in areas such as New Jersey, close to the very end of the Cretaceous. This animal of course, is Dryptosaurus aquilunguis. It is only known from fragments of the dentary, maxilla, as well as a somewhat well preserved forelimb, some caudal vertebrae, and a well preserved hindlimb. It is more basal than Appalachiosaurus, and also smaller, an adult measuring 7.5 metres and weighing 1.5 tonnes. The forelimbs are actually quite large, and only seems to have two fingers, like tyrannosaurids, but unlike them might actually have used them to capture prey. Still, this might suggest that forelimb reduction occurred after the evolution of a two digit manus in tyrannosauroids. In any case, we know nothing of the environment it lived in and like Appalachiosaurus, must rely on climate models. And these climate models seem to suggest a MAT of about 6°C, which is fairly cool. So, I'm on the side of a mostly feathered Dryptosaurus.

Bistahieversor

And now we are getting to the closest relative to tyrannosaurids, Bistahieversor, commonly found just outside of tyrannosauridae, but more derived than Appalachiosaurus, and sometimes even found as within tyrannosaurinae, although a position just outside of tyrannosauridae seems to be more well supported. Anyways, Bistahieversor is known from the Kirtland Formation in the San Juan Basin of New Mexico, a woodland coastal plain, which separates Bistahieversor from tyrannosaurids due to the geographical barrier of the newly forming Rocky Mountains. It is also incredibly warm, about 20-26.8°C + or - 2.24°C.

Bistahieversor has the basic, robust build of tyrannosaurids, including a very deep skull like that of Tyrannosaurus. Known from adult and juvenile remains, an adult Bistahieversor measures about 9-10 metres and probably weighed more than a tonne. Due to the coastal environment, the ocean would've influence a fairly stable temperature, so due to that, and the larger size of Bistahieversor, I conclude that Bistahieversor either had a reduced coat of feathers, or a completely scaly body. And this is very important to our understanding of tyrannosauroid integument evolution.

Several things to note here; one is that there does seem to be a steady trend in the size of tyrannosauroids, especially apparent at Appalachiosaurus and Dryptosaurus. Second, is the evolution of reduction in arm size and increase in skull size. Dryptosaurus already had the two functional digits which was kept in the more derived tyrannosauroids, but the arms were still pretty large and likely important in prey capture. By Bistahieversor, arm size had reduced, and the skull size did increase, indicating they that the head based predation was a trend right around the Appalachiosaurus/Bistahieversor clade. So things such as large size and reduction/increase in arm/skull size were already occurring in derived non-tyrannosaurid tyrannosauroids.

The third, and probably most relevant thing to note perhaps, is that feather loss already seemed to be occurring before tyrannosaurids, right at their sister clade. As I’ve already noted, Appalachiosaurus and Bistahieversor might have started or already reduced their coats, and this correlates with the trend of increase in size. In addition, both species appear to live in very warm environments as well, which were probably a factor into their integument as well.

Tyrannosaurid Integument Evolution

Latest phylogeny as per Carr et al. 2017. Right in between Appalachiosaurus and Bistahieversor is where I believe tyrannosauroids really reduced their feathering.

So, it seems to be pretty consistent that tyrannosauroids kept their feathers for the majority of their evolution. But as they got more derived, and especially as they got closer to tyrannosaurids, they began to lose their feathers. This is due to several reasons; one is gigantism. While some forms such as Yutyrannus did reach huge sizes, they differed from the later tyrannosauroids in several ways; one is body build. Later tyrannosauroids not only increased in length, but also increased in girth as well. Tyrannosaurids are typically very wide, robust animals, and so was Bistahieversor. Thus, while they are the same length as Yutyrannus, they weighed a lot more, probably due to a different sort of predatory lifestyle which demanded a more powerful and differently built animal.

The second reason for the feather loss proposed here is environment. Yutyrannus lived in a high altitude woodland which ranged anywhere from 6-14°C annually. This is pretty cold, and probably allowed for the larger animals to still keep their feathers (also because as stated above, Yutyrannus was gracile and more like a carcharodontosaur than a tyrannosaurid). However, here we see tyrannosaurids and their sister clade originating in the southern United States, which had a very different feel, with forested to woodland floodplains that are usually in the mid to high 20s, and thus may have driven the need to lose their feathers in addition to increased size.

Conclusion

Here I have put forward a hypothesis on the variability of integument within tyrannosauridae, as well as a hypothesis on the origins of feather loss within tyrannosauroidea, and to go further beyond the discussion of this post, also put forward a hypothesis on the evolution of arm size and skull size in tyrannosauroidea. What I have discussed in my opinion supports evidence of ancestral feather loss in tyrannosauridae, and also a mostly scaly tyrannosauridae, with a few species suggesting that feathers secondarily re-evolved for insulation and thermoregulatory purposes, mainly in high latitudes and in smaller bodied species. But those that lived in lower latitudes and were larger tended to be more scaly than the higher latitude species due to, in part, a warmer environment.

Another topic to add on to variability within tyrannosauridae is to consider the integumentary differences in different populations and in different ontogenetic stages. Some tyrannosaurid species (such as Tarbosaurus(?) and Tyrannosaurus) seemed to have had quite a large distribution, covering both lower latitudes and higher latitudes, experiencing warmer tropical weather and colder semi-arctic conditions. Thus, higher latitude populations of tyrannosaurids may have had more feathers whilst the lower latitude populations having more scales. During ontogeny, tyrannosaurids went from small bodied, gracile individuals to large bodied, robust individuals. The small size and gracile nature of hatchlings and juveniles, as well as a quick growth rate, suggests that tyrannosaurids were born mostly or fully feathered, to help regulate their high metabolic rate and growth demands, and as they entered into maturity, lost the need for feathers as they halted growth and had a large enough body mass to help regulate their body temperature, which lessened the need for feathers.

For the origin of feather loss, I propose that feather loss occurred in the sister clade to tyrannosaurids, which include Bistahieversor and Appalachiosaurus. The reason for feather loss is thought to be due to size increase in warmer environments, which in the case of Bistahieversor/Appalachiosaurus as well as Teratophoneus and Lythronax, is on average around 23°C. In addition, two other results not related to integument have bearings on tyrannosauroid evolution. First is tyrannosaurid origins. We find Bistahieversor (the closest relative to tyrannosaurids), as well as Teratophoneus and Lythronax (the oldest known tyrannosaurids) in southern Laramidia and thus suggests southern Laramidian origin of tyrannosaurids, which is consistent with several papers published in the last 6 years.

The second non-integument related discovery was the evolution of tyrannosauroid arm reduction and skull increase. The earliest we see evidence of digit loss is in Dryptosaurus, but arm length then was still long and comparable with previous tyrannosauroids. Not until Appalachiosaurus and Bistahieversor did the arms shrink, and it is at that time that we find large skulls with powerful bites, which hints at a change of predation style, from “grapple and bite” to “bite and crush” predation. It should be further noted that the expansion to the back of the skull is taken to the extreme in tyrannosaurines, which have the strongest bite forces of any terrestrial animal.

And this wraps up my megapost on my thoughts on tyrannosauroid integument and evolution. I worked my fucking ass off on this, and I can say that I am very proud of what I’ve written. Since I’ve been in a tyrannosaur mood lately, I might actually do a blogpost on tyrannosaurid ontogeny and ecology, but… depends what I feel like doing. I would like to thank Mark Witton for allowing me to use his illustration s(support him at Patreon), and my good friend Christian for providing the lateral view illustrations of the tyrannosauroids you have seen in this post (you can find more of his work here).

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