Insulation Without Feathers - A New Hypothesis Addressing Thermoregulatory Strategies of Naked Skinned Dinosaurs

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The startling and unprecedented cavalcade of evidence of definite non-scaly integumentary structures on dinosaurs stands as one of the most exciting developments in dinosaur research over the last several decades. Whether in the form quills, feathers, pynofibers or any of the variously described features put under the banner of "dino-fuzz" these features continue to bedazzle us, further blur the line between dinosaur and bird, and offer up important new vistas and possibilities as to what and how dinosaurs looked and behaved. A speculative movement towards "enfluffening" - that a more widespread and encompassing integumentary parade of dinosaurs of all sizes and stripes was likely - has come into vogue. At the acme of this development are speculative depictions of shaggy coated arctic ceratopsids, woolly sauropods, and furry ankylosaurs.

However even a cursory review of known feathered dinosaurs shows an astonishing bias towards theropods and especially smallish theropods (insert discussion on taphonomic bias). Among ornithischians we have evidence of quill structures on Psittacosaurus, "protofeathers" on the heterodontosaurud Tianyulong, and of course the startling, unexpected and weird mash-up of scales and novel assorted integumentary structures found on the basal ornithopod Kulinadromeus. Never the less this is a paltry selection compared to the abundance of evidence in theropods. Lack of evidence arguments can be applied to a failure to find structures in sauropodomorphs, iguantodontids, marginocephalians, and thyreophorans but these arguments - in my opinion - sound more and more like special pleading. It is more likely that these groups either never evolved such structures, minimally featured them, or secondarily lost them. Long and short of it is that I think that the signal we are getting from the fossil record is, in broad strokes, correct. Theropods were primarily the fuzzy, feathered dinosaurs and such structures were rare, reduced or absent in many of the more derived sauropods and ornithischians. However these structure may have been quite endemic and numerous in small and/or basal members of several ornithischian clans. 

Wherever the truth lies among who wore their hair long, shorn, or not at all among dinosaurs is still very contentious - chances are you might agree with some of what I said but probably not all of it. That is ok. This post is really not out to settle that debate. What this post is about is offering an alternative to "dino-fuzz" that accounts for the notable lineages of dinosaurs that so far lack good evidence for extensive insulatory pelages and that also lived in some pretty chilly environments. 

That some dinosaurs lived in what can only be described as cold climates has been known for some time now. While the Mesozoic was for the most part a hothouse climate with equitable climates penetrating much highter in latitude than modern ones, these areas still went through months of darkness as well as heavy cloud cover during immense precipitation events. To say that the Cretaceous north slope of Alaska was Minnesota cold is probably an overstatement but maybe as chilly as Seattle or even Tierra del Fuego might be more accurate. You would want to wear some layers in such environments. While presumably fuzzy theropods (and possibly some ornithischians) were important components of Mesozoic arctic faunas these environments also hosted dinosaurs that we have no evidence of integumentary structures. Examples include a titanosaur and the ankylosaur Antarctopelta  from Antarctica, Muttabarasaurus from polar Australia, and of course the abundant remains of ceratopsids, hadrosaurs, and ankylosaurs from the Cretaceous north slope of Alaska, among others. Integumentary insulatory structures are either wholly or almost completely unknown in these lineages. Most notable is the complete lack of structures found in hadrosaurs, despite the abundant, remarkable, and compelling skin preservation record for this group without even a stitch of integumentary structure found in their ranks. Now that is something to think about.

Despite the paltry or non-existent record of non-scaly integumentary structures in these groups we do have a pretty good record of skin impressions, scutes, osteoderms, and especially for hadrosaurs direct skin "mummifications" if you will. What I am going to suggest is a novel method of thermoregulation that these structures converge upon and that may in fact negate the need for an extensive outer insulating coat of feathers.

Before I get to the fun part just a little basic physics and assumptions that I am working from. First off I am working from the assumption that dinosaurs had a heightened level of internal thermal control - whether you want to call if "warm-bloodedness" or endothermy - they could generate internal heat on their own. However, ecotherms or "cold-blooded" physiologies are going to play a crucial part in this theory when it is all said and done... hint, hint. Next, just a little background on insulation and the physics of thermal flow. In modern day furred or feathered birds and mammals insulation works by trapping a layer of warmed air close to the body surface which retards the loss of heat from the body. Now it is more complicated than that of course with recent studies pointing to other uses of fur/feathers such as diffusing thermal radiation. But for simplicity's sake I am going to ride with the idea of trapping a layer of air being crucial to modern furry endotherms, people wearing clothes, and dinosaurs as well. That air plays such a crucial role in insulation is due to the fact that it does not conduct heat very well or more precisely temperature from a hot body to a cold surrounding or vice versa hot surroundings into a relatively cold body. This is why you can be fairly comfortable walking around with minimal clothing in 55 degree fahrenheit weather (13 celsius) but jump in water of that same temperature and you will be singing a different tune!! A liquid medium, or increased air flow, is going to whisk away heat from a body more quickly than a still layer of air - better yet for insulation would be a vacuum seal of air with minimal molecules. One way you can picture it is the less molecules bouncing around and "stealing" warmth from a body the better for insulation. That organisms that generate their own body heat find it evolutionarily advantageous to insulate themselves is fairly obvious. Not doing so would mean that extra calories are getting burnt to keep the house heated. Such large heating bills require an organism to eat that much more to stay at an optimal temperature. And endotherms, more so than ectotherms generally, want to keep their temperature in fairly narrow band that is optimized for their own enzymatic and physiologic pathways.

So how does a scaly, osteoderm studded, naked hided dinosaur trap a layer of warm air around itself to retard heat loss without an outer layer of insulation? 

The answer is simple and should be familiar to anyone who has installed insulation in a house. Dinosaurs did not trap a layer of warm air on the outside of the skin, but inside the skin.

In this scenario the outer layer of keratinized skin, osteoderms, plates, and other assorted scaly structures - largely dead, keratinized material anyway - act as the outer insulatory layer shielding the all important layer of warm air directly underneath them. Evidence of such thin ampullae, or hollowed out spaces visceral to the outer integumentary layer of dinosaurs, is very prevalent and I will go through them in a moment. What creates such hollow air spaces is simply shunting blood away from the exterior of the animal. Vasoconstriction is a simple and elegant mechanism known in all tetrapods that will withdraw blood away from extremities leaving behind, essentially a vacuum sealed outer layer of warm, insulating air with these structures. As I mentioned earlier a vacuum sealed air layer is the best type of insulatory layer (less molecules bouncing around stealing warmth).

Triceratops internal skin mould. credit Rapid City Journal

Above is the quite startling and unexpected internal skin mould of a Triceratops still unpublished in any sort of meaningful venue. But understand you are looking at the skin from the inside out. What you will notice is the large raised scales creating sort of a honeycomb effect. Also some of the scales have what has been described as a "nipple" like projection arising from them, best seen in the photo below of the external skin. To feed and grow such structures blood flow would be necessary, but in cold weather or especially hot weather blood flow would be shunted away from these outer extremities leaving the convex scales with their nipple like projections with a layer of warm insulating air underneath. The flipside is also a possibility. In extremely hot conditions blood flow to the extremities could be shut off insulating the cooler insides from the outside temperature extremes. When the animal wanted to cool off - in water, shade, or a prevailing wind - blood flow could be opened up engorging the outer scales and nipples, the large surface area allowing efficient conduction of heat away from the body.

External mould Triceratops skin. Rapid City Journal

The analogy to insulating a house is useful but let us also think about how a thermos works. A thin outer metallic layer separated by a thin layer of air, another layer of metal and the inside fluids. A thermos works to keep its fluids from heating up or cooling down, i.e. they work to keep cold stuff cold in hot outside temperatures and warms stuff warm in cold outside temperatures, All because of that thin layer of internal insulating vacuum sealed air.

But wait it gets better....

Remember that giant titanosaur osteoderm from Madagascar? Probably not, I had to do some internet sleuthing to rediscover it. But it is amazing.

The adult-sized osteoderm is the most massive integumentary skeletal element yet discovered, with an estimated volume of 9.63 liters. Uniquely, this specimen possesses an internal cavity equivalent to more than half its total volume. Large, hollow osteoderms may have functioned as mineral stores in fecund, rapidly growing titanosaurs inhabiting stressed environments.

I am not saying such structures did not serve as mineral stores but the possibility for thermoregulatory air space is a potential use too.

However the real pantheon of evidence for a subdermal insulating layer of air comes from the hadrosaurs, dinosaurs that for whatever reasons left us with a lot of skin evidence.

hadrosaur skin from museum quality fossil casts. inside and outside texture

Now take a good long look at this fossil cast chunk of hadrosaur skin. You will notice two general morphologies; in the upper section a nice bit of what has been described as hadrosaur "pavement" scales, non-overlapping convex polygonal; but trending down and to the left you see a distinctly different type of scale that has a raised outer rim and is concave. Go back and compare this morphology to the internal skin mold of the triceratops I discussed earlier. No, you are not looking at an internal mold of hadrosaur skin but in fact are looking at the layer of skin directly underneath the outermost layer of scales. Another way to put it is that the outer layer of skin has been stripped away through some taphonomic process in the lower quadrant leaving us with an image of what lies directly beneath the outermost layer of scale. What we see is that the interstitial space on the outer scale become an raised interstitial border on the visceral layer - in affect a mirror image - and what we have is a nice little hollowed out area immediately underneath the outermost layer of skin.

But don't take my word for it in describing what you are seeing in this unpublished chunk of hadrosaur skin taken from a fossil cast store. It is merely a good visual representation for what has already been described in the chapter on hadrosaur skin in the epic tome Hadrosaurs (2014, Indiana Univ Press): A Review of Hadrosaur Skin Impressions.

In describing Sternberg's famous mummified Lambeosaurus magnicristatus Evans & Reisz (2007) describe (pp 583):

"...The authors also noted and unusual relationship between raised polygonal scales and scales demarcated by raised insterstitial tissue, similar to what would be expected in a negative impression. In the area of the neck, the raised scales lie immediately external (superficial) to the grid of raised interstitial tissue. This suggests the preservation of both epidermal and deeper (dermal) tissues (cf. Manning et al., this volume). Regardless of their relationship, the size and morphology of the scales and deeper integument are consistent with one another. A similar relationship between raised scales and interstitial tissue is present also on an specimen of Edmontosaurus regalis (ROM 801) from an unknown region of the body (fig 34.2)." 

Catch all that? Evidence positive of a dermal pocket immediately beneath the outer dermis and which could potentially serve as a vacuum sealed layer of air when blood flow was shut off to the extremities.

Here is another good graphic of this system from the massive Baja lambeosaurine Magnapaulia.

Magnapaulia skin preservations. credit Prieto-Marques, Chiappe & Joshi. CC 2.5

Here is that same hollowed out morphology in the internal cast photos of Saurolophus skin (Bell, 2012).

credit Phil R. Bell. CC

And guess what, it get's better. Check out Dave Hone's website here where you can quite clearly see that Kulinadromeus exhibits the same morphology!!

I want now to take the liberty to name this integumentary feature.

S.ubdermal
I.nterstitial
G.ridded
I.nsulatory
L.ayer(s)

SIGIL

The acronym spells out SIGIL or to maybe use it in sentence: "Does the integument show evidence of SIGIL?" or "Are there SIGI layers?"

As I alluded to earlier in this post I believe that modern ectothermic reptiles have a strong role to play in all of this. Given that dinosaurs are nestled between modern crocodilians and birds - scaly hided and feathered respectively - for those dinosaurs that most likely were scaly hided modern reptiles prove a useful proxy model for investigating thermoregulatory functions in these dinosaurs. Wouldn't it be useful if there were some studies addressing this very issue? 

Turns out there is and a just recently published paper by Witmer labs (Porter  & Witmer 2015) that looks at this question through the lens of an ancestral diapsid condition that is remarkably precise in it's ability to fine tune the flow of blood for thermoregulatory functions.

What the study shows is that the ability to move and shunt blood - especially with relation to the cranio-cephalic region - is highly refined in modern diapsids

 and likely ancestral to the whole group. In other words reptiles can bask longer in hot conditions without cooking their brains or eyeballs due to their ability to move around blood and cool it off at select spots. The take home message is that this system was likely ancestral to dinosaurs.

For our interests here there is no reason to preclude dinosaurs from having a similar system of blood control, especially with regards to active peripheral control of blood movement.

However the study is really a refinement of what has been known for a while. That reptiles are not passive recipients of their thermal environment but active harvesters and, to an extent, hoarders of thermal energy. What this boils down to is that once a reptile has achieved its optimal temperature it can  maintain this temperature - despite what the ambient temperature is - for a lot longer that would be possible if it just had an open door policy with its environment.

It is worthwhile now to mention one man who is most responsible for how we now view reptile thermal physiology more than any other, Raymond B. Cowles. 

From his seminal paper from all the way back in 1957 "Possible Origin of Dermal Temperature Regulation" on the assumption that temperature regulation via insulation and vasoconstriction/dilation originated with endothermy:

What underpinned much of Cowles' work was a strange notion that dinosaurs - which at the time were believed to be ectothermic - died during a hot flash at the end of the Mesozoic. Cowles investigation into desert reptiles overturned the prevailing notion that desert reptiles had insatiable tolerance and proclivity towards heat - so much so that strange tails of snakes crawling into campfires were commonplace. Instead, Cowles found that desert reptiles had no such special proclivity towards heat and would die if left exposed to high temperatures for prolonged periods. Going with this strand of thought Cowles speculated - with no supporting evidence - that a sudden hot flash at the end of the Mesozoic killed off the dinosaurs. This "serendipitous flash" of inspiration that guided much of Cowles research into reptiles has not been borne out with prevailing evidence in the decades since. Never the less his work on reptile physiology that dovetailed with this "serendipitous flash" and then revolutionized the field is a fascinating story documented by Scott J. Turner titled Raymond J. Cowles and the Biology of Temperature in Reptiles (1984). I highly recommend taking the time to read Turner's paper on Cowles and his research.

Some excerpts from Turner's paper (Pp 433):

And finally (Pp 434): 

So what of this slight aside into the story of the iconoclastic reptilian physiologist Raymond J. Cowler? While I doubt there is any validity in his "serendipitous flash" theory of dinosaur extinction I do think that his life's work - showcasing reptiles as "masters not slaves of their thermal environment" is instrumental and prescient in establishing the success - not the failure - of dinosaurs.

What I suggest was going on with a great many dinosaurs, especially those that evolved layers of SIGI and equivalent integumentary features as I have outlined on this post, is that they were double-dipping. That is dinosaurs were harvesting thermal resources as an ectotherm would when it suited them but also generating internal heat when conditions were not favorable in their environment. In essence a blending of the best of the two physiologies of an ectotherm and endotherm with neither of the "perceived" limitations of either. In other words dinosaurs could conserve energy that would otherwise go into heat production when they were harvesting thermal energy to warm themselves. To retard heat loss they would take advantage of the thermally insulating potential in their layers of SIGI. Alternatively they could avoid overheating by either withdrawing blood away from the peripheral tissue (utilizing their insulatory layer) or by shifting it peripherally for heat loss through convection via wind, water or other cooling mediums. When the environment shifted towards cool conditions - at night, during monsoons, high latitudes etc. etc - endothermic generating gear would kick in, again, with heat loss minimized via SIGIL(s). 

There is numerous implications inherent in this idea with respect to dinosaur growth rate, efficiency and size attained that should be immediately apparent. Additionally there is the obvious benefit in the ability to sequester a free and abundant resource - solar thermal energy - that was plentiful during the often times hot-house world of the Mesozoic.

Gee, this is all very neat and interesting you are probably thinking but wouldn't it be nice if we had at least one reptile that displayed this "double-dipping" of both physiologies at hand? Well, in what I can only describe as "serendipitous" what came out this last week but a paper describing just such a reptile that utilizes both classic reptilian ectothermy and what can only be described as a primitive seasonal "warm-bloodedness" in tegu lizards (Salvator merianae).

credit Tattersall et al, 2016 Creative Commons

Pretty darn cool. This study even suggests the impetus to evolve endothermy for reproductive success. I wonder if there could be any similarities in the subdermal anatomy of tegu skin and dinosaur skin... hmmm looks almost like a "skin envelope" where have you heard that term before (hint, hint dinosaur mummies).

Salvator genus tegu. credit Donar Reiskoffer CC3.0

Before I dig into several studies suggesting thermoregulatory functions in the dermal structures in  armored dinosaurs a brief word on the concept of "mesothermy" and nuance in dinosaur temperature range. A recent study (Grady, 2014) pointed to widescale mesothermy in dinosaurs (mesothermy implying temperatures generally lower than modern mammals and birds but higher than the environment). Furthermore a study looking at isotopes in eggshells as a proxy for dinosaur temperature surprisingly found oviraptors maintaining lower body temperatures than titanosaurs. I don't intend to do a full review of the literature on dinosaur temperature only suggest that a wide range was possible and probable. We should not insist on or expect a "one size fits all" approach to optimal dinosaur temperature.

In an extensive review of ankylosaur dermal armor Hayashi et al. (2010) found that dermal structures that differed in morphology had a remarkably consistent pattern of histology. Although some dermal structures, especially in later ankylosaurs showed increasing cortical strength and integrity (implying use in combat) other osteoderms seemed remarkably thin and underequipped for defensive purposes. "Pipe" like structures and extensive vascularization in all of the dermal armor implied a potential for thermoregulatory purposes which the authors compared to the use of osteoderms in crocodiles.

If, as the above study suggest, such highly vascularized structures had blood flow restricted via vasoconstriction the numerous ampullae "pipe" like structures and other hollowed out features would now serve as a buffering insulatory layer as I discussed with layers of SIGI. However the main function was of course to exchange heat with the environment.

Another study, this time looking at stegosaur plates, found the exact same pattern of extensive vascularization and large "pipe" like structures (Farlow, Hayashi, & Tattersall 2010).  Note that Tattersall is the same Tattersall from the work on Tegu lizards I discussed earlier, hmmm very interesting coincidence there.

From the abstract:

"In Stegosaurus the potential thermoregulatory role of the plates may have been greater than in other thyreophorans, by virtue of their extensive internal and external vascularity, their large size, thin cross sections above the plate base, dorsal position, and alternating arrangement."

Again, the authors make the connection between the stegosaur plates (which are actually modified osteoderms) and crocodile osteoderms. The study also shows experimental evidence that the osteoderms scutes on the back of crocodilians - in this case a Caiman latirsostris - are fundamentally important in the animals' thermal bank account.

As I mentioned earlier different dinosaurs may have varied in the relative amount of internal heat that they generated and external heat that they absorbed. It was likely not a one size fits all situation. Note also that uses for display and combat are not mutually exclusive with uses of thermoregulation.

I also noted this cryptic little sentence in the paper:

"Our observations on plate vascularity say nothing about whether heat exchange with the environment primarily involved heat gain or loss."

Hmmm, I wonder if Hayashi, Dodson & Tattersall know or suspect more than they are letting on. In either case they are being good, conservative scientists about it - not really speculating beyond the data. An approach I will not take.

Now finally onto the crocodile question. Both of the last two studies kept coming back to a comparison of the dermal structures in thyreophorans to crocodile osteoderms. This is a little weird because thyreophorans likely had some degree of endothermy while crocodiles do not. Wait a second, let me correct myself, modern crocodiles are cold blooded but several lines of evidence - a four chambered heart, avian style unidirectional breathing , erect gait - point to a history of endothermy in crocodylomorpha.

Actually what I think was going on was that "endothermic" crocodylomorphs of the past were actually  switch hitters like naked skinned dinosaurs. They could, if needed, generate heat internally but were more than happy to let their intricate osteoderm soak up rays like a built in solar panel. Later, when crocs became more specialized towards an ambush, semi-aquatic existence they abandoned the expensive physiology of endothermy and simply co-opted their solar panel osteoderms for a fully cold-blooded lifestyle.

What has muffled the issue of dinosaur physiology for naked skinned species is that from the extant phylogenetic bracket for dinosaurs - birds and crocodiles - the naked skinned crocodilians have possibly secondarily lost their capacity to generate heat. If endothermic crocodilians were still around we would have a much clearer analogy to the thermoregulatory strategy of many dinosaurs. However since the only extant endothermic archosaurs around are birds - their physiology and obligate insulatory feathers have dominated thought on how and what it means to achieve a heightened metabolism in dinosaurs.

Well then, I think that is a lot to digest. I have taken you through my thought process and how I arrived at this theory as best as I can. It doubtless needs refinement and additional testing. I might be totally right, partially right, or way off. But it is something to think about and offers some new perspective and thought on the issue of dinosaur integument, physiology and what it really means to be warm or cold blooded.

In conclusion,

Since the gradual and accumulating evidence for widespread endothermy or at least mesothermy in many dinosaurs it has become somewhat anathema to compare dinosaur thermal physiology to ectomthermic reptiles. While mounting evidence suggests the potential for widespread insulatory coats in many, if not all theropod lineages, such evidence is much thinner in many other dinosaur lineages and completely absent in several despite abundant casts and direct skin preservations. Research, thought, or even speculation has been lacking in terms of explaining how such naked skinned dinosaurs insulated themselves given wide distributions up into polar regions. Here is presented a novel hypothesis addressing insulation in such naked skinned species. An anatomical feature referred to as the subdermal interstital gridded insulatory layer(s) - or SIGIL - is outlined and referenced via several lines of evidence. Through vasoconstriction blood flow can be diminished to this layer creating an insulating, vacuum sealed layer of air visceral to the outer skin and which insulated dinosaurs from temperature extremes. Additionally, this layer could be vasodilated and engorged with blood to facilitate heat shedding or heat uptake into or from the environment respectively. This ability to control blood flow to the extremities is likely ancestral to all tetrapods and is a simple co-option of known capabilities in extant ectothermic reptiles. A novel ability to both absorb thermal energy from the environment and create internal heat is inferred for many dinosaurs via the efficient capacity for heat exchange and insulation through SIGIL. The extent of this dual functionality likely varied significantly across families and genera and offers potential insight into efficiently achieving gigantism and fast growth rates with minimal or non-existent parental provisioning of food and at rates much more efficient than other endotherms in terms of food intake. The intricate vascularized osteoderms of several types of dinosaurs are argued to represent the acme of this "dual functionality" and crocodylomorphs are inferred to have a congruent thermal function for their osteoderms as well as secondarily losing this "dual funtionality" in their evolutionary history that they once shared with their extinct archosaurian brethren.



References

Bell, Phil R. (2014) A Review of Hadrosaur Skin Impressions. Hadrosaurs Univ.  of Indiana Press editors David A. Eberth & David C. Evans

Bell PR (2012) Standardized Terminology and Potential Taxonomic Utility for Hadrosaurid Skin Impressions: A Case Study for Saurolophus from Canada and Mongolia. PLoS ONE 7(2): e31295. doi:10.1371/journal.pone.0031295

Cowles, Raymond B. (1957) Possible origin of dermal temperature regulation. Evolution Vol 12 No. 3

Grady, J. M., Enquist, B. J., Dettweiler-Robinson, E., Wright, N. A. & Smith, F. A. Evidence for mesothermy in dinosaurs. Science 344, 1268–1272 (2014)

Farlow James O, Hayashi Shoji, & Tattersall Glenn J (2010) Internal vascularity of the dermal plates of Stegosaurus (Ornithischia: Thyreophora) Swiss Geological Society

Hayashi S, Carpenter K, Scheyer TM, Watabe M, & Suzuki D (2010) Function and evolution of ankylosaur dermal armor. Acta Paleontologica Polonica 55 (2): 213-228




Headden, Jaime. The Enfluffening. The Bite Stuff. February 10, 2013

Naish, Darren. Life as a Stegosaur: the SJG stegosaur special part III. Tetrapod Zoology. January 10, 2011

Porter WR, Witmer LM (2015) Vascular Patterns in Iguanas and Other Squamates: Blood Vessels and Sites of Thermal Exchange. PLoS ONE 10(10): e0139215. doi:10.1371/journal.pone.0139215

Prieto-Márquez A, Chiappe LM, Joshi SH (2012) The Lambeosaurine DinosaurMagnapaulia laticaudus from the Late Cretaceous of Baja California, Northwestern Mexico. PLoS ONE 7(6): e38207. doi:10.1371/journal.pone.0038207

Tattersall Glenn J., Leite Cleo AC, Sanders Colin E, Viviana Cadena, Andrade Denis V, Abe Augusto S, Milsom WIlliam K (2016) Seasonal reproductive endothermy in tegu lizards. Science Advances Jan 22 Vol. 2 No. 1

Turner, Scott J. (1984) Raymond J. Cowles and the Biology of Temperature in Reptiles. Journal of Herpetology Vol 18 No. 4.

Witton, Mark. Dinosaur Skin: Some Thoughts for Artists. MarkWitton.com December 24, 2015

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