Wednesday, January 27, 2016

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

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 dinosaursScience 34412681272 (2014)


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



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


"A Long habit of not thinking a thing wrong, gives it a superficial appearance of being right, and raises at first a formidable outcry in defense of custom". Thomas Paine

Support me on Patreon.
Like antediluvian salad on facebook. Visit my other blog southlandbeaver.blogspot
Watch me on Deviantart @NashD1Subscribe to my youtube channel Duane Nash.

35 comments:

Kevin Franck said...

Actually although there are clearly reptilian similarities of Dinosaurs to today's reptiles, I have never thought of their skin as being scaly or them necessarily all being cold blooded. I believe their skin was probably quite similar to Rhinos and Elephants. Take the Triceratops as an example. Looks like a Rhino on steroids.

Anyway, interesting article, thanks

Duane Nash said...

Thanks for comment Kevin Franck. Most evidence of preserved skin so far points to non-overlapping scales like in crocs, gila monsters, monitor lizards and tegu lizards. Kulinadromeus had some weird stuff going on with its scales too. So far no evidence of leathery hides as in rhinos or elephants although that look was for a while quite often depicted in paleoart.

khalil beiting said...

Wow. It all actualy makes a lot of sense as to how scales could be used for thermoregulation. Thank you for answering the question of "why do massive herbivorous non avian Dinosaurs lack large amounts of feathers?". I've had that question for years now, yet nobody could give me a solid answer. But I am curious as to multiple things that should be addressed some other time:
1. Why is it that Kulindadromeus has large amounts of feathers, yet also has these thermoregulatory scales?
2. Why is it that this feature is only prevelent in medium to large sized herbivores, yet isn't common in equally large sized Theropods (although Carnotaurus and other Abelisaurs might be themajor exception to this rule)
3. Why is this feature not too prevelent in small sized animals?
4. Why/how are these scales more beneificial then feathers?
5. Why aren't these scales prevelent in Avian Dinosaurs (and Theropods in general) when they are so beneficial?
And just a side note, but taphonomy plays a greater role then you portray it as. It's incredibly hard for an animal to be fossilized with it's integument. Sure, scales are much easier to be preserved because they are incredibly durable, but fiborous integument (especially feathers) are incredibly hard to be preserved since they are so dainty. It's settled that these scales serve a thermoregulatory purpose that may or may not be more efficient than feathers, but that doesn't rule out large amounts of feathers for other purposes. Intimidation, attraction, communication, camoflauge, resistence from the elements, etc. are major factors in determining feather covering on an animal. And like I said, it's still incredibly hard for feathers to be preserved. Even in animals that have vast amounts of feather covering, if the envirnment isn't good for preservation then even in cases where feathers are preserved they aren't even that abundant. One of the most recent feather remains from a North American Ornithomimosaur has only 3 very small patches of feathers found on random areas of the body, even though in reality it had about the same amount of feathering as modern day ratites.
This is just another question, but I was a little confused in the post. Are you saying that there are subdermal, partially hollow scales underneath the skin that could be used in thermoregulation, or are you saying that there are two layers of scales i.e. the outer layer and then subdermal scales that control temperature?
Yet another question, but this has to do with overheating. Could it still (hypothetically) be possible for an animal the size of a Hadrosaur or even a Sauropod to possess large areas of feathers throughout the body without overheating? The feathers might not nesecarilly be used for thermoregulation, but maybe for something else. Could that be possible without the animal overheating?
And this is my last question, but couldn't cold weather Dinosaurs that only possess these scales still die/become ill due to frostbite or other weather factors? One way to get around this is by covering yourself up with a pelage of feathers or fur so that the frost can't actually get to your skin, but I don't know if such a problem would occur in the case of scales.

Bk Jeong said...

I do get this, but considering most arguments on integument revolve around theropods (which are feathered-be quills, true feathers or simple fluff)....

Also, this post may potentially be used as an argument for BANDits.

Personally, I think theropods were almost always feathered (even the earliest ones), and that sauropods and ornithischians lost integument multiple times.

Em Miller said...

"...highly refined in modern synapsids and likely ancestral to the whole group..." did you mean diapsids here?

Duane Nash said...

Great comments and questions.
@khalil 1. I don't think it is settled that Kulindadrkmeus had feathers. We can discuss in email further.
2.I don't think we can say which species did or did not have SIGIL. Not too hard to evolve this feature from the ancestral condition. Hadrosaurs have the best skin record so I focused on them.
3. Same reason as #2.
4.I don't know if which is more efficient it would be an interesting test. I did find it interesting that titanosaurs maintained a higher temperatures than oviraptors.
5. Maybe some theropods did have them. Probably quote easy to evolve.

As I alluded to in the post towards the end where I suggested SIGI layers also occurred incrocs I suspect that this was the ancestral arcjosauroan conditions. Feathers evolved later for whatever reason you want to insert and made the SIGIL redundant because the success of the system requires open access to the environment ie no or minal fur or feathera.

Duane Nash said...

Em Miller thanks fixed it.

David Marjanović said...

but were more than happy to let their intricate osteoderm soak up rays like a built in solar panel.

Wait. In the caiman pictures you show, the osteoderms are colder than their surroundings, fitting claims in the literature that turtle shells act as a noticeable degree of thermal insulation. I thought the passages about large hollow spaces inside osteoderms was leading up to that conclusion – and suddenly you state the opposite?

And did I get that right – you think there were air spaces so deep in the skin that there were blood vessels around them, meaning that blood flow to the outside of the air spaces could be shut off for insulation? If so, how did the air get in there?

David Marjanović said...

(Air spaces between successive layers of shed, dead skin are of course easy to imagine. That's what a rattlesnake's rattle is.)

Duane Nash said...

Thanks for comments/questions David

per the caiman picture showing that the osteoderms are colder than the surrounding skin - which is taken from the Farlow et. al paper I referenced and linked to - the paper quotes Seidel who said about alligator osteoderms: " dense bone of the osteoderm is riddled with small holes that are presumably arterioles. Any heat picked up by the surface of the osteoderm will be rapidly transmitted throughout the entire osteoderm via its compact mineral composition. The arterioles in the osteoderm could readily pick up the absorbed heat and carry it to other parts of the body. To control heat absorption, simple vasoconstriction could reduce blood flow and thereby reduce heat transfer rate." Granted I should try and hunt down that paper by Seidel in its entirety..

Additionally the authors address this very observation of the osteoderms that you noted (they are cooler than the surrounding skin) From the Farlow paper: " In both caimans the temperature of the animal's skin overlying the osteoderms was cooler than that of skin between the osteoderms. This suggests that relatively cool blood may have been preferentially flowing to the osteoderms. Had the osteoderms been an insulating thermal barrier, they should have been expected to be as warm, or even warmer than, the skin in other parts of the back. Thus the osteoderms of these caiman may have been involved in heat collection."

So it is a good observation that you made David and I see how it can cause confusion but it is very consistent with the osteoderms acting as active sites of thermal exchange.

"You think there were air spaces so deep in the skin that there were blood vessels around them, meaning that blood flow to the outside of the air spaces could be shut off for insulation?"

I am not sure where you got this idea from what I wrote, sorry if I confused you somewhere. I think the "air layer" was immediately subdermal. See the photos of the triceratops and hadrosaur skin I provided. As first noted in the chapter from the Hadorosaurs on skin impression, many of the concave impressions in hadrosaur skin are not negative impressions but a record of subdermal features. So what I am suggesting is that this thin layer - SIGIL - could be fully engorged with blood to diffuse heat or bask and in this situation vasodilation would occur. In order to set the insulating layer in action vasoconstriction would occur creating the slight hollow insulating layer as blood was shut off from its extremities. In essence it would be a vacuum seal - which is better for insulatory purposes since less molecules bouncing around - the tough, keratinized scale would maintain its shape even with the vacuum seal. The role of osteoderms, scutes, plates etc etc especially of the thyreophora augmented this system, but did not supplant it. They were extensions of it and may in fact highlight a range of thermoregulatory strategies in naked skinned dinosaurs. All had the ability to generate some internal heat but - to various degrees - they also harvested solar thermal energy as well. The ultimate hybrid.

Duane Nash said...

edit "This suggests that relatively cool blood from the core was flowing to the osteoderms."

The Eurypterid said...

Are there any differences you could hypothesise between feather and SIGIL insulation? Tissues with varying temperatures (eg feet, tails, noses, body surface in large-bodied animals) seem to be more likely to be scaled than those that require a more constant environment (eg body surface in small-bodied animal, main body in mid-sized).

Duane Nash said...

It definitely needs testing the question of SIGIL vs feather insulation.

RyanKoopa said...

This is a really interesting article, it makes a lot of sense when you think about it. I'm surprised nobody thought of this theory in application to dinosaurs before you did! I want to look at this from a Speculative Evolution viewpoint though. Was this integument system more or less efficent at heating up feathers then feathers&fur? If non-avian dinosaurs had survived to the present day, would feathers be more common in the temperate&polar regions then they were in the Mesozoic? Would scaled lineages become feathered, or need to somehow further refine these insulatory scales?

Alexander Ruger said...

Very interesting, very nuanced article on dino thermoregulation. I appreciate that you didn't automatically assume Kulidadromeus' filaments were feathers. The enfluffening pendulum has swung a bit too far, and I'd like to see a return to the middle.

Is there really a case to be made for an endothermic crocodile ancestor? Of course the four chambered heart, one-way breathing, and gait are suggestive, but is it maybe possible that croc ancestors and pseudosuchians were simply more active? Monitor lizards have a modified heart to let them be more active, but they still have a metabolism in line with the majority of reptiles. Could we maybe be falling prey to Bakker's old "mammalian bias in thinking ectothermy precludes activity?

Let me know your thoughts, and thanks for the quality blog!

Alex Ruger

Duane Nash said...

@RyanKoopa see my earlier comments on feathers vs SIGIL for insulatory benefits, testing needs to be done (someone should harass Witmer labs into looking at this). As I mentioned in the post a thermos is an apt analogy to what I am describing. You don't see people carrying around feathered or furred mugs to keep their liquids warmed do you?

@Alexander Ruger As I am leaning more and more towards for a great many dinosaurs, I suspect mesothermy might be the ancestral metabolism of many croc ancestors. As I laid out in the post an ability to generate some internal heat - which allowed giant scaly hided dinosaurs to make it through arctic winters - combined with an ability to harvest thermal energy and then to "hoard" both forms of energy via SIGIL shows much promise. Studies on croc ancestor metabolism are needed... are there any? I got to look into that!!

On Bakker's "mammalian bias" Well Bakker did a lot of work directly with monitors looking at their gait, breathing efficiency, and was well aware that ectothermic reptiles could be active so I don't think that HE had a bias. What he didn't subscribe to was that gigantothermic ectotherms were what the dinosaurs were - which I agree with and which he marshaled numerous lines of evidence against. Not the least of which is: if this is such a successful strategy where are all the giant multi ton terrestrial reptiles today or even in the past?

I think the tegu study on seasonal endothermy in that reptile is very intriguing. It makes me wonder if there are not more examples like it. I mean its not like there are detailed metabolic studies done on >every< reptile. And given the congruent ecology between tegus and monitors I would not be surprised in the least if some types of monitors were shown to have some incipient heat producing endothermic abilities. If you were to ask me where we should look I would definitely start with them!!

strangetruther said...

Brilliant idea Duane!

https://sciencepolice2010.wordpress.com/2016/01/29/the-dinosaur-wore-bubblewrap-duane-nashs-brilliant-sigil-idea/

Alexander Ruger said...

Thank you for the response!

Just to pick your brain (because I think I agree with you on mesothermy and Bakker on gigantothermy), do we have to necessarily prescribe dinosaurian success to their metabolism? After all, mammals were/are endothermic, and they never grew to multi-ton (or even single ton) sizes during the Mesozoic. Couldn't it have been their gait, respiration, or factors that weren't physiological?

Apologies if I'm getting a bit outside the scope of the post here. I appreciate you being open here in the comment section.

Alex Ruger

strangetruther said...

Some further thoughts, e.g. on exactly how such gas spaces might have worked etc...

https://sciencepolice2010.wordpress.com/2016/01/30/the-dinosaur-wore-bubblewrap-pt2-further-thoughts/

Duane Nash said...

@Alexander Ruger I tend to give metabolism a lot of merit because it touches upon pretty much every aspect of an animals life -foraging, growth rate, reproduction, movement, gait. On the mammal question - and that is a good one that comes up a lot - first of all I would question the assumption that Mesozoic mammals were as fully warm blooded as most modern ones - many could have very likely been more in the range of echidnas. The difference I presume, and your question just led me to this, is that naked skinned dinosaurs were more efficient than furry mammals at harvesting solar energy. This dual functionality allowed more efficient growth rates, higher temperature, and generally a more thrifty and energy efficient design for the conditions of the Mesozoic than mammals.

The Eurypterid said...

I had a (probably wrong) thought- could dinosaurs' bare skin be more resistant to infection? Off the top of my head, bare-skinned animals like humans, hippos and seals tend to be more resistant to injury than others, and allosaurs are known for frequent injury survival...

strangetruther said...

Yup - sunbathing easier without a fur coat, and presumably less of a problem with parasites.

Daniel Hartig said...

Why are these features necessary for thermoregulation in 'cold' climates? Why is harvesting sunlight an advantage in a 'hothouse' world?

A paper cited on the wikipedia page https://en.wikipedia.org/wiki/Polar_forests_of_the_Cretaceous suggests that seasonal variation in high latitude cretaceous forests ranged from 10 to 22 °C. That makes the cold season close in temperature to Cape Town, Algiers, or Rawalpindi; all places that elephants lived in the near past. Elephants are comparable in size to some of the non-insulated dinosaurs and are similarly non-insulated.

Shouldn't we be looking at how the increased surface area to air interface of some of these structures could aid cooling? I think that modern birds/reptiles have a comparative advantage in the heat compared to mammals. The distribution of large land-based birds/reptiles in in the later Cenezoic has been strongly correlated with areas that don't freeze often. Look at the northernmost distribution of alligators, the strength of varanid lizards in Australia, Terror birds like Titanis not being found farther north than Texas/Florida, etc. The ostrich's niche is apparently being able to hang out in the mid-day sun in seasonally-arid africa, free from worries about predators (incidentally, some think that was also the early human's niche).

Cretaceous tropical temperature estimates vary widely, but most seem to indicate surface temperatures at last 5C hotter than the present. Elephants are heat limited on hot summer days (http://www.sciencedaily.com/releases/2011/09/110930071659.htm). How much harder for an animal twice as massive in much hotter temps? If elephants drop their body temps at night to compensate, how much better would a not-strict-ectotherm be able to drop their body temp at night?

An osteoderm based heat exchange system seems more likely to be competitively advantageous for for shedding heat in the sweltering tropical shade than storing heat overnight in the polar regions, where, due to the size of the creature, that just might not be necessary.

Duane Nash said...

Per "cold climate" and thermoregulation in a hothouse world. Because dinosaurs were not all elephant sized and even the big ones started small.

If you can steal warmth from the sun that is less calories you have to burn to maintain a stable temperature during polar winters or long monsoonal storms. More efficient and also allows more food is going into growing bigger rather than staying warm.

If you read the whole post you will see that I directly mention that SIGIL works for both heating and cooling the blood depending on the circumstances. The increased surface area is useful but what is ultimately the key is the ability to both shunt and flush blood to the outer layers to exchange temperature with the environment, whether it be relatively hot or cold.

khalil beiting said...

In the post and in the comments, you mention mesothermy and possible gigantothermy in extinct non avian Dinosaurs. One of the highlights of the TPA 2015 meeting shows something else going in:
Cracking dinosaur endothermy: palaeophysiology unscrambled

Max. T. Stockdale, Michael. J. Benton and Octávio Mateus

The amniote eggshell is a respiratory structure adapted for the optimal transmission of respiratory gases to and from the embryo according to its physiological requirements. Therefore amniotes with higher oxygen requirements, such as those that sustain higher metabolic rates, can be expected to have eggshells that can maintain a greater gas flux to and from the egg. Here we show a highly significant relationship between metabolic rates and eggshell porosity in extant amniotes that predicts highly endothermic metabolic rates in dinosaurs. This study finds the eggshell porosity of extant endotherms to be significantly higher than that of extant ectotherms. Dinosaur eggshells are commonly preserved in the fossil record, and porosity may be readily identified and measured. This provides a simple tool to identify metabolic rates in extinct egg-laying tetrapods whose eggs possessed a mineralized shell.

Duane Nash said...

I don't think gigantothermy is very likely in dinosaurs. Eggshell porosity is interesting I should read that. I wonder if the study takes into account differing gas levels in the Mesozoic?

khalil beiting said...

Oh I thought you were suggesting that Gigantothermy was possibly present in non avian Dinosaurs earlier on. I also don't find that likely, but I do find mesotherym and differing levels of endothermy most likely for the whole of Avemetatarsalia (with many forms of ectohermy, gigantothermy and mesothermy in Psuedosuchia and Phytosauria). Let me know what you think of the article after you've read it by commenting here.

Duane Nash said...

I can just find an abstract for the Benton project.

Nick Fonseca said...

Just a weird analogy I was thinking of for this idea other than the thermos would be bubble wrap. I believe I have heard of people using bubble wrap to insulate windows and the like. They also use it to wrap perishable items such as food for delivery. I also wonder what effect, if any that color would have in addition to the thermal qualities of the scales. I'm thinking colors with heat reflective/absorptive properties i.e. light or dark. I am also shocked that people assume insulation can only be useful for one temperature. You mention in your post that the thermos like the scales has the ability to keep things warm and cool.

Duane Nash said...

Yes i think the bubble wrap idea is a god one and people are using it. As dinosaurs either flushed their skin with blood or shunted it away their skin probably did change color. Walrus turn white in cold water. Also built in camo for when it snows!!

lyuti said...

Darren, I'm sorry and a thousand more times sorry. I have not many time to read now, and your posts deserve a slow reading, also i am a slow thinker. So, it is not fair with your work to make any statement right now. Just that the them of the post reminded me of this paper http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2736124/, titled A new scenario for the evolutionary origin of hair, feather, and avian scales. The author is a skin embryologist..well, it seems that some of them write popular papers even today, he concludes that feathers "may have evolved independently of squamate scales, each originating from the hypothetical roughened beta-keratinized integument of the first sauropsids. The avian overlapping scales, which cover the feet in some bird species, may have developed later in evolution, being secondarily derived from feathers."

So...feathers and reptiles scales have different origins, which is very interesting. I don't know why, probably because it can tell something about the extant reptiles we can see in our current world, but it sounds interesting if this guy is right.

Duane Nash said...

Hi Lyuti remember I am Duane Nash but that confusion between myself and a certain British blogger~researcher has happened before lolz. Yes that feathers did not evolve from scales has been in discussion and that they have separate origins has been out there.

The Eurypterid said...

Had some more thoughts on the relationship between SIGILs and feathers.

Basically, if you had a surface made up of vasoconstricted SIGILs and erectile feathers, the SIGILs would be more metabolically expensive. Erection in feathers can save energy through a variety of methods such as leverage and neutral positions. Vasoconstriction in SIGILs, on the other hand, must apply muscular force directly to the veins on their entire length. This means that actively-thermoregulating SIGILs have an increased metabolic cost all on their own, making it only worth having them on a surface where rapid heat exchange (SIGILs' main feature) can take place. In addition, this has to be advantageous heat exchange.

We can see this by how there is a much greater correlation between presence of scales (assumed to be SIGILs) and feathers in two carnivorous dinosaurs with extensive regions of each- Carnotaurus and Yutyrannus. Carnotaurus lived in Cretaceous Argentina 70MYA, which (as far as I can tell from a look at a map) was likely a warm, mild climate.

This would've created an ideal environment for the presence of SIGILs. When inactive, Carnotaurus could relax its SIGILs to absorb heat during the day, or constrict them to retain heat during the night. When active, it could constrict its SIGILs in the day to stay cool, and relax them to release excess heat at night. Seeing as most predators prefer to remain inactive during the day and active at night, this would have made the default state of the SIGILs- relaxed- advantageous.

Yutyrannus, on the other hand, probably lived in a temperate environment where it was too cold to easily absorb heat. This would mean that SIGILs, for the most part, would have to remain constricted, making a coat of feathers much more metabolically efficient.

For very large dinosaurs, such as hadrosaurs and sauropods, it is possible that their skin-to-surface-area ratio made the vasoconstriction of SIGILs metabolically negligible. Gigantothermy could also be used to make SIGILs more advantageous, using their surface biology as a 'heat sink' in unfavourable temperatures. Whenever these unfavourable conditions for heat transfer were lifted- in a rainstorm when it's hot, for example, or when it's sunny but not windy in the cold- the SIGILs could be used to rapidly empty this 'heat sink'. Non-organ tissues in smaller dinosaurs, such as the tail, could possibly be used as a heat sink as well- and we see that, in mixed feather/scale species, it's the hardier limbs and tail that are generally covered in scales.

For small dinosaurs in a favourable climate, feathers would have an advantage outweighing SIGILs- the size of the thermoregulatory area. Feathers can stick up from the skin by a large margin, making them lightweight relative to the pocket of insulation. SIGILs would be much bulkier, seeing as they are attached to the skin at all points. The fact that small animals expel or absorb heat proportionally faster (surface area/volume and whatnot) might also be a reason to retain feathers, as the advantages of absorbing or expelling heat with SIGILs would be mitigated if small body size meant feathers could expel or absorb heat before the conditions allowing that ended.

Does this sound suitably science-y?

Duane Nash said...

Great comment Eurypterid. I see your reasoning here I want to think about it some more. A few other points of inquiry; small hadrosaurs, sauropods etc etc growing up in polar environs that are - as far as I know unfeathered. I also am intrigued by several reports that show basal birds, paravians, various maniraptorans having fairly low "mesothermic" temperature ranges. Additionally, titanosaurs showing the best signal for high temperatures? Maybe gigantothermy does play a part?

The Eurypterid said...

Thanks! :) Do note that I'm some random teenage dino nerd, so corrections on assumptions may be required.

For the low body temperatures in mesozoic avialans, perhaps it's a derived trait in neornithines, and more of an oddball offshoot than the norm? If I remember correctly, a waterfowl-based origin for neornithines is likely, and modern waterfowl (ranging from geese to waders, so it's probably convergent) tend to be skilled migrants. These migrations need a stable and high internal temperature in often-harsh conditions, making primitive neornithines develop a high metabolism, well-suited for these long journeys, at the cost of their ability to conventionally thermoregulate using the environment. This resistance of external conditions, instead of going with the temperature flow, could be a reason why they survived the KPg and other avians died out.

I think gigantothermy would be reasonable for titanosaurs- or, at least, a resistance to their poor thermoregulation due to their low area-to-mass ratio. High temperatures would also assist in digestion and fermentation, possibly making sure that the added food needed for endothermy wasn't quite as much. Younger animals would need a high metabolism to get to that state, due to growing so fast.

A seperate point in regards to large, feathered theropods- another factor on why theropods seem to prefer feathers is that they need rapid exertion, which increases heart rate and volume of blood being pumped. This would create high blood pressure, further increasing the exertion of vasoconstriction in SIGILs.

Related Posts Plugin for WordPress, Blogger...