Thursday, 10 January 2013

Skin-deep: the 'One Skin Fits All' approach to integument reconstruction in palaeoart

The snowy, chilly plains of Maastrichtian Alaska, where Pachyrhinosaurus perotorum roamed. But were they scaly like other ceratopsids, or covered in protofeathers, as shown here?
So... no pressure here, then. I innocently replaced my festive Facebook profile picture with some detail from the painting above last Sunday (07/01/13) and quickly found a storm (well, gusty conditions) of discussion, 'likings' and shares, with several folks mentioning their anticipation of this post to see what all the fuss is about. The Facebook responses have been interesting and mixed: nods of approval, some head-slapping 'why didn't I think of that', a revelation that Tom Hopp already did this last year and, perhaps more predictably, scepticism from a number of individuals who consider the whole concept very silly indeed. In short, there seems to be a certain amount of expectation in the air about this image, and I wouldn't be surprised if some virtual beer bottles are hurled at me across the Internet should this explanation not prove convincing. Here's hoping I've done my homework properly, then.

The painting in question shows a family of the Alaskan centrosaurine Pachyrhinosaurus perotorum, a species notable for its existence in rather chilly, latest Cretaceous climates at palaeolatitudes of 80-90°. It differs from other pictures of this species by having its Muskox Quotient upped by 500%, replacing the scaly hides of more traditional Pachyrhinosaurus reconstructions with a blanket of fuzz analogous to the fuzzy, unkempt feathers of modern ratites. Fuzzy polar dinosaurs are not unusual in palaeoart nowadays and they result in animals that look immediately more at home in icy, subfreezing climates than their scaly brethren. This image, however, directly contradicts what most folks will say we know about horned dinosaur integument. Some comments on Facebook have already wheeled this argument out: known ceratopsian integuments were predominately scaly, so the concept of a shaggy pachyrhinosaur is nonsense, right? Well, I'm going to argue here that it's not, or at least not a concept that is easily dismissed. Before we go any further, it's worth stressing that I'm not presenting this image as the new 'standard' for Pachyrhinosaurus perotorum: I don't know of any new evidence that confirms the shaggy hides shown here, be it soft-tissue remains of ceratopsids or a new interpretation of dinosaur evolution that suggests super-fuzzy ornithischians were common. Nor, for that matter, do I have the heads up on research indicating that latest Cretaceous Alaskan palaeoclimates were much lower than expected. Instead, across four points, I'm going to argue that, based on what we know of dinosaur evolution, the responses of modern animals to their environments, and - importantly - the vast gulf of unknown data regarding dinosaur appearance, that this concept is as plausible as our scaly variants and, in some respects, may be more plausible. On the way, I'm going to suggest that, as with some other considerations in palaeoart, we may be too conservative when it comes to depicting animal integuments, because we focus too much on their evolutionary relationships without considering their likely adaptations to habitats and lifestyles. Hmm... this is all starting to sound very All Yesterdays, isn't it? That's not a coincidence.

1) Ornithischians were fuzzy, and some were probably fuzzier than others.
First up, the least controversial pin in this case. Thanks to Tianyulong and the early ceratopsian Psittacosaurus, we know that ornithischians were covered in more than just scales, the former being covered in filamentous structures akin to early feathers and the latter possessing long quills (Mayr et al. 2002; Zheng et al. 2009) Accordingly, it's now fairly fashionable, and by no means unreasonable, to restore even large ceratopsids with at least a smattering of quills across their bodies like those seen on Psittacosaurus, reflecting a relict integument from an earlier phase of their evolutionary history. It naturally follows that we should expect some taxa to have been more densely adorned with filaments and quills than others, just as fur and feathers are of variable densities in modern species. Accordingly, while a shaggy pachyrhinosaur is certainly at the 'extreme' end of our predictions for an ornithischian integument, it does not directly contradict anything we know regarding dinosaur evolution. Ceratopsids probably had the appropriate genetic blueprints to produce a shaggy animal, so long as the right conditions promoted its expression. There is a question of how appropriate it is to cover a ceratopsid in shaggy integumentary structures however, in light of preserved skin impressions of other ceratopsids. How likely is it that any horned dinosaurs were fuzzy?


Fossil integument of Chasmosaurus belli. From Sternberg 1925.
2) Is the extrapolation of preserved integuments to other species that reliable, really?
Skin impressions and the remains of other integumentary structures are Holy Grails to palaeoartists, and we use them extensively in restoring extinct animals. Through phylogenetic bracketing, or use of their basic phylogenetic proxy when less data is available, we stretch these remains over entire clades so that the known integument of one species becomes the norm for an entire group - what I'll call 'One Skin Fits All' approach. Thus, because we have scaly skin impressions for three ceratopsids - Centrosaurus, Triceratops and Chasmosaurus (see image, above, of the latter. From Sternberg 1925), it's assumed that scales were common to the entire clade. There's nothing necessarily wrong with this assessment and, one may argue, it's the most parsimonious way to interpret this data. A stick in the mud, however, is that another dataset, the diversity of integuments in modern animals, suggests that integuments can vary wildly within groups, and that we could be vastly underestimating the integumentary variation in extinct animals.

Consider the different varieties of fluff, fur, feathers, hair, bristles and other fuzzes in a group of modern animals and then think how a future palaeoartist would reconstruct all varieties of that group if they only had access to only one or two examples of integument. Perhaps all reconstructions of bovids would have woolly coats like those of sheep, or, conversely, the sparse, almost naked skin like a water buffalo? We may deck all primates out in the long capes of colobus monkeys, all pigs with boar-like fur, or cover every inch of birds with feathers. We know such approaches are wrong because these groups demonstrably show variation in the distribution, length and structure of their varying integuments, and yet we maintain a One Skin Fits All' approach to fossil clades. We can't even play the 'extremely close relationship card' in this game as the likes of woolly mammoths, and the fuzzy Sumatran and woolly rhinos, show vastly different integuments to their closest, naked relatives.

One could counter this point by arguing that the relative abundance of scaly remains in certain dinosaur lineages suggest that most, if not all members of that clan were scaly. Perhaps, but we should consider both the sample sizes here and the taphonomic window through which fossils are passed to the modern day. We have, at best, skin impressions from a handful of species compared to the group diversity, so statistical support for the 'One Skin Fits All' approach is low. Moreover, which types of skin are more likely to be preserved? Taphonomic observations on modern animals suggest that fur and feathers are easily removed from carcasses by biological or physical processes, so their preservation potential in ancient animals is low outside of fossil Lagerstätten. Is it a coincidence that the only skin impressions we find outside of Lagerstätten are scaly, leathery hides? I don't have the answer to that question, but it's worth chewing over.

With all this in mind, I wonder if applying the fossil integuments of one species to all its relatives, even close ones, is a questionable practise. I'm not saying that skin impressions are useless and that we should pay them no attention, but we should remember that they only highlight possibilities and perhaps some degree of probability for integumentary structures in a related species. They may well also have no bearing whatsoever on the appearance of their relatives. We're dealing with a great amount of unknown data when reconstructing ancient integuments, and we know how complex this issue is through modern species. When applying this thought to horned dinosaurs, we can say that the scaly skin impressions we have for a few species demonstrate that some bore scales, but we cannot rule out the possibility that others were covered in entirely different structures, like the quills and fuzz that seem deeply rooted in dinosaur ancestry. It does seem likely that many centrosaurines heads, including Pachyrhinosaurus, bore heavily keratinised scales and pads (Hieronymus et al. 2009), but, of course, this doesn't tell us much about the rest of the body. The majority of skin in Pachyrhinosaurus could be scaly, fuzzy, or anywhere inbetween. Without skin impressions to directly tell us the integument of specific species, there's no way to be sure. We need to be careful that we do not afflict ourselves with palaeoartistic phyloblindness here, by only considering these animals as denizens of cladograms and evolutionary hypotheses. Phylogenies may tell us what is possible for integument reconstructions, but other factors may help us decide is more probable.

3) Phylogeny is far from the only factor controlling integument, and can be readily overruled
It's funny to think that, for all the time we spend looking at the phylogenies of extinct animals, we're often missing much about the raw power that drove their evolution: adaptation. This is probably because we're lacking so much anatomical detail in their fossils that their responses to even broad environmental changes are largely undetectable, so understanding why they change through time is not always as certain as how. Nevertheless, we can be sure that different environments drove modification to the anatomy of extinct lineages on small and large scales, and integuments were likely to be one of the most affected tissues. Animal integuments are critical interfaces between body and environment, and have to be appropriately adapted for given habitats. This is a readily observable phenomenon in modern animals, because their integuments reflect all sorts of environmental factors including sun exposure, temperature, local vegetation types, water availability, parasite prevalence, and their local predators. Presumably, this is why such variation in integument exists in even closely related species. But we frequently reconstruct fossil species as if they all live in the same place. Sedimentological and isotope data reveals that closely related ancient species sometimes lived in starkly contrasting settings, but because we frequently take the 'One Skin Fits All' approach, our animals look very similar, irrespective of the requirements of their habitats.

Hot Fuzz: a reference to the condensing breath of the animal, the controversial concept depicted here, or just an excuse for a bad pun? Whatever: it's an excuse to link to this clip from Hot Fuzz: Best. Granny. Kick. Ever.
Behaviour may also have an effect on integument. Sun-shy, non-aggressive and cursorial animals may well bear thinner integuments than slower, frequently exposed or bad-tempered species, for instance. Morphology, too, will have an influence, with larger animals having lessened needs for insulation or being capable of carrying heavier, armoured hides. These are all things palaeoartists should be considering when reconstructing extinct animals: they should look adapted to the lifestyles we predict for them, rather than being based on phylogenetic hypotheses alone. Perhaps some desert-living dinosaurs had elephant-like, deeply wrinkled skin to help heat dissipation, while smaller desert-dwellers had extremely short feathers, or none at all, to prevent overheating. Maybe large theropods had heavy scales on their faces to defend themselves during bouts of head biting. We don't know for sure, but we can be certain that these species had to be appropriately adapted for wherever and however they lived. In short, we need to be wary reconstructing our ancient species by cladogram alone, and realise that integuments, and soft-tissue anatomies in general, should reflect a combination of phylogenetic data and possible adaptations to habitats and lifestyles. This, undoubtedly, involves some of the 'informed speculation' that has been discussed so much with All Yesterdays, but the results are more consistent with our knowledge of modern animals and evolution generally, and arguably producing a more convincing look into the ancient world ('convincing' is the right word here: I'm not sure we're ever going to get 'accurate' results in this game). In fact, after mulling this over for some time, I find the typical and conservative, 'One Skin Fits All' approach much harder to defend than the more open minded, environmentally-influenced reconstructions argued for here.

Bringing this back to our fuzzy pachyrhinosaurs, we again have to question the how applicable the currently available ceratopsid skin impressions are to this Alaskan species. The scaly hides of Chasmosaurus, Centrosaurus and Triceratops represent animals living in more southerly regions than Pachyrhinosaurus, which were at least temperate to subtropical in climate. The former two taxa also lived somewhat before Pachyrhinosaurus, when global temperatures were, on average, a little warmer. These sub-arctic coastal plains encountered by Pachyrhinosaurus perotorum, by contrast, were much cooler, and sometimes genuinely cold. Accordingly, the selection pressures on integument may have been very different for P. perotorum compared to these warm-climate ceratopsids, and we have to wonder how suitable the skin impressions of Chasmosaurus et al. are for reconstructions of Pachyrhinosaurus. We wouldn't, after all, expect the integument of a yak to resemble that of a African buffalo, or consider the fur of a lithe gazelle a suitable model for mountain goat fur. With this philosophy in mind, the question of shaggy pachyrhinosaurs shifts focus from arguments about the cladograms and the skin impressions of their relatives, and on to whether or not Late Cretaceous Alaska was cold enough to promote the development of an extreme integument adaptation in a large dinosaur species.

4) Late Cretaceous Alaska: a struggle for any tourist board
Although nowhere near as bleak as our modern Alaska, the dinosaur faunas inhabiting the northern reaches of latest Cretaceous Alaska would have experienced fairly grim weather for much of the time, perhaps akin to that experienced by modern animals living on the northwest coast of Canada or the more depressing parts of Scotland. In a recent review based on palaeobotanical data from the Cretaceous Arctic, Spicer and Herman (2010) suggested that uppermost Cretaceous Alaska experienced mean average temperatures around 6°C, with summer months attaining a comfortable 14.5°C, but winter months dropping to an average of -2°C. The Pachyrhinosaurus perotorum-bearing Prince Creek Formation may have been a little cooler than other parts of the Late Cretaceous arctic circle, with a mean average temperature between 2.5-5°C. Winter lasts a long time at 80-90° latitude, with 5 of darkness bracketed by 2 months of twilight. A permanent cloud cap over the Late Cretaceous Arctic (detected by the oversize nature of the fossil plant leaves from Cretaceous Alaskan localities) acted as a atmospheric blanket for the region, prohibiting temperatures from plummeting below freezing low for long period. The lowest temperatures - perhaps -10°C - may not have lasted more than a few weeks. Evidence for deep freezes is absent however, with neither the sedimentological or palaeobotanical record indicating nothing more severe than week-long frosts and light freezes. Despite this, rain and snow were probably common, with relative humidity averaging about 80% and even the driest months of the year experiencing over 180 mm of rain. The collective three wettest months, by contrast, collected almost 800 mm. (To put this in perspective, rain-soaked England has an average annual rainfall of 854 mm, according to the UK Met Office [via Wikipedia]). Such a climate was capable of supporting a rich array of plantlife, and evergreen taiga-like forests were common, as were swamps, rivers and other bodies of water. Such conditions seem fairly common right the way through Late Cretaceous Alaska, with conditions a full 8° south of the Prince Creek Formation seeming similarly cold and wet. I should add that this consideration of ancient Alaska  isn't particularly controversial, the palaeobotanical data mentioned here matching palaeoclimate models based on isotope records, animal distribution and sedimentology.

So the Prince Creek palaeoenviroment wasn't exactly an ideal holiday spot, but was it 'extreme' enough to promote the evolution of an fuzzy coat in a 1.5-2 tonne dinosaur species? Given the dense furs we see in large mammals found in similar climates, I think it's certainly a possibility. It would certainly be far weirder if polar dinosaurs of the Late Cretaceous didn't respond to their climate somehow, and a thick coat of protofeathers is one possible adaptation to their cool, wet habitat. A layer of insulating fat would be another (cue an image of some tubby pachyrhinosaurs). Of course, the picture may be different if these animals hibernated or migrated in and out of Alaska annually, enjoying the brief mild period before heading south to escape the winter. The latter is perhaps the most widely discussed concept, but direct evidence for such migrations in Alaskan dinosaurs is as sparse as evidence for their fluffy integuments or fat layers. In the concept proposed here, the protofeather coat may have acted as an insulator against the cold, prevented wind, rain and snow from hitting the naked skin of the animal, or both. The latter function would benefit from the coat being thick and fluffy, but this may not have lead to overheating even in a big animal like Pachyrhinosaurus: the ragged, loose coats of hot-climate adapted ratites appear similarly thick and massive without overheating their owners. Perhaps the concept of shaggy coats in these Alaskan dinosaurs doesn't seem so as implausible as it may first appear, then.

To bring this more-mammoth-than-intended essay to a close, then, I again stress that I'm not saying 'this is what Pachyrhinosaurus looked like!', but attempting to present it as a product of both its phylogenetic history, its environment and habits, and not simply reconstructed via a cladogram. I think there's a lot of scope for these sorts of palaeoartistic renditions, even it does mean more reliance on the 'informed speculation' principle of All Yesterdays. Of course, this whole argument flows back to the core idea behind the All Yesterdays movement: the conservative, 'One Skin Fits All' approach to integument reconstruction is just as likely to be wrong as our more speculative concepts, but at least the use of informed speculation lines the reconstruction up with our knowledge of modern animal diversity.

References
  • Hieronymus, T. L., Witmer, L. M., Tanke, D. H. and Currie, P. J. 2009. The facial integument of centrosaurine ceratopsids: morphological and histological correlates of novel skin structures. Anatomical Record, 292, 1370–1396.
  • Mayr, G., Peters, D. S., Plodowski, G. and Vogel, O. 2002. Bristle-like integumentary structures at the tail of the horned dinosaur Psittacosaurus. Naturwissenschaften 89, 361–365.
  • Spicer, R. A. and Herman, A. B. 2010. The Late Cretaceous environment of the Arctic: A quantitative reassessment based on plant fossils. Palaeogeography, Palaeoclimatology, Palaeoecology, 295, 423–442.
  • Sternberg, C. 1925. Integument of Chasmosaurus belli. The Canadian Field-Naturalist, 39, 108-1 10.
  • Zheng, X., You, H., Xu, X. and Dong, Z. 2009. An Early Cretaceous heterodontosaurid dinosaur with filamentous integumentary structures. Nature, 458, 333–336.

28 comments:

  1. Nice to see someone arguing what I've argued for years.
    I suspect the main problem for those using phylogenetic bracketing in tegumentary reconstruction is the use of false dichotomies like scales vs feathers and a bad interpretation of the fossil record. The fact that we know scaly skin in ceratopsids says nothing about the presence and distribution of feathers in that clade. Presence of scales is absence of feathers. People arguing that phylogenetic bracketing suggests absence of feathers in pachyrhynosaurs because other ceratopsids are known only for scaly skin are just doing phylogenetic bracketing wrong.

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    1. I meant: "presence of scales is NOT absence of feathers".

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    2. Yes. Dinosaur palaeoartists should print out that sentence in a massive font and stick it on their studio wall.

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  2. Yes very cool. Regardless whether they had feathers or no, it makes for a much fresher reconstruction when they do have the fuzz!

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  3. Interesting points. I definitely agree about making assumptions on the form of extra-integumentary structures such as scales and features and fur. I definitely agree with the recognition of cold temperatures during the North Slope winters, while specific variation from Great Plains Montana/Alberta taxa suggests endemism and non-migratory patterns of the larger taxa (though some taxa may have still migrated, and thus caused fluctuations in the gene pools), though it is far from certain that the taxa in the Prince Creek died during the times of the cold winters, but not the warm summers. I also agree with the problems of trying to pretend the EPB is the-all/end-all for biological reconstruction, but there is FAR too much push against phylogenetic bracketing when it seems inelegant to someone, or restricts them from "speculation." The EPB and typical phylogenetic bracketing arguments are tools, and nothing more, and should be considered as such by EVERYONE.

    Where I disagree, however, is the conflation of the "dinofuzz" as a singular structural feature among all dinosaurs, something that (oddly enough) requires parsimony to affirm, and phylogenetic bracketing to proscribe to other taxa. The structure of the quills in Psittacosaurus sp. are different in both form and size to those of filaments in Tianyulong confuciusi, and it takes a jump in reasoning to presume they are the same, or fall within the same category of structures. There is a presumption that the little heterodontosaurid represents a basal motif for ornithischians, but that as we ascend into larger forms, especially given direct evidence for integument in a diversity of clades, we find scaly, pebbly integument instead. Rather, Psittacosaurus sp. might seem to represent part of a trend away from filamentous integument, and the quills are isolated to the dorsal midline of ONE section of the body, while the rest was squamous, the same region where the extra-long and -thick quills in Tianyulong confuciusi are located (I've speculated this is based on marginocephalian affinities of heterodontosaurids, but the current phylogenetic data doesn't support this). Absence of skin-impressions in arctic sediments for large-bodied animals in the Campanian-Maastrichtian of ANYWHERE makes it very difficult to assess the integument of ceratopsians directly, especially if one considers that the basal ceratopsians didn't seem to have fuzzy hides. Moreover, no animal group varies so widely as to go from totally naked and squamous to covered in fluff -- the extreme present, one might compare to rhinos and elephants/mammoths, but is distinguished by the presence of hair over the bodies of all the mammals noted, while in ceratopsians, low-latitude taxa exhibit NO evidence for filaments. We might say something about dispersing a few fine threads here and there in their hides, but they'd not be visible at much distance, as in rhinos and elephants.

    Anyways ... no objection to the picture, just the debate points. Fun stuff! I definitely like that you, John and Niroot are illustrating extreme fluffy ornithischians. The idea is sexy, but I fear caution is being thrown to the wind and validated on opposition to the "mainstream" -- which I acknowledge has its own problems (thanks, swamp lizards!).

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    1. Thanks for the thought, Jaime, and point taken about the differing nature of Tianyulong and Psittacosaurus filaments. However, I am sceptical of identifying any trends in ornithischian integument - such as your proposed movement towards scales in Ornithischia - at present: the dataset is too small to spot any meaningful patterns and could be overturned with just a few contradictory discoveries. For instance, your state that 'basal ceratopsians didn't seem to have fuzzy hides'. I would argue that you’re possibly over-stretching that single occurrence to the whole group, as we have no idea how typical the skin of Psittacosaurus is among early ceratopsians. And even if we ignore the statistical problems of skin remains, we have to acknowledge the preservational biases affecting skin preservation. Matt Davies’ work on hadrosaur integument seems relevant here, with his conclusions that their skin impressions are relatively abundant because they bore unusually tough hides compared to other dinosaurs. As such, their integument has been identified as atypical among dinosaurs, so perhaps it’s not a great model for other ornithischians either? What I’m saying, I guess, is that we should be wary of interpreting the data we have on ornithischian skin at face value when we know how biased and sensitive that dataset is, and how wrong we would be when reconstructing the integuments of modern animals with comparable data.

      On the second point, that few lineages have swapped scales or naked skin for extensive fuzz: don’t your own examples counter that point? Rhinos and elephants are pretty good examples of naked, or almost entirely naked, lineages going feral with fuzz. Of course, the integument in the common ancestor of the Sumatran and Woolly rhinos isn’t known, but mammoths at least sit somewhere between effectively naked species. Near-naked babirusa species seems to sit between some very fuzzy suid lineages like peccaries and hairy members of Sus too, and then you’ve got oddball mammals that replace most of their fur with scale-like structures – pangolins and armadillos. It would be nice if birds could help us more here for obvious reasons, but I think their propensity for flight and low disparity compared to their ancestors makes them poor models in this argument. They do show that feathers and scales can be swapped around liberally in part though, thanks to their diverse pedal integuments. Accordingly, it seems to me that integuments can be swapped about fairly freely under the right conditions, if modern animals are anything to go by. Saying that fuzzes cannot replace scales or naked skin seems to invoke Dollo’s Law, but compared to the complexity of some re-developed features we see in animal lineages, switching on feathers or hair from naked beginnings seems fairly trivial.

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    2. Mark, thanks for the reply.

      I do not mean to imply or say that when one loses "fuzz," one cannot regain it. I do in fact suggest that all theropods retain "fuzz" early in ontogeny, but lose it. Birds retain it, perhaps as a consequence of a clade where integument was becoming more elaborate. The expense is paid, and the clade doesn't readily revert, so even at large body size would retain extensive integument. Certainly, this might restrict environments to where the animals can readily control parasites and temperature.

      But it suggests that the reasons for loss are also important, and not to be taken lightly: if a clade reduces expression at adult size of "fuzz," what qualities should be use to infer it present? Obviously, you suggest environment can require the integument in even ceratopsians, but there the next step should be considered:

      Are filaments of a density one expects in, say, birds, effective enough as a temperature regulating organ as in mammals? Consider the density and structure of bear fur versus avian feathers, and further whether avian feathers become denser in colder environments.

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    3. Feathers are undoubtedly fine insulators in cold climates. As penguins, Arctic owls, albatross chicks and other permanent* avian residents of high latitudes show, feathers are excellent insulators. Modern birds show clear alterations to plumage that reflect their use as insulators in cold climates. Feather density and body coverage increase with latitude in owls, which enhances their use in this regard. Adult penguin feathers are heavily modified to be excellent thermal regulators while their chicks retain downy coats that are more typical of other birds, but still still allows them to withstand some of the most extreme temperatures and weather on Earth. Bear in mind that some avians put themselves in the line of fire of cold temperatures, when many mammals tuck themselves into cozy dens to hibernate. There is a host of literature testing mammal and avian integuments as thermal conductors, concluding that they are equally good insulators and stressing how important feathers are to avian thermoregulation. Naked birds lose heat two or three times quicker than their feathered counterparts, so feathers are clearly essential items for keeping them warm. I really don't think the ability of feathers to keep birds warm, even in cold climates, is in any doubt.

      *or semi-permanent, in the case of the slow-developing albatross chicks.

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  4. Adipose tissues likely also pose a major issue in regards to integrument. Modern birds - and most non-avian sauropsids, for that matter - have adipose tissues that are very shitty when it comes to thermoregulation, while mammals have numerous types of insulating fat, which turns leads to more relaxed fur presence (compare warthogs to cassowaries. The former have very short fur, the latter are fuzzy enough to survive in cold temperatures).

    Is it possible that some unique adipose tissue in ornithischians helped them to preseerve temperature so suberbly that more derived forms had more sparse "feathers".

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    1. Are fats really that bad for avian insulation? I don't think there's anything about avian fat that makes it an intrinsically bad insulator. To the contrary, fats are known to be important insulators for some diving birds. Studies on cormorants, for instance, conclude that their fat layers (up to 10 mm thick) significantly reduce heat loss during dives. Penguins are also reported to use fats for thermoregulation, and retain a significant fatty layer even after fasting. These birds also use air trapped within their feathers to insulate themselves, and this is generally more common mechanism for avian insulation. This is obviously more conducive to minimising weight for flight, which may explain its prevalence: maybe more bird species would use fat to keep themselves insulated if they had given up flight. I will agree that most non-avian reptiles don't seem to rely on fats to keep themselves warm, but their heterothermic metabolisms don't lend themselves to heavy investment in insulation.

      I'd argue, then, that no living sauropsids are really suitable models for possible dinosaur uses of fatty insulation. Still, we can be very confident that dinosaurs possessed fatty tissues of phylogenetic bracketing , and there's no reason to think that they could not use them to keep themselves warm. Because thick layers of run-of-the-mill subcutaneous fatty tissues work so well at insulation, I don't know that there's any need to assume that naked ornithischians, or any dinosaurs for that matter, developed unique types of fatty tissues to do the job instead.

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    2. Well, I've read somewhere that brown fat is the main reason mammals loose fur more easily than birds loose feathers. Even mammals supposedly without brown fat, like us, appearently have films of brown fat around the most vital organs.

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    3. That's as maybe, but cold-adapted birds - even flightless ones - live fine in their respective habitats using just feathers and avian fats, which are most similar to mammalian white fats. Combinations of these tissues allow penguins to withstand the coldest temperatures on our planet. The blubbery hides of seals and whales are comprised of tissues more similar to white fat than brown, too. White fats must be fairly good insulators, then, even if they don't generate heat like brown fats. I'm really not sure that ornithischians needed unique types of fatty tissue to help them stay warm.

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  5. Very, very cool image... wooly Pach illustrates your ideas fantastically.

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    1. "Very, very cool image"

      Yes, it does look cold there. (Sorry, to obvious not to pun about...)

      Thanks for the nice words, David!

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  6. Mark, I'm very much with you on this one. The climate must have been punishing for herds that far north. So, if an artist wants to portray a scaly Pachy herd on a snowy day, then he has got to come in for some criticism. In today's world, are there any fully scale-covered creatures in snowy environments? Not that I can think of. So you and I have been thinking along the same lines (thanks for the link). Let me aid and abet you a little. Given the known presence of predators of the furry and feathery kind in the northern reaches, how could a scaly, lethargic-in-snow, horn-faced lizard ever have survived, even if it weighed two tons? The answer is simple. With active predators, it had to be active or else. So it needed insulation when the wind chill factor went down to -20. Otherwise, even the small troodonts and dromeosaurs already documented in the northlands would have made short work of it. And Chinese fossils already tell us there was a cold-adapted tyrannosaurid, so watch out. I'm going to write a blog entry on these subjects, probably today, so keep an eye out. Hint: as you have already stated, modern animals provide some very intriguing examples.

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  7. Mike from Ottawa12 January 2013 19:00

    Is the preservation on those fossils of ceratopsian skin (Centrosaurus, Triceratops and Chasmosaurus) so good that the inability to see fuzz/feathers/'hair' demonstrates these animals did not have fine filaments between the scales?

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  8. Mike from Ottawa12 January 2013 19:07

    Oh, and I got so wrapped up in the discussion, I forgot to include: Awesome pic.

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    1. Thanks for the nice words, Mike. I don't think any pits for fine filaments have ever been noted in published descriptions of ceratopsid skin. There are reports that undescribed Triceratops skin bears perforated scales which may have anchored quills of some kind, though.

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  9. "Is it a coincidence that the only skin impressions we find outside of Lagerstätten are scaly, leathery hides?"

    Well, now we have the Dromiceiomimus brevitertius with feathers. But besides that nitpick, I agree with your general conclusions.

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    1. Yes, very true. Thinking about it, I suppose there are a number of saurischian integument fossils that have been found outside of non-Lagerstätten deposits. I was very ornithischian focussed in this post. Still, if my understand of the saurischian integument record is correct, our knowledge of non-coelurosaurian saurischian integument is not much better of that of ornithischians.

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  10. To me, on a complete gut level of "animals looking correct", this image works. Certainly better than the mangy versions with patches here and there.
    These resonate.
    It's really hard not to steal your image and make my own version!
    Instead it'll be time to update Leaellynasaura(for the umpteenth time).

    Naked polar dinosaurs have always struck me as particularly jarring.
    Peter Trusler, a very talented Australian artist has been the benchmark for representing dinosaurs in local publications, has painted Leallynasaura like this: https://c479107.ssl.cf2.rackcdn.com/files/11978/area14mp/5jfnjchw-1340237966.jpg (of course you're dead! Look at you, it's cold and you're a *lizard*!)
    For a long time and it's boggled my mind that a polar animal would be represented this way. Peter is extremely talented as both an anatomist and illustrator, but I've always been shocked by this really conservative representation.

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    1. Thanks Matt. I was surprised at how 'naturally' the pachyrhinosaurs took to thick coats too. I thought they'd look far sillier, but they look quite at home in their pelts. And yeah, scaly dinosaurs in cold climates, especially small ones, just seem ridiculous now. Trusler's Leaellynasaura is lovingly rendered, but looks so out of place in that setting.

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  11. I like your illustration, however, it appears, based on your "Perotorum" skulls horn placement and lack of accurate nasal boss, that your image was not based on the Perotorum fossil evidence that actually made it a new discovery. I find it difficult to take your assumptions seriously when even the accuracy of the basic anatomy is just not there, needless to say, there is "no evidence" for making them "muskoxen" save the controversy driven attention you might receive, (which has already been done by other artists) and the possibility of selling lots of T-shirts and mouse pads. - cheers

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    1. Thanks for the comment Anonymous, although you get zero points for politeness. I don't think the nasal boss is off, but you're right about the morphology of the posterior frill being out. It's more like that of P. lakustai, not P. perotorum. My bad. And yes, I clearly did this for all the money and fame that controversial palaeoart brings. The 3000 word explanation behind my rationale was just for fun.

      By the way, you don't capitalise species names. I find it difficult to take your comment seriously when even the knowledge of basic scientific writing is just not there.

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  12. I could be wrong, but this is just a thought. In all of your examples, the animals have the same basic integument. Mammoths clearly had more hair or fur than modern day elephants, but they both have mammalian skin with some form of hair. Same with yaks and cows, colobus monkeys and humans, or in terms of birds, hens and ostriches (clearly different types of feathers on either one, but feathers nonetheless). While there were clearly dinosaurs with feathers evolving along similar lines to birds, both Saurischians and Ornithischians evolved from Archosaurs, which didn't have feathers, and it is more commonly the Saurischians who have feathers. Research by Mayr, Gerald, Peters, D. Stephan, Plodowski, Gerhard & Vogel, Olaf (2002) on Psittacosaurus's quill-like tail structures said that "there is no convincing evidence which shows these structures to be homologous to the structurally different feathers and protofeathers of theropod dinosaurs." So wouldn't it seem to be a stretch of theory to envision Pachyrhinosaurus with feathers of any kind? I would think the likeliness of dinosaurs being endothermic would make integuments like thicker hides or fat deposits more likely. - Derek

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  13. Hi Derek,

    Thanks for the comment. I'm not sure I agree with you that mammals and birds each have the same basic integument. There is some commonality in each group, sure, but they're incredibly diverse in terms of integumentary structures. Birds possess naked skin, countless variations on feathers, scales, keratinous sheaths, hairlike-beards and so forth. Mammals are probably less diverse, but still show variations in hair morphology and density, possess cornified structures (nails and claws, and scale-like structures in some species) and substantial variations in skin thickness (e.g. the toughened, armoured skin of rhinos). The underlying genetics may be similar for many of these, but the variation is so high that I don't think we can say they're basically the same. And particularly so we we start to think about their functionality.

    On the dinofuzz issue: what about the integument of Tianyulong? This incontrovertibly demonstrates that ornithischians bore some sort of filamentous integument, which bears more than a passing resemblance to some early grades of feather evolution. And the filaments of dinosaurs aren't a million miles away, morphologically speaking, from the hair-like structures of pterosaurs, either. While homology of these structures remains a little murky, the possibility filamentous integuments being ancestrally common to all ornithodirans cannot be excluded, and is perhaps a likely possibility.

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  14. The image is fantastic, Mark. I wonder, though, if other Pachy features might not speak to its plausibility, if Pachyrhinosaurs were uniquely adapted for cold weather among Ceratopsia. Shouldn't we expect to see facial or limb proportion changes, cold-specific nasal passages, or smaller more heat-conserving frills among cold-adapted species? I know that arctic animals tend to be thick-bodied and thick-limbed, with a maximization of body volume to surface area ratio, but I don't know if such adaptations are present in Pachyrhinosaurus perotorum, or Pachyrhinosaurus generally.

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  15. There is evidence to suggest that all dinosaurs had the genetics for feathers yes. But I doubt that they all had feathers. But many basal dinosaurs may well have had feathers

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