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Friday, 16 June 2017

Revenge of the scaly Tyrannosaurus

Reworked version of my 2012 Tyrannosaurus painting, now in it's third guise. There's something about this painting which recalls reconstructions from 1906 rather than those of 2016.
The skeletal anatomy of Tyrannosaurus rex is probably better known and studied than the skeletons of many living animals, but its soft-tissues - and thus much about its life appearance - are poorly represented by fossil remains. Thus, virtually all of our ideas about muscle bulk, soft-tissue body shape and integument have to be reconstructed by phylogenetic proxy and functional prediction. As with all dinosaurs, we've historically felt pretty confident that Tyrannosaurus was entirely scaly, but relatively recent discoveries of filamented tyrannosauroids in China (Xu et al. 2004, 2012), as well as a growing mountain of fuzzy coelurosaur fossils, point to a different conclusion: that Tyrannosaurus was adorned in simple filaments - hair-like equivalents of feathers. Skin impressions for more derived tyrant species - the tyrannosaurids - have proven rare in fossil record (Hone 2016) and, though rumours have circulated about some, they have largely escaped formal description and publication. In the absence of better evidence, the most parsimonious modern takes on everyone's favourite tyrant have involved a fuzzy covering.

In the recent months two papers have challenged this idea. The first, by Thomas Carr and colleagues (2017), purports to find osteological correlates of scales on the facial anatomy of the tyrannosaurid Daspleteosaurus, which they argue (along with other lines of evidence), to suggest crocodylian-like facial tissues and sensitivity. The second, by Phil Bell et al. (2017), describes scaly skin impressions from multiple postcranial regions of a Tyrannosaurus skeleton, and argues that the distribution of these impressions implies a uniform (or near uniform) covering of scales across the body, without much in the way of fuzz.

Because this is Tyrannosaurus, media sites and bloggers have spilled great amounts of ink over these stories. The scientific press has often been far from objective or unbiased. Popular articles have suggested Jurassic World fans might have 'won' the debate over scientists, that science fans are 'due' a return to scaly tyrants after 'losing' Pluto, and that the findings mean 'all is well in the dinosaur world'. The implication is a ridiculous one, like evidence of scalier tyrants is a moral victory rather than a test of a scientific hypothesis. But while the popular press has been celebrating the new papers, members of the palaeoblogosphere have been less enamoured with the findings. Trey the Explainer suggests that Bell et al.'s work doesn't really change what we already knew about tyrant integument, and thus does not invalidate many existing reconstructions. Andrea Cau posits that interpretations of scaly tyrants reflect our prejudices more than science, and that taphonomic factors may explain the absence of filaments. Brian Switek has concerns that the skin patches are too small and spread too widely to give a complete picture of the integument, and echoes concerns about taphonomic interference. The collective response seems to be a defensive one, protecting concepts of filamented tyrannosaurids from a resurgence of a more traditional, scaly model. Would any other dinosaur get this treatment? Perhaps not: as Brian explains in his recent post, this reaction is the T. rex celebrity effect at full bore.

Supermegafluffy Tyrannosaurus, from 2015. They were simpler times.
I've painted many fluffy Tyrannosaurus in the last few years (above) and quite like the idea of everyone's favourite 6 tonne dinosaur bonecrusher being a giant plush toy. However, we also have to concede that our ideas of Tyrannosaurus skin have been largely informed by prediction, not direct data, and that popular, long-held notions are as ripe for scientific revision as any other (lest we forget other famous examples of this - Brontosaurus and Ornithoscelida). Moreover, although some critics are suggesting the papers don't tell us anything new - rumours of scale impressions have been circulating for years - these recent studies give us the first rigorously documented, peer-reviewed glimpse into Tyrannosaurus skin anatomy. This is new, allowing us to form our own opinions on Tyrannosaurus appearance based on actual data, not hearsay. So, rather than putting our gloves up to defend our prior model, I wonder if we should be exploring how this new data might transform our perception of Tyrannosaurus life appearance. That these new studies present conflicting data to our expectations is not grounds to be upset, annoyed or defensive. To the contrary, they allow us to use real data - not predictions - to refine our ideas of tyrannosaurid appearance and evolution. For those of us interested in dinosaurs as real entities, and not movie monsters, that's a good thing.

What, exactly, has been argued about scaly tyrants?

A lot of the popular write ups of these recent papers include errors and misrepresentation, so let's recap what is actually being argued about Tyrannosaurus skin. A common social media reaction to Bell et al.'s work is that they've presented 'a patch' of skin, and are extrapolating from that. We need to debunk that right away: they've not described a single patch, but multiple small patches from the neck (alas, exactly where on the neck isn't reported), the top of the pelvis, and the base of the tail (below). All the samples stem from the 'Wyrex' specimen (HMNS 2006.1743.01). The most extensively represented area is the tail base, which has the largest single piece of fossil skin - 30 cm². The other skin samples are not as large, some being just a few centimetres across. Each patch shows the same skin type: uniform, tiny 'basement scales', each less than 1 mm across (Take note, artists: you would not see Tyrannosaurus scales until you were being eaten by their owner). Similar scale patches, also described by Bell et al. (2017), have been found on the torso and tail regions of other tyrannosaurid species, implying similarly scaled regions in these taxa.

Tyrannosaurus skin patches from the neck, pelvic area and tail of the 'Wyrex' specimen as illustrated by Bell et al. (2017). The scale bars for the scale imagery are 5 mm (b - e) and 10 mm (f-h). These things are tiny, and we can assume the skin of the animal would look smooth or leathery in life.
Some folks are suggesting that the size of these skin patches allows us to dismiss their scaly signal, or that even that they're anomalous, reflecting unusual taphonomic conditions that cloud their significance. I'm unsure about these ideas. Most skin impressions are small patches (even scaly skin gets a rough ride during fossilisation) and the fact they're small doesn't diminish the fact that each records a cluster of scales. We have to assume these are not unusual or 'special' areas on the body but generally indicative of surrounding skin fabrics. The fact that each patch is consistent with regard to scale size and texture hints at them being part of a continuous, unbroken integument, and not isolated scaly pockets in a sea of fluff.

But what about arguments that the scale patches are tissues stripped of filaments before preservation, like so many 'monster' carcasses? Filament/scale combos do have precedent in dinosaurs, being present on the tail of Juravenator and those scales of Kulindadromeus with fibre-like tassels (Chiappe and Göhlich 2010; Godefroit et al. 2014). We know from modern animals that fibrous epidermal structures are especially vulnerable to decay and physical weathering, but is there evidence that this has taken place on the Wyrex Tyannosaurus skin patches? At present, it's hard to say because we have no idea what tyrannosaur skin looks like as it decays. It might be significant, however, that the scale patches look very similar across the Wyrex specimen, and that they resemble other tyrannosaurid skin impressions closely. We might expect some variation if taphonomy was really distorting these specimens in a major way, and we're not seeing that. Moreover, the Wyrex skin impressions, though small, are pretty high-resolution. The scales, and their intervening areas, have sub-millimetre proportions and sharply defined edges. There's no tatty scale margins, no obvious spaces for filament attachment, or linear structures crossing the scales to imply a rogue filament impression. We'll remain uncertain if these are anomalous, taphonomically-altered samples until we find other examples of tyrannosaurid skin, but there's no reason to be unduly suspicious of the the samples we have.

Of course, the adage that 'absence of evidence is not evidence of absence' is always important when dealing with the fossil record, and it applies here as a sensible caveat. However, we shouldn't wield this phrase as a definitive counter-argument to reasonable interpretations of available evidence. Palaeontologists have to work with data, not suspicions or gut feelings, and the data we have does not include, or hint at, the presence of filaments. I'm not arguing that taphonomy isn't worthy of consideration here (indeed, the omission of details about 'Wyrex' taphonomic history is an issue with the Bell et al. 2017 paper) but we must beware the logical fallacies of appealing to probability (i.e. taphonomy could explain the lack of filaments, so it does explain the lack of filaments) or special pleading (excluding Tyrannosaurus from the same logic we would apply to other fossil animals when presented with this data).

Tyrannosaurus skull AMNH 5027 - note the 'hummocky' textures on the side of the snout, above and below the orbit, and atop the rostrum, likely indications of scaly skin. Image in public domain, sourced from Wikipedia.
Carr et al. (2017) present a different form of evidence for scales: osteological correlates. I consider some aspects of their study problematic in that it only looks to crocodylians and birds for comparative tissues, despite the clear value other tetrapods have in deducing facial tissue types (Knoll 2008; Morhardt 2009; Hieronymus et al. 2010); it lacks illustrations of the bone textures correlated to scaly integuments; and the conclusion of tyrants bearing crocodile-like face scales is flawed: crocodylians do not have face scales, but a tight, highly cracked sheet of facial skin - Milinkovitch et al. (2013). Nonetheless, I think Carr et al. (2017) are right in concluding the bony textures of tyrannosaur skulls seem indicative of scaly skin. These findings echo previous interpretations of bosses and rugosities in tyrant skulls (e.g. Brusatte et al. 2012; Sullivan and Xu 2016) and aren't controversial. Scales closely associated with bone either leave a 'hummocky' surface texture, which is seen on tyrant snouts (specifically their maxillae and nasals) or small bosses and hornlets, which are found in all tyrannosaurid skulls above their orbits (lacrimal and postorbital bones) and on their 'cheeks' (jugal bones). Hornlets and bosses represent the locations of specific scales in living reptiles (Hieronymus et al. 2009) and can thus give especially good indications of life appearance (check out chameleon skulls for especially good correlation between skull and scale features). The presence of hummocky bone textures and hornlets is a strong correlate for scales, as they rule out coverings of naked or feathered skin. Such skin types do not alter the underlying bone surface (Hieronymus et al. 2009).

These osteological correlates combine with the skin impressions to collectively show Tyrannosaurus as scaly across much of its face, somewhere on its neck, over the pelvic region and along the tail base (below). So far as we can tell, this picture seems consistent with osteological correlates and skin sampling from wider Tyrannosauridae. That's pretty extensive coverage, ruling out the presence of fibres in places that we know other dinosaurs - including other tyrannosauroids - were fuzzy, and implies that tyrannosaurids were mostly scaly. I'm particularly startled at the scales over the hip region as they curb even the long 'fibre capes' we see in some modern tyrant reconstructions, like the famous Saurian Tyrannosaurus. The fact that the scales occur in places known to be ancestrally filamented for tyrants is also intriguing: Bell et al. (2017) speculate that they may be modified feathers - that is, the same as bird scales - rather than a reversion to lizard or croc scales. Hold that thought, we'll come back to it soon.

Everyone's doing maps of Tyrannosaurus with integument details nowadays, and I want in. Note that this is Tyrannosaurus specific, and does not feature scale data from other tryannosaurids.

What's in the gaps?

The million dollar question is what was present between these scaly regions: more scales, or fibres? This is a major point for many respondents to the Carr et al. and Bell et al. papers, as it decides whether we keep our interpretation of Tyrannosaurus as an - at least partly - fuzzy animal. With our scale distribution map as a starting point, several options are available. The first is that fuzz was present in regions not yet represented by skin remains or osteological correlates. This would mostly imply the top of the torso (Bell et al. 2017), but may also be parts of the back of the head, some aspects of the neck (depending on where the neck skin impression came from) and maybe the end of the tail. Over on Twitter, Patrick Murphy has presented a reconstruction which shows what this might look like. I must admit to finding it quite amusing, sort of like T. rex has put on a shawl to visit the opera.

But how dense could these fuzzy patches have been? Bell et al. (2017) suggest that dense fibrous coverings are doubtful, noting that large living mammals avoid patches of thick insulating fibres to aid heat loss. This has not gone down well with some critics, who cite studies of feathers preventing over-heating instead of facilitating it. An oft-cited study in this regard is Dawson and Maloney (2004), who found emu feathers block virtually all solar radiation from the skin, preventing them from overheating in solar exposure that causes similarly-sized hairy mammals to seek shelter.

Feathers: great at blocking solar radiation, also great at trapping body heat. Note how cooking hot these ostriches are on their necks, heads and legs, while the feathers are mostly ambient temperature. This isn't because the body isn't warm, but because the feathers block the heat signature entirely, trapping all that heat around the body. As surface area:volume ratios drop as animals get larger, it stands to reason that the benefits of blocking solar radiation give way to a need shed heat. Image from Wikipedia user Arno / Coen, CC BY-SA 3.0.
Feathers, however, are not magic structures that defy fundamental physical laws of insulation, nor do they liberate animals from the challenges of heat loss at reducing surface area:volume ratios. Beyond a certain size, shedding excess body heat is difficult for any terrestrial animal, and it gets tougher as they get larger. King and Farner (1961, p. 249) described feathers as having "an extremely high insulating value to the feathered surfaces" and a rich literature of studies on modern birds shows that feathers are as effective at trapping body heat as they are blocking solar rays (e.g. King and Farner 1961; Kahl 1963; Philips and Sandborn 1994; Dove et al. 2007). We can almost see them as a little too effective, leading many birds to develop heat-dumping adaptations to circumvent their own insulation, such as highly vascularised, non-feathery body parts as well as a repertoire of postures and behaviours (maximising exposure of unfeathered body parts; flapping wings; urinating on their legs) that aid cooling (e.g. Kahl 1963; Arad et al. 1989; Philips and Sandborn 1994). So yes, feathers are terrific at protecting birds from environmental heat, but that limits their ability to release metabolic heat from their own bodies.

If living birds find feathers a little warm, despite their relatively high surface area to volume ratios, we have to assume a theropod weighing anywhere between 6-14 tonnes is going to find big areas of dense filaments a challenge to thermoregulation too. It is not unreasonable to assume blankets of fibres could be a problem for big tyrants. The counterargument here is that Yutyrannus huali, a largish tyrannosauroid, does have dense fibres everywhere. But Yutyrannnus seems more lithe than Tyrannosaurus - perhaps just 10-25% of its mass, depending on the estimates (Bell et al. 2017) - and lived in a more vegetated, and thus shadier, habitat (Bell et al. 2017). A neat comparison Bell et al. (2017) make along this line uses living rhinos, where hairier species live in shadier settings than the virtually naked ones. In light of this, the reduction of filamented regions, and perhaps lessening their density, is a reasonable inference for animals of the size and habitat of Tyrannosaurus, and would reflect thermoregulatory responses to scaling and shade availability seen in living animals.

Large tyrannosauroids, like Yutyrannus huali, show that dinosaurs weighing perhaps 1.5 tonnes could be covered in feathers. But does this reflect the fact that this animal lived in shadier, vegetated habitats than the tyrannosaurids? This idea isn't silly: adaptation to specific circumstances has a major role to play in shaping animal skin anatomy, and could well explain why some tyrants are fuzzy, and others seem less so. (If you want to see the rest of this picture, check out this Patreon post)
Could Tyrannosaurus have had extremely fine, widely-distributed filaments - perhaps similar to something like elephant hair? This isn't entirely falsified by the new data, although the skin impressions we have show no evidence of such a covering despite preserving tiny integument details. Granted, animal filaments can be extremely fine, and they might be beyond the preservation potential and mechanics of even these high-res impressions. However, if we're arguing for filaments of this size and patchiness then - certainly for artistic purposes - we should concede that the animal would be essentially scaly, in the same way that most rhinos, elephants and hippos are essentially naked (below). From a thermoregulatory perspective, short, sparse filaments could make sense as these have the surprising ability to draw heat from the body in modern elephants, helping them stay cool (Myhrvold et al. 2012). Given the potential for overheating under dense filament coats in giant animals (Bell et al. 2017), I see this as more plausible than a 'cloak' of fibres between our scaly waypoints.

Scaly, minimally-filamented Tyrannosaurus. There's some tufts on the neck, but that's it. Is this model more consistent with the thermoregulatory requirements of a 6-14 tonne animal?
A last interpretation of this new data is that Tyrannosaurus was actually just scaly, with no fibres whatsoever. This is the most contested suggestion made by Bell et al. (2017), but it's not unreasonable with our current knowledge. Existing skin data, representing seven parts of the body if you pool all the distinct skull correlates and postcranial points (add several more if you want to extrapolate scale patches from other tyrants), shows enough scales and consistency in the scalation pattern that uniform scale coverage is not a ridiculous or indefensible concept. I appreciate that some folks will point to regional fuzziness of animals like Kulindadromeus in response, and its sharply defined areas of different integument types, and that's valid point. But we can also point to plenty of dinosaurs with extensive or entirely scaly hides and - if there's any value to linking body size and thermoregulatory regimes - they're a better match to Tyrannosaurus body mass than any known fuzzy species. For the time being, wholly scale models fit our existing data just as reasonably as partly fuzzy ones so, archaic and counter-intuitive as it seems - a scaly Tyrannosaurus is not an unreasonable interpretation for the life appearance of this animal, given our current data.

Beyond Tyrannosaurus: 'unlocking' dinosaur skin constraints

My take-home from these new papers is that our models of Tyrannosaurus skin have not crystallised, but we're a little more constrained in how we can imagine this animal, and have to concede a scalier appearance than many of us thought likely. But the implications of the Bell et al. study go beyond Tyrannosaurus in implying new ways to think about dinosaur skin evolution. With incontrovertibly fuzzy animals lining much of the the tyrannosauroid tree and its root, our scalier Tyrannosaurus gives us one of the best examples of a dinosaur replacing fuzz with scales. This is a far-reaching conclusion for those of us interested in dinosaur life appearance, complicating the already confusing evolutionary pattern of scale and fuzz distribution within the group. Ideas that some dinosaurs could be 'secondarily scaled' are supported by this discovery, and we have to wonder if classically fuzzy lineages - including many other theropod lines - are as tightly locked into fuzz, fibres and feathers as we once thought. Could large dromaeosaurs be a little lighter on fuzz than we imagine? Did Therizinosaurus look less like a giant pigeon and more like a walking Christmas dinner? We don't know, but now have reason to wonder.

Fluffy Tyrannosaurus juveniles, one of the possibilities created by the idea that tyrannosaurs might have avian-like 'dynamic' skin. The recovery of scales in non-scaly clades is not as simple as it might first appear!
Furthermore, the notion that Tyrannosaurus scales could be modified feathers (Bell et al. 2017) opens possibilities about mixes of filaments and scales. It's important to realise that not all scales are alike: 'reptile'' scales' are developmentally and genetically distinct from those we see in birds, which are actually secondarily modified feathers (Chang et al. 2000; Dhouailly 2009). Reptilian skin cannot be forced to grow feathers or filaments (Chang et al. 2000) and is developmentally static: once scales are formed, they're with them for life. Bird skin, however, is far more dynamic, and allows for all manner of ontogenetic and even seasonal variation in scale:feather ratios, changes to feather types, and modification of scale size (Lennerstedt 1975; Stettenheim 2000). If, as suspected, our tyrannosaurid skin samples represent fibrous integument masquerading as a scaly one, is this a sign of a bird-like 'unlocked' skin configuration where epidermal dynamism was possible? If so, Tyannosaurus could have changed appearance considerably with age (fluffy when small, scaly when big - above) or season (reflecting changes in climate or behaviour)? It must be stressed that we don't have any direct insight into these sorts of changes at the moment, and the hypothesis of tyrannosaurid scales being modified feathers needs testing. But the irony - we might have data indicating Tyrannosaurus could change its appearance readily, vindicating debaters on both sides of the scaly and fuzzy debate - is not lost on me. Maybe, just this once, everyone wins?

Summing up time

Let's tie this all together. A lot of ambiguity remains about the skin of Tyrannosaurus and its relatives, and it's not wise to hold any opinion about their life appearance too strongly at present. However, unduly downplaying the creep of scaly evidence into the tyrannosaurid fossil record isn't useful or logical. The skull skin correlates and fossil skin patches show that scales were present in numerous, widely-distributed parts of the body, and - until we see evidence to the contrary - this is good reason to assume scalier Tyrannosaurus than we might be used to. And yes, this does mean that some of our favourite, fluffier interpretations are now directly contradicted by fossil data, and consigned to our ever growing book of historic, discredited reconstructions. But this is always a possibility in palaeontology: our views of these animals are only ever hypotheses based on a sparse, biased fossil record, and every new discovery risks overturning someone's favourite concept. The fact we're able to move on from these reconstructions is positive, as it means we're a little less uncertain about the past, and a little closer to the truth.

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References

    • Bell, P. R., Campione, N. E., Persons, W. S., Currie, P. J., Larson, P. L., Tanke, D. H., & Bakker, R. T. (2017). Tyrannosauroid integument reveals conflicting patterns of gigantism and feather evolution. Biology Letters, 13(6), 20170092.
    • Brusatte, S. L., Carr, T. D., & Norell, M. A. (2012). The osteology of Alioramus, a gracile and long-snouted tyrannosaurid (Dinosauria: Theropoda) from the Late Cretaceous of Mongolia.
    • Carr, T. D., Varricchio, D. J., Sedlmayr, J. C., Roberts, E. M., & Moore, J. R. (2017). A new tyrannosaur with evidence for anagenesis and crocodile-like facial sensory system. Scientific Reports, 7.
    • Chang, C., Wu, P., Baker, R. E., Maini, P. K., Alibardi, L., & Chuong, C. M. (2009). Reptile scale paradigm: Evo-Devo, pattern formation and regeneration. The International journal of developmental biology, 53(5-6), 813.
    • Chiappe, L. M., & Göhlich, U. B. (2010). Anatomy of Juravenator starki (Theropoda: Coelurosauria) from the Late Jurassic of Germany. Neues Jahrbuch für Geologie und Paläontologie-Abhandlungen, 258(3), 257-296.
    • Dawson, T. J., & Maloney, S. K. (2004). Fur versus feathers: the different roles of red kangaroo fur and emu feathers in thermoregulation in the Australian arid zone. Australian Mammalogy, 26(2), 145-151.
    • Dhouailly, D. (2009). A new scenario for the evolutionary origin of hair, feather, and avian scales. Journal of anatomy, 214(4), 587-606.
    • Dove, C. J., Rijke, A. M., Wang, X., & Andrews, L. S. (2007). Infrared analysis of contour feathers: the conservation of body heat radiation in birds. Journal of Thermal Biology, 32(1), 42-46.
    • Godefroit, P., Sinitsa, S. M., Dhouailly, D., Bolotsky, Y. L., Sizov, A. V., McNamara, M. E., ... & Spagna, P. (2014). A Jurassic ornithischian dinosaur from Siberia with both feathers and scales. Science, 345(6195), 451-455.
    • Hieronymus, T. L., Witmer, L. M., Tanke, D. H., & Currie, P. J. (2009). The facial integument of centrosaurine ceratopsids: morphological and histological correlates of novel skin structures. The Anatomical Record, 292(9), 1370-1396.
    • Hone, D. (2016). The Tyrannosaur Chronicles: The Biology of the Tyrant Dinosaurs. Bloomsbury Publishing.
    • Kahl Jr, M. P. (1963). Thermoregulation in the wood stork, with special reference to the role of the legs. Physiological Zoology, 36(2), 141-151.
    • King, J. R., & Farner, D. S. (1961). Energy metabolism, thermoregulation and body temperature. Biology and comparative physiology of birds, 2, 215-288.
    • Knoll, F. (2008). Buccal soft anatomy in Lesothosaurus (Dinosauria: Ornithischia). Neues Jahrbuch für Geologie und Paläontologie-Abhandlungen, 248(3), 355-364.
    • Lennerstedt, I. (1975). Seasonal variation in foot papillae of wood pigeon, pheasant and house sparrow. Comparative Biochemistry and Physiology Part A: Physiology, 51(3), 511-520.
    • Milinkovitch, M. C., Manukyan, L., Debry, A., Di-Poï, N., Martin, S., Singh, D., ... & Zwicker, M. (2013). Crocodile head scales are not developmental units but emerge from physical cracking. Science, 339(6115), 78-81.
    • Morhardt, A. C. (2009). Dinosaur smiles: Do the texture and morphology of the premaxilla, maxilla, and dentary bones of sauropsids provide osteological correlates for inferring extra-oral structures reliably in dinosaurs? (Doctoral dissertation, Western Illinois University).
    • Myhrvold, C. L., Stone, H. A., & Bou-Zeid, E. (2012). What is the use of elephant hair?. PloS one, 7(10), e47018.
    • Phillips, P. K., & Sanborn, A. F. (1994). An infrared, thermographic study of surface temperature in three ratites: ostrich, emu and double-wattled cassowary. Journal of Thermal Biology, 19(6), 423-430.
    • Stettenheim, P. R. (2000). The Integumentary Morphology of Modern Birds—An Overview 1. American Zoologist, 40(4), 461-477.
    • Sullivan, C., & Xu, X. (2017). Morphological diversity and evolution of the jugal in dinosaurs. The Anatomical Record, 300(1), 30-48.
    • Xu, X., Norell, M. A., Kuang, X., Wang, X., Zhao, Q., & Jia, C. (2004). Basal tyrannosauroids from China and evidence for protofeathers in tyrannosauroids. Nature, 431(7009), 680-684.
    • Xu, X., Wang, K., Zhang, K., Ma, Q., Xing, L., Sullivan, C., ... & Wang, S. (2012). A gigantic feathered dinosaur from the Lower Cretaceous of China. Nature, 484(7392), 92-95.

    73 comments:

    1. I've been among those who have cautioned against overextrapolating from the skin impressions described Bell et al. (largely in response to popular news coverage claiming as "conclusive proof" that feathers were absent in tyrannosaurids), but I concur with nearly all the points you raise here. Though I don't think Bell et al. falsified the possibility of feathered tyrannosaurids, the impressions we have are at least highly suggestive that they were scalier than commonly restored in the past few years. Certainly the evidence is not in favor of Yutyrannus-style feathering in tyrannosaurids.

      On a different note, for some time I've wanted to blog about the "feathers are utterly different from fur as insulatory structures" meme that appears to be popular in the paleo community. I might still do it, but you've already made it almost superfluous here using fewer words.

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      1. I think a lot of the "feathers are utterly different from fur as insulatory structures" folks are simply incorrectly extrapolating. I and others have pointed out that feathers can work quite differently as insulators due to their structure. But I was generally talking about pennaceous feathers, which can exist in patches and tracts, and still cover sognificant portions of the body, AND be highly mobile. The ostrich is a prime example. It does have a seemingly dense coat of insulation but those are just a relatively few, large feathers covering much of the body compared to the number of hairs in a comparably voluminous mammal. Further, the feathers can be lifted to expose large naked patches for shedding heat. A lot of the arguments about "large animals can have more feathers at greater size than furry animals" were in response to arguments against scaly DROMAEOSAURS (remember when those were still a thing)?

        I agree that these arguments do not apply to down or protofeathers, which would function not very differently than hair.

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      2. Indeed, Matt. Specific differences in how fur and feathers function in thermoregulation do exist (which I've written about in the past myself), but I frequently see it distorted into "unlike feathers, fur can only keep an animal warm but not keep them cool", which is not at all true.

        I think you're right on in tracing it back to the (now thankfully rare) arguments about dromaeosaurids. In addition to the differences between expected feather structure in dromaeosaurids and tyranosauroids, it's also worth remembering that even using only mammals as a guide, the largest known dromaeosaurids were still not at the size threshold where we'd expect essentially-complete loss of filamentous integument to have been widespread, whereas most tyrannosaurids got well up into that size range.

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      3. Thanks for the comments guys, very interesting. I think there's definitely room for more discussion of thermoregulation with feathers, in large dinosaurs etc.: it's very clear that much of the online palaeo community is confused about this.

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      4. There are a few issues to consider, however, namely the possibility that tyrannosaurs could have had complex feathers

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      5. Sinosauropteryx may have had stage III feathers, and it's a relatively basal Coelurosaur right? So Tyrannosauroids having stage III isn't out of the question. Dilong certainly seems to have had branching filaments.

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      7. I think, still, as Witton points out, certain feather positioning effects cooling abilities, whilst similar fur positioning will not cause the same result.

        As for the notion that Rex was more concerned with cooling than staying warm, let's remember that the radiator effectiveness requirement for large animals, i.e. the most fundamental constraint on cooling, surface area to heat generation ratio, is proportional to the 1/12th power of mass. The difference between a 2-kilogram predatory therapod and an 8200-kilogram predatory therapod with a similar set of proportions and metabolic constant would be a factor of two.

        The environment, however, is far more critical. Rex lived in an environment with a mean annual temperature of something like 11 Celsius and no prolonged freezing. Think the modern climate of the Oregon coast or Portland, Macedonia, San Francisco, and more.

        Quite simply, the dry skin temperature relative to the environmental temperature is constrained by the power dissipation and exposed skin area. It cannot simply take arbitrary values or the animal will freeze or cook itself. The skin temperature of a human in 23-Celsius static air is such that it can minimize power usage in cooling off a 36-Celsius body with a 29-Celsius skin temperature. This 6-degree difference is expanded to 8.5 degrees if we multiply the human body weight by 100 times and keep the proportions constant and the idle power at 3/4ths power of body mass.

        Was T. Rex at the same allometrically-scaled metabolic activity levels as humans? Probably not. In what direction this error goes, I do not know. On the one hand, the modern ecological analogues like the more northern subspecies of tiger have higher metabolic activity levels than humans, allometrically speaking. On the other, many (possibly-misguided) people suggest that therapod dinosaurs were not as warm-blooded as mammals or birds are. Whether this is true or not, it appears that flightless birds alive today need very slightly more energy than comparably massive mammals with similar diets.

        So overall, we would expect that if Rex wanted to keep a reasonably normal skin temperature relative to body temperature, that it would NOT look like African rhinos or elephants that live in an environment at least 20 Celsius warmer. It probably wouldn't even look like humans because we evolved in much the same environmental conditions as the African megafauna, and those of us who left used clothing to stay warm in colder climates.

        This isn't to say the evidence is wrong, just that we would expect T. Rex to need insulation most of the year. It might be possible that this insulation was subsurface, like blubber or simply a system to prevent heating of the surface tissues, but why we don't see such adaptations in other cold-dwelling animals that aren't aquatic is beyond me if this is the case.

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    2. Hello Mark Witton. My name is Davide. By the way, what do you think about these Andrea Cau's posts on the theorical motivation for the presence of adult plumage in Tyrannosaurus ?

      http://theropoda.blogspot.it/2010/06/motivazione-teorica-per-la-presenza-di.html

      http://theropoda.blogspot.it/2011/08/lista-dei-futili-motivi-per-cui.html

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      1. Andrea seems to be assuming a mass of 6 tonnes for Tyrannosaurus, which is at the lower end of current mass estimates for this animal at full size. As with all estimates of extinct animal mass, predicted figures range widely and it's difficult to know which ones are most reliable. However, masses exceeding 6 tonnes are now common for big animals (7.5 - 9.5 tonnes are argued for 'Sue' in some papers) and, if these values are right, the surface area:volume ratio is going to be lower than he predicted. Also, we might note that elephants do have particularly low SA:V ratios, and we might want to look at other hairless species for insight on this matter.

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      2. Ok Mark, but, ignoring weight, have you noticed that body shape of Tyrannosaurus is more tapered than stocky body of elephant ? Usually, a tapered body shape allows to desperce heath well. Furthermore, dinosaurs have a methabolism less wastefull than mammals...

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      3. Can you elucidate "methabolism less wastefull than mammals"? I'm curious to know 1) what this means and 2) what assumptions you are making about non-avian dinosaur metabolism. This is a highly controversial area of research, so any assumptions have to be treated with a grain of salt.

        And yes, dinosaur shape might be better at dumping heat than an equivalently sized elephant (they have, as noted above, quite poor body shapes for shedding heat). But, if your dinosaur is heavier than an elephant, which seems possible in the case of big Tyrannosaurus, at least, then the surface area to volume ratio is going to be lower in actuality. Moreover, as alluded to in the article, we see animals much smaller than elephants showing trends of shedding fuzz to help dissipate heat. Big rhinos are less than half the mass of a good sized elephant, but are also naked. Even large birds, which are a fraction of the size of a rhino, elephant or tyrannosaur, have bare patches to help thermoregulation. The observation that animals lose insulation with increasing mass is not a controversial one and reflects fundamental aspects of physics: we have to assume that dinosaurs responded to issues of heat exchange in some way, and shedding insulation is a likely tactic.

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      4. Big rhinos live an environment that's much hotter than the Hell Creek environment, which as far as I'm aware was probably closer in temperature and humidity to San Francisco. Humans in the same environment as the aforementioned rhinos are puny in comparison but still almost completely nude. Humans in San Francisco are usually layered in a thick coat of synthetic fabric.

        I think the clear takeaway is that T. Rex was covered in Polyester or nylon.

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      5. Probably with garrish fluorescent yellow pinstripes and a hideous brand logo on the side.

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    3. I've been arguing against the conclusive headlines (I feel like everyone tries to do their own version of damage control whenever a press release over exaggerates the results of a scientific study) but admittedly I am a little more swayed - or at least less defensive - by your take on this. Until recently I've personally always imagined thin filaments with an elephant hair-like distribution with denser clusters on parts of the neck and torso as my personal favorite reconstruction until I fell in love with the _Saurian_ design, so it's a bit of a reversal in form to imagine them as scaly again. (Oh how the pendulum of paleontology swings.)

      That said, it does bring me to inquire more about your idea of 'dynamic' bird skin and how you at least think that could have worked. Are you suggesting that the feather follicle falls out as the animal grows and is replaced by a scale?

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    4. I agree with a lot of the points and I second Albertonykus' thoughts on "magical feathers properties" discussions online. Yes they can shield heat from outside but also trap heat inside, which is a big problem for a big animal.

      I do have to wonder your thoughts on phorusrhacid skulls as you are pretty much in line with hummocky skull texture being indicative of scaled skin. These birds do have a fairly textured, rugose skull and presumably did not have scaled faces. Though they may have had keratinized faces. https://naturalishistoria.files.wordpress.com/2013/03/skull-terror-bird-scientific-american.jpg

      I have no problem with largish theropods being mainly scaled (possibly secondarily derived) but am a bit more leery of assuming scaled faces where both crocs and birds lack scaled faces. Bosses, hornets, and ridges are also present on crocodiles - which of course lack facial scales.

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      1. But don't Hadrosaurs have facial scales?

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      2. Indeed yes, but I do have to wonder about scales on the face and why both modern birds and crocs lack them? What both birds and crocs have and what hadrosaurus presumably lack is a highly tactile face. And again, hummocky bone texture is being construed as indicative of scales but phorusrhacids have some pretty hummocky skull texture but presumably naked skin. Naked skin, it is argued by Mark here via the Hieronymus paper, does not produce such hummocky patterns.

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    5. I wonder if there is a body weight/air temperature ratio that would predict when feathers and filaments would begin to disappear? At what point would feathery integument become a liability? Would a 3,000 lb Yutyrannus overheat in the the Horseshoe Canyon paleoclimate vs. that of the Yixan? I believe I have read that the climate of the Horseshoe canyon formation was cool and dry. Albertosaurus is about the same size and weight as Yutyrannus and is thought to be featherless as per this study. I wonder to what extent their climates/environments and body size relate or differ?

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      1. http://translate.google.com/translate?u=http%3A//theropoda.blogspot.it/2010/06/motivazione-teorica-per-la-presenza-di.html&hl=en&langpair=it|en&tbb=1&ie=UTF-8

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      2. Thanks! I usually keep up to date with this blog. How timely

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    6. Really well-written article, but I have a few questions:

      1: What about the argument that carnivorous animals tend to produce less body heat than herbivorous ones?

      2: What do you think about the idea of giant ground sloths like Megatherium lacking fur?

      3: Wasn't the idea of young tyrannosaurs having a feathery coat that they lose in favor of scales when they grow up debunked, since no known tetrapod completely replaces its integument with another as it grows?

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      1. If giant ground sloths had slow metabolisms, they may have needed fur to slow heat loss despite being huge.

        Some bird species grow thick feathers over their foot scales seasonally, and wood storks shed feathers from their necks as they grow scale-like plates in their place.

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      2. We have many sloth fossils with associated fur, and not just a light covering but some pretty shaggy coats.

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      3. Those preserved fur impressions come from species like Nothrotheriops and Mylodon, which...

        1: Lived in completely different climates (North American deserts and Argentinian pampas respectively, which both get cold at night).

        2: Were a lot smaller than Megatherium.

        3: Weren't even of the same family, so it's like comparing a horse to a rhino.

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      4. Mass estimates for the largest ground sloths suggest they were smaller than the biggest tyrannosaurs, too. If I recall correctly, Megatherium might be something like 4-6 tonnes, which puts it at or below the lower estimates for Tyrannosaurus size. They're also not going to be doing the sort of strenuous, metabolically-intense predatory behaviour expected of tyrannosaurs, so don't have to worry about heat stress so much. Combined with the points about metabolism and habitat made here, I wonder if ground sloths are looking like doubtful counter-arguments to the idea of scalier tyrants.

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      5. Sim, what species of bird are you referring to? I understand Ptarmigans seasonally grow a thicker coat of feathers on the feet during the winter but they do not molt off the entire covering of feathers and replace them with scales in the warm season.

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      6. Willow ptarmigans have scaly feet that grow feathers in the winter; their claws grow longer too. You can see coat comparisons in field guides. I've also seen pics of their feet without feathers in the summer, and they have scales.

        Feathers grow through or between scales in some other bird species, like doves.

        Wood storks are the most fitting example here: They go from fuzzy headed chicks to scaly necked adults.

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    7. What about their pulmonary airsacs?

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    8. What about their pulmonary airsacs?

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      1. These do seem to aid cooling, but living birds are just as pneumatised as Tyrannosaurus, if not moreso, and still face cooling challenges. It might give dinosaurs a higher threshold for losing fuzz than animals without air sac systems, but it doesn't eliminate the problem.

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      2. Also, the total interior surface area of human lungs is the size of a tennis court. Imagine that of a rhino or elephant.

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    9. I do wonder what influence Deinocheirus has on this discussion. It's Tyrannosaurus sized, lived in the same habitat as Tarbosaurus, and had a pygostyle and thus had at least some feathers. So based on that, I don't think it's reasonable to say giant tyrannosaurs were featherless due to gigantism. Am I on the right track here?

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      1. Maybe Tyrannosaurids just had higher metabolic rates than other lineages of giant theropods thus producing more body heat and having less need for insulation.

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      2. Matthew: If that structure in Deinocheirus is a feather-spouting pygostyle (and I'm a little sceptical, just because lots of animals fuse their tail vertebrae. Two fused verts is only possible evidence of feathers in my book) it could indicate feathers just at the tail tip, not necessarily elsewhere on the body. I don't think anyone is ruling out filaments in some regions on these large animals: the question is whether gigantism limits potential for entire animal could be covered.

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      3. Thing is, though, recent Ornithomimid feather impressions show a covering roughly similar to that of an Ostritch with a Kulindromeus-like bare tail, so I'd think tail might not be as likely for Deinochirus as it was before.

        The real question to the rest of its body depends entirely on the temperature, climate, environment and Plantlife of the Nemegt formation, which as far as I'm aware, we know very little about. I'd say they'd, at the most extensive feather loss, would at least keep wing fans for display.

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    10. And now scale fanboys will use this as evidence.


      What about mammalian brown fat? That'/ insulation archosaurs lack.

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      1. Birds still have fat. Fat is stored around the sternum,flanks, etc. And large animals are capable of generating body heat to the point where they do not need much insulating integument (i.e. elephants and rhinoceros). But then again Andrea Cau calculated that Tyrannosaurus' surface area was about equal to that on a one ton mammal (http://translate.google.com/translate?depth=1&hl=en&ie=UTF8&langpair=it%7Cen&nv=1&rurl=translate.google.com&tbb=1&u=http://theropoda.blogspot.com/2010/06/motivazione-teorica-per-la-presenza-di.html%3Fm%3D1), and most one ton mammals still retain fur, ranging from sparse (Sumatran rhinoceros) to not as sparse (Bison and giraffes).

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      2. I believe BK was referring to brown adipose tissue which is distinct from white adipose tissue. BAT "brown adipose tissue" has a thermoregulatory function i.e. it generates heat. I found one paper on research gate that concludes birds do not have "BAT" (I don't know how to properly site the paper in blog context) I am assuming he was implying that large mammals with brown fat would face more danger of overheating in certain environments than an equivalent sized dinosaur if they lacked BAT as in birds.

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    11. Also, are we still confident Nanuqsaurus was feathered, since it lived in a colder climate?

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    12. Dear Dr. Witton,
      There was apparently a Tarbosaurus preserved with a "wattle or bag of skin" under the skull showing only naked skin, neither scales nor feathers(http://z13.invisionfree.com/Hell_Creek/ar/t19.htm). Perhaps the face merely had thickened skin, like that of Corvus frugilegus?
      Also, Albertosaurus, Gorgosaurus, Lythronax, etc are closer in size to Yutyrannus. And, according to the Bell et. al. paper, their environments don't seem too different in temperature. Perhaps these smaller Tyrannosaurid genera kept a fuzzier visage while the "giants" Tarbosaurus and Tyrannosaurus reduced their plumage to a sparser covering?
      -MrCrow

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      1. And, the bony bumps along the snouts of Tyrannosaurids...could they be attachment points for a keratinized or fleshy crest? Like in some Pterosaurs?

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      2. There isn't, to my knowledge, any reliable or peer-reviewed takes on the alleged Tarbosaurus dewlap, so I think it's best not to comment on that until we know more about it.

        The comparison to rooks is complicated by the fact that they have feathered faces, with the feathers lost after their first Spring. Do you have a reference for thick skin in these animals? To my knowledge, it's just naked, not reinforced. Of course, we do have correlates for super-thick, armoured skin, and these don't seem present in any tyrannosaur. The same also applies to skin outgrowths: these leave scars on animal skulls which are quite distinct from those of tyrannosaur nasals. Check out the details here:

        http://markwitton-com.blogspot.co.uk/2017/05/armoured-theropod-faces-rhino-horns-and.html

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      3. I'm a bit confused now. Carr et al. (2017) mention correlates for "armor-like skin" on several regions of the Daspletosaurus skull, and based on your earlier blog post, I assumed these to be the armored skin you wrote about. Was I mistaken in thinking the two terms were synonymous, and should the armor-like skin instead be understood as scaly or similar cornified tissue?

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    13. This article is very well-written indeed. However, I have a problem with the rule that animals living in shadier, vegetated habitat tend to be hairier, as there seem to be some notable exceptions like the African forest elephants. They are slightly smaller than their bush-dwelling relatives, yet they are not hairier.

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      1. Forest elephants might be an interesting exception as, in a roundabout way, they actually support the ideas of Bell et al. African rainforests are surprisingly new - they only developed in the last 12,000 years - and much of its fauna are species that evolved in savannah settings. The ancestors of forest elephants were presumably near-naked because of their ancestrally savannah lifestyle, and have taken this trait into the forests with them. As of yet, they have not re-developed much in the way of hair, but it's early days for these 'new' forest animals and they might not have been put under sufficient selection pressures to re-grow hair yet (if they ever will - it's not a given that forest animals have to be hairy, it's just that shadier habitats might lessen pressures for hair reduction).

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      2. If I was to hedge a guess, it would be that the hot climate, combined with increased humidity in the forest, means the elephants retain acceptable temperature easily enough without needing extra insulation. They also have less exposure to direct sunlight than savannah elephants, and this may also encourage an absence of hair to maximise UV absorbing potential.

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      3. 12000 years really doesn't sound like enough time. Humans and elephants are similar in their generation times and 12000 years for us is evidently not enough to cause speciation, as the physical and genetic diversity within Homo Sapiens, across a wide variety of environments, is quite subtle.

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      4. And also humans have a mean common ancestor >120,000 years ago, not 12.

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    14. Just throwing in my 2 cents, Hell Creek, where tyrannosaurus lived, would have been around 51 degrees on average, which is no where comparable to the large African mammals that you continue to refer to. It should also be noted that Mastodons ranged throughout all of North America, from the frozen north to the much warmer southern climes, and still retained a relatively thick coat of fur. Now, I'm aware of the size difference between mastodons and Tyrannosaurus, but my point still stands. I'm not saying that a fully scaly covering is implausible, but neither is a "jacket" of feathers.

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      1. Actually, Hell Creek was much warmer in the upper part, with temperatures around 20 degrees celcius on average, comparable to Northern Florida or the lower end temperature of the African Savannah.

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      2. Like k have said though, not very comparable to some blistering conditions that those same animals live in, which can be over 90 degrees,

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      3. The average high and low temperature for the Serengeti where most of the mammals people are referring to live ranges between 52F(11C) to 82F(54C) I've visited the Masai Mara which is the northern part of the Serengeti in Kenya and it can get pretty cool in the mornings and evenings. It is cool enough to warrant long sleeves and pants. I have no way of knowing what the cut off temperature/body mass for losing any kind of filamentous integument is.

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      4. BTW, 54 Celsius isn't 82 Fahrenheit, it's 127 Fahrenheit.

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    15. Dear Dr. Witton
      Aren't scales usually more..uniform? These bumps do not look very uniform. Perhaps is is more like bald bumpy elephantine skin? Then again, bird reticula isn't always uniform either...

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    16. Barn owls (Tyto alba), have admixed filamentous feathers and scales covering tarsi and toes. Each scale has a tiny notch in the middle of its distal edge, surrounding the feather. Some griffon vultures (Gyps fulvus) that show an anomalous partial feather covering at the tarsi have the same arrangement. I'd suppose that, if those Tyrannosaurids had a similar admixed filament-scale pattern, they should have a similar scale notching.

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      1. Looking at some photos of barn owl feet (and the odd Matthew Martyniuk blog post) it looks like the feathers originate from the tarsal scutes rather than the reticula. Given some facts and assumptions:

        - owl tarsal feathers originating from the edge of tarsal scutes, not between scutes
        - the possibility (likelihood?) of tarsal scutes being modified beta-keratin feathers
        - reticula being arrested pre-feather, alpha-keratin structures
        - tyrannosaurid scales being a form of, or related to, reticula (particularly if secondarily featherless)
        - tyrannosaurid scales showing well-defined preservation, with no apparent filament attachments between or within scales

        ... I'd view owl tarsal feathers as developed continuations of the tarsal scutes, rather than completely separate structures, and that such a thing would be much less likely with tyrannosaurid reticula. (When I see folk arguing for filaments sprouting between tyrannosaurid scales, I wonder why that would be if [if] the 'scales' are the 'feathers')

        I'll admit that's a fairly uneducated guess on my part, though it seems reasonable to me. ('Continuations of tarsal scutes' might be a bit much, though) More densely-feathered bird feet like those of snowy owls (Bubo scandiacus) and ptarmigans (Lagopus mutus) give me pause, though experience of the ability of feathers to wrap around and conceal unfeathered skin give me a little more confidence that their reticula (non-plantar reticula at any rate) don't explode with white fuzz every winter.
        I'd need to go check, though.

        dinogoss.blogspot.co.uk/2013/09/youre-doing-it-wrong-dino-foot-scales.html

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      2. Tarsal feathers of barn owls are real feathers, plucking them one can see a folicle (I've done it today), sheltered by the notch in the scute. I think feathers were the ancestral structures, later modified into scutes, surely some regulatory genes are involved in thedevelopment of feather vs scutes, and sometimes the regulation fails and we can see oddities as tarsal feathers in Griffon vultures.

        Chabier G.

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    17. An interesting thought... Did Rex sweat?

      It would make sense considering that it probably could not straight up outrun its prey, but that insofar as we can tell, it ought to have been pretty lethal.

      Maybe it was, as an adult, not an ambush predator OR a fast pursuit predator, but a persistance hunter, oriented toward pushing its prey to heat exhaustion like humans or wolves do, whilst simultaneously having far superior sweating ability to its prey?

      Just an idea.

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      1. If T. Rex could basically operate on continuous overdrive like humans can, and its kill mechanism was, like is the case for humans and wolves, extremely lethal with a quick attack on an exhausted animal, that would make for a clearly very capable predator.

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      2. Sweat is a mammals only thing. Sauropsids (lizards, crocodiles, birds, turtles) won't sweat. At all. Their are other ways to cool down. The air sacs system of theropods and sauropods helps. So does panting, bathing, and moving at night when it's cooler. I too have wondered if Tyrannosaurus was a persistence hunter. A study from the recent SVP indicates that Tyrannosaurids have more stamina than other large theropods (http://nmpdn.blogspot.com/2016/11/svp2016-presentation-highlights.html).

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      3. Hmm... Interesting. Did not know birds couldn't sweat. I wonder then, if they were persistence hunters, how they dealt with heat budgeting.

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    18. Hmm...well I definitely see your point. Personally, I really can't shake the idea of a feathered tyrant, but by now I'm willing to concede that it was certainly less feathered than we thought at first. My favorite interpretations as of late are the sparse "peach fuzz" concept and the feathery "cape" idea. With both forms of plumage being still present, but small and fairly vestigial, no longer serving a purpose to the animal's survival. I mean, T. rex wouldn't be the ONLY animal to have that going on (not just the already mentioned rhinos and elephants, but humans too--seriously, what has our hair done for us lately? Besides make us look good, and even that is entirely subjective).

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      1. Bald heads are dangerous if you have light skin and live in, say, the American desert or South Africa. All the deadly deadly sunlight and such.

        But yeah. For the most part, I think human hair is more in the "elaborate sexual display features" category. That's why it grows to feet in length on our heads, something that happens almost nowhere in other mammals, but grows to a short sensory-enhancing coating or is entirely absent everywhere else.

        An interesting point with humans, however, is also that hair and fire don't mix. You slightly wonder if maybe some of our not-quite ancestors may have experienced natural selection first hand by being too flammable?

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      2. Hmm.. I apparently forgot a few cases. Namely horses, lions, etc. Even so, the point still stands that gigantic feet-long hair is not common in our relatives the apes.

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    19. I think people put too much stock in phylogenetic bracketing when thinking about the integument of a given dinosaur lineage. We know from elephants and rhinos that closely related taxa - even ones with slow generation times - can evolve huge differences in the length and density of filamentous integument within a few million years. Combine this with the potential interplay of body size and climate, and I think ideas such as 'basal tyrannosauroids had a pelt and tyrannosaurids were naked skinned' are barking up the wrong tree. When it comes to multi-tonne dinosaurs, I think this is going to turn out to be a species to species thing based largely on habitat, it's not going to be big areas of the dinosaur tree that are either EXCLUSIVELY filamentous or EXCLUSIVELY naked. When it comes to small dinos, I imagine most of them retained an extensive pelt inherited from basal ornithodirans. That's my two cents anyway.

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      1. "I think ideas such as 'basal tyrannosauroids had a pelt and tyrannosaurids were naked skinned' are barking up the wrong tree."

        Personally, I can agree with you that an example like Nanuqsaurus - small size, high latitude - might have retained (redeveloped?) a filamentous covering, but I don't think anyone's arguing against that in particular. (It'd be like a smaller version of the 'coelurosauria ergo fuzzy tyrannosaurs' bit, at this point) Though looking at the fossil evidence, it could be hard to argue that it wasn't an outlier in that regard, barring an undiscovered radiation of Nanuqsaurus & chums.

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      2. Yeah Nanuqsaurus does seem like an outlier at the moment, I'm currently thinking most tyrannosaurids were basically naked skinned or somewhere close to naked skinned; though again, I think that is more likely to be because of body size and the climate of their habitats, rather than cladistics. Or to put it another way, if another polar tyrannosaurid was found, I would expect it to have a filamentous covering whether or not it was a sister taxon to Nanuqsaurus. And I daresay the Prince Creek Ugrunaaluk and Pachyrhinosaurus may have had some kind of pelt, perhaps moulting with the seasons.

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    21. Great article, as usual! Always informative reading your articles.

      While metabolic rates would be an issue with feather coverings, it is important to note that almost all the animals researched for this (Elephants, Ostritches, etc) are herbivores (well, mostly herbivores for Ostritches). Herbivores have a more complicated digestion process than carnivores and require much more energy to process plants, hence herbivores produce much more metabolic energy than carnivores. We sadly don't have any giant, protofeathered carnivores living in warm environments to really compare Tyrannosaurids with. Please tell me if I'm wrong.

      Also, I'm still not completely sold on the lack of a "cape," as you put it, at least to some extent on the body. If it covered a third or a fourth of the animal, wouldn't it not matter how thick it was as a large portion of the body was still bare and body heat could escape out of that? Lions, while not as big as T.rex, show that perhaps predators in warm environments can pull off large coverings on intugement on some sections of their body, keeping them for display purposes or for extra protection against rivals in some instances. Dinosaurs, being stem-birds would likely be much more big on visual display than mammals.

      Also, they don't specify where on the neck the scales are. It could probably be on the wattle/dewlap for all we know.

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