|credit Hyrotrioskjan aka Joschua Knüppe|
The battle of out times - scales versus feathers. I imagine some epic Lord of the Rings battle in middle earth: scale loyalist Orcs massing against feather nazi elves (feather nazi elves lolz). Various banners of the opposing forces drawn up with resounding battle cries and triumphant bellows of warfare. I paint this picture to obviously poke fun at members of both parties and to set a tone of jovial banter. Cuz it is kinda funny when you step back from it and realize 99.999% of people don't really care... Of course I care and so do my readers but I think discussions would be more fruitful if both factions stood down a bit and realized that there are resounding points that both sides make and, as I will highlight in this piece, there are some points and arguments to be made that have largely been circumscribed in the exchange.
But if you find yourself getting heated while reading this just pinch yourself, and then remind yourself that in a world of of growing nationalism, fundamentalism, misinformation campaigns, cyber-attacks, and frank talks of nuclear bombardment what dinosaurs wore for apparel does not really matter much...
The long running scale versus feather argument, largely played out online and largely converging on theropods (especially T. rex) integument has become mired down into two factions.
The pro-scale minions, seemingly on the defeat and defensive suffering wave after wave of feathered theropod discoveries and to top it off ornithischians with filaments. Most damning of all, Yutyrannus a largish tyrannosauroid with abundant feather impressions over large parts of the body. This and other feathered basal tyrannosauroids creating the valid and likely position that more derived tyrannosaurids also sported (some amount of) plumage. Still the pro-scale contingent has reason to remain vocal. Scale or scutes is all we really have for many lineages of dinosaurs; Carnotaurus; derived tyrannosaurids and even T. rex. Scales and scutes certainly have a big role to play in theropod and dinosaur integument and they did so for the duration of the Mesozoic.
|credit Chris Masna|
Diametrically opposed to the scale loyalists are the self proclaimed (I kid you not there is an actual satirical deviantart group) ... "feather nazis". Theirs is the grand and exciting enfluffening movement where everything from sauropods to ceratopsids is gowned and strutted with honest to goodness flowing manes of insulatory integument. Feathers for all!! Feathers right down to the base of dinosauria!! What was considered overly speculative by many in the 90's has now metastasized into a dogma all of its own. What the feather nazis lack in outright evidence they more than make up for in passion and online advocacy (shaming?).
A more succinct analysis would be that nothing triggers a scale loyalist like a fluffy T. rex and nothing triggers a feather nazi like a JP stylized raptor.
My main beef with both factions is that they have perpetrated a bit of a false dichotomy. Lost in the garbled rhetoric of both parties is a third option that has largely been eschewed, silenced, or simply ignored. That third option is nekkid skin itself. As I will lay out nekkid skin perhaps highly keratinized or fleshy - the most basal, germinal layer of all - on the head and even neck in many cases is a quite defensible option for theropods and perhaps even other dinosaurs.
But first I must slaughter some feather nazis.
Now my issue with the "feather nazis" is not one of complete disagreement. I think there is good reason to argue feathers or at least stage one quilly type things all the way back to the origin of theropods or more basally to stem archosaurs. However somewhat embedded in this suggestion is that such insulatory "protofeathers", "dino-fuzz" or "quilly" type structures are wholly and completely superior to "naked" scutes and scales in terms of insulatory potential. Or just negating the potential for thermoregulatory capacity in scaley structures at all and dismissing them as "inferior" or harkening back to more "primitive" gestalts.
I believe that scale loyalists have really missed or conceded an important argument in favor of abundant and diverse scaley type integument in dinosaurs. S.I.G.I.L. "subdermal interstitial gridded insulatory layer" an idea I suggested here a while back makes the case that scales, scutes, even osteoderms, have important potential for insulatory adaptions. Here I suggested that small pockets underneath the outer layer of skin in naked hided dinosaurs created an insulatory vacuum sealed layer when blood was withdrawn internally, basically the same concept of a thermos or people insulating their house with bubble - wrap. So I am not so sold that naked hided dinosaurs are a bad thing of lacking in insulator capacity even at small sizes or in cold climates. If certain basal prosauropods sported insulatory coats (no evidence yet) or basal ornithischians that we know did (Kulindadromeus yes!!) S.I.G.I.L. might offer a reason why this coat was lost and basically not needed in more derived forms like hadrosaurs, ceratopsids, ankylosaurids etc. etc. or maybe never there to begin with!!
No, my main beef with the "feather nazis" is not just a certain arrogance and condescension in their ranks but a certain amount of committing "sins of omission". We should always be careful to consider are we making the best argument or are we asserting what we want. And I don't really care if you are asserting what you want but don't pretend not to be!!
Time to take the feather nazis down a peg or two.
For best effect the arguments of the feather nazis should be read in the voice of Gollum. Like Gollum, whose mind was poisoned by the ring and all that it brought, the feather nazis have also came in possession of an ancient and powerful artifact - the feather - that has likewise poisoned and twisted their mind.
"But, my precious, feathers protect from harmful UV rays..."
This is true. But it is a bit of a half truth. What it is suggesting is that if it was not for feathers naked skin would be burnt to a crisp like extra crispy Kentucky Fried Chicken. This is simply not true and you know what protects from UV the best... melanin found in feathers and skin. You don't really need feathers to shield from UV rays, simply put melanin in your skin (as nekkid skin birds do already) and the problem is solved.
"Yes but feather have unique insulatory properties that keep birds thermally protected from extremes in heat or cold my precious..."
This is true. But again the assumption is that scales or scutes (esp. as I outlined in my S.I.G.I.L. hypothesis) could not do that job aptly too. Scales/scutes are rather unexplored as unique thermoregulatory organs in their own right and need not be assumed as vastly inferior to feathers in terms of insulatory properties in light of the lack of study on them. Especially if we assume a lower middle metabolic rate, scute and scale having dinosaurs might have done just fine… sort of like armadillos (more on xenarthans later).
A BIG problem in feathers covering the totality of large archosaur bodies in hot climates (read large dinosaurs in hot climates) is the issue of heat exchange. If the whole or most of the body is draped in feathers - which have excellent insulatory capacity - and body heat rises too high there is nowhere to shed this excess heat. Except, of course, by allowing for large tracts of scaley or nekkid skin (as modern birds do).
"Precious, you have forgot about the heat shedding abilities of the avian lung system..."
Ah... the coup argument against large tracts of naked skin or scales for large feathered dinosaurs in hot climates. The problem is that the poultry ranchers of California don't agree with you.
A bit ago I taught traffic school and one of the more obscure laws on the books is one concerning objects that can fall out of a moving vehicle and not be considered litter (absolving one of a $1000 fine). Clear water and feathers from live birds.
Spilling Loads and Damage to Highway.
(VC §§23114 and 23115) It is against the lawto operate on the highway a vehicle which is improperly covered, constructed, or loaded so that any part of its contents or load spills, drops, leaks, blows, sifts, or in any other way escapes
from the vehicle. (Exception: clear water or feathers from live birds.)
Turns out that transporting live poultry requires open air containers in order for adequate airflow and avoiding heat stress. In fact a cursory investigation into the issue of heat stress and birds (just google birds overheat) is rife with accounts of heat induced stress and death.The mighty avian lung system might not be the panacea we assumed it was in terms of preventing overheating. Additionally that turkeys and chicken have both avian lungs and fleshy display structures shows that even with both of these adaptations overheating still occurs. Overheating occurs quite a lot in birds actually. So I would not put much merit in this argument to counter abundant and extensive tracts of nekkid skin or scales.
The better bet for being a large feathered archosaur in hot climates is to clear up large tracts of nekkid skin on the face, around the eyes, the neck, even the chest. Get nekkid. Like Lappet-faced vultures. Or marabou storks. Or ground hornbills.
Pelage Loss: For Some But Not All?
There is one important concession that I have never seen feather nazis make. If insulatory apparel goes way back to the base of dinosauria or even earlier then we must presume that it was secondarily lost in some lineages : sauropods, hadrosaurids, ceratopsids are the most commonly cited potential examples of this. However if we presume that they could have lost their plumage then why can't we do the same for large tyrannosaurids? I don't think it is an argument that the pro-scale loyalists have been vocal enough about. I don't myself necessarily agree with it but I also suspect that tyrannosaurids had a heck of a lot of scale coating and nekkid skin on them.
In fact if people are allowed to have the boldness and audacity to depict "feathered" sauropods (and I believe that they should although I don't necessarily think it likely) they should also be allowed the boldness and audacity to depict a giant, perhaps mainly scaled and nekkid skinned maniraptoran that has lost almost all of its plumage. Developmentally it could happen. Personally I doubt it and I think a wooly sauropod and a mainly featherless maniraptoran are long shots, but if we allow speculation to one extreme why not the other? Boy would that trigger the feather nazis.
The Nekkid Skinned Archosaur
The argument is rather simple and straight forward. More so than that it is bolstered by the patterns we see in living feathered archosaurs. Go read this excellent online summary from an ornithology class on avian integument
Ye shall know it when ye sees it...
What was once completely scaled shall henceforth be completely feathered...
From the tip of the tail to the tip of the noggin' ye shall know only feathers until...
Conditions henceforth arise that necessitate a shedding of select feathers revealing...
a nekkid primordial skin... rippling, oozing, and stimulating with a lustful passion for display!!
Or to put it another way:
1) Scaled theropod evolves feathers. Not just a few feathers, but a full on coat covering the whole body as evo-devo studies suggest. Because feathers and scales are somewhat competitive developmentally what was skin covered with scales now just becomes skin covered with feathers.
2) Later on certain members of this feathered tribe get big or move to hotter climes or both. In order to shed heat, feathers retract from certain areas - the head, feet, the tail. In certain spots that are prone to impact or abrasion against the ground scales reemerge. This is exactly like the "scaled" feet and legs of modern birds. In other areas the feather retract to reveal nekkid skin.
3) And when modern archosaurs shed the feathers on their head, around eyes, the neck... when birds get nekkid they freaking play with that skin in all kinds of weird and wondrous ways. Big time freaks.
4) Bird skin is thin. If you eat some bbq chicken it comes with the skin. If you order some bbq beef it does not come with the skin. So that when feathers are lost one potential way to toughen up skin is to add more layers, wrinkles and weird growth type thingies. Cancerous looking caruncles and tufts of skin, waddles.
|helmeted Guinnea fowl. credit Bob CC3.0|
|Abyssinian ground hornbill. female. credit Jerry Thompson CC2.0|
|credit Eric Kilby wattled crane CC2.0|
A frequent dismissal of this generous endowment of soft tissue display/thermoregulatory structures onto non-avian theropods is parroted at me as such:
"Such skin display structures are not that widespread in birds, in fact statistically speaking we probably have not even found a dinosaur with such display structures."
For starters it is a text book case of bad statistics. While it is true that if you measured the prevalence of naked skin/fleshy display structures in modern birds you would find a fairly low percentage - I would hazard less than 5% of birds if not less have such structures - but numbers without analysis and context is meaningless. What you have to do is look at the likely ecology of dinosaurs and find what birds best match. And if you look at the birds that best match dinosaurs in ecology I would pick the ones that are large (for a bird that would be partridge size), mainly terrestrial or at least feed on terrestrial items, and live in temperate to hot climates. I would not put aquatic birds and passerines high up in ecological correspondence but even some passerines and waterfowl sport naked skin structures. Among the birds I would put forth closest to non-avian theropods ratites, galliformes, "vultures", storks, hornbills, cranes and especially ground hornbills as some of the best matches. Once you start looking at the birds that best fit non-avian theropod dinosaurs ecologically and environmentally you start to see that such nekkid, fleshy, and skin derived display/thermoregulatory features are quite common. Nothing is 100% and not all of these birds fit the bill - but a large enough proportion of them do that we should be highly confident that such displays were at least as common in feathered dinosaurs. More common I would argue because non-flighted dinosaurs could afford heavier skin structures while birds have to be a bit more thrifty in terms of such heavy building material. Also, the compacted lifespan of dinosaurs and hyper-competitive sexual politics of dinosaurs would encourage the elaboration of skin derived display structures on both sexes. Also, and most importantly, because they tend to piss off some feather nazis and anything to take the piss out of them.
|Large male Kori Bustard. Gular Display but no nekkid flesh. An important exception to the trend|
credit David Berkowitz C2.0 http://www.flickr.com/photos/davidberkowitz/5698290596/in/photostream
In fairness there are notable exceptions to this large, terrestrial bird with naked skin/fleshy faced pattern. Biology is after all notoriously squishy and as soon as you push down on something and proclaim "this is the answer" exceptions start to arise. Kori bustards are fully feathered and siriemas only have a little eye patch of naked skin. Cranes don't have loads of fleshy skin derived facial structures but they are highly migratory (remember skin weighs more than feather) and principally temperate. Far be it to me to insist on fleshy faced, skin display structures on all theropods even maniraptorans - I don't really see that this issue is as clear cut as even I would like it to be. Herons are another group that doesn't fit the trends as they have fully feathered necks and faces. However in their case they need to use rapid neck strikes to catch prey and skin weighs more than feathers so better to keep the feathers for display. Certainly smallish microraptorines types likely had fully feathered faces, or species that lived in cool temperate climes (i.e. Liaoning). But gigantic oviraptorines, dromies, ornithomimids, tyrant lizards, therizinosaurs, megaraptors and other largish coulurosaurs I would really hedge my bets towards such gnarly skin & flesh derived facial and forequarter display/thermoregulatory structures as fairly common. How far back you want to take such skin derived structures depends somewhat on how far back you want to take feathers in theropods. In other words the evolution of feathers begets naked skin derived structures.
The other point lost entirely on some is that I don't have to speculate about nekkid skin and derived structures - we already have proof.
Pelecanimimus polyodon. How come nobody talks about Pelecanimimus anymore? This is a basal ornithomimosaur and therefore a coelurosaur and therefore feathered. The soft tissue preserved around the head and neck as interpreted by the authors is very telling. No scales present. No feathers present. Instead impressions revealing wrinkles (possibly desiccated caruncles?) of skin lacking integumentary structures i.e. no feathers or scales. A soft tissue crest or flange of skin or keratin on the back of the head. A "pelican like" gular pouch. Just as I would predict and just as we should expect for a good number of feathered dinosaurs in hot climates. Soft tissue structures and nekkid skin. Also Edmontosaurus.
*update Darren Naish (see comment) let me know Pelecanimimus soft tissue interpretations are a bit of a wash. What did I say about taking the piss out everyone (including myself)? Maybe the wiki page needs an update too...
Of course if an ornithomimid the size of Pelecanimimus is showing a tendency to clear up feathers for nekkid skin and display structures you can imagine how far Deinocheirus could have taken things...
Let's not to forget to feed the rumor mill. If you dig deep enough there are some accounts of nekkid facial skin on tyrannosaurids and ol' sexy rexy himself. Now I don't want ol' Rexy to dominate this discussion but there are consistent rumors of "plucked chicken" looking skin, bits of skin with reduced scalation and completely naked skin. Feather nazis should rejoice at the notion of naked skin because as I highlighted earlier naked skin implies it was once feathered and there is likely reduced feathering somewhere on the body. But the sheer amount of retrieved "gila-monster" like scalation does suggest that scales were an important and potentially widespread ectodermal covering for tyrannosaurids. Probably more widespread than in the secondarily evolved scalation of modern birds.
I and a few other artists and thinkers that are increasingly embracing nekkid skin on the head & forequarters of theropods are a threat to both the scale loyalists and some feather nazis. I put scale loyalists in a frenzy because this look completely alters the iconic typography of how a theropod head "should" look - primarily portrayed as a tightly adhering scaled shellac over the theropod skull. And I know I really get under the skin of feather nazis because I argue - based on what modern birds do - that the attractive, graceful, and highly feathered facial countenance as depicted in many non-avian theropods is substantially different than what they have become beholden to.
Some feather nazis deserve to be put in their place a bit because some are guilty of the same crimes that they charge the scale loyalists with. What am I saying? Feather nazis often charge the scale loyalists with being emotionally and culturally clinging to an old outdated 90's nostalgia - the T. rex and raptor of Jurassic Park fame. However from my point of view when I advocate abundant fleshy and skin display features in theropods - inspired by birds of course - who do I get the most push back from? Feather nazis. Why? Even though nekkid skin presupposes a fully feathered ancestral state once you go the path of nekkid skin then you pretty much have to go the path of outrageous skin derived structures. And this completely ruins the fully feathered and attractive plumage depicted on the head of most feathered dinosaurs and calls into question how ubiquitous the "ground hawk" look was for dromies. Feather nazis don't like me for primarily the same reason scale loyalists don't like feather nazis. I call into question a certain look that they have grown fond of. Feather nazis can't outright dismiss me based on an adherence to past interpretations as they can do with the scale loyalists.
Confounding the situation is that many paleoartists give theropods a veneer of feathery type insulation but capitulate to the scale loyalists by giving theropods a scaly head. Why do they do this? If you look at what modern birds do when they lose the feathers on the head all you are left with is naked, not scaly, skin.
So far I have taken it a bit easy on the scale loyalists. That is gonna change.
(best read in an awesome - bro voice, whatever that means to you)
"But, yeah, hey man what about like Carnotaurus it had like scales like top to bottom man. Fully bruh."
Ahh I'm glad we get to talk about Carnotaurus. And it was literally shellacked with scales, scutes, nodules galore. Not a feather to be found. Carnotaurus is indeed the clarion call for scale loyalists and they should not budge from what it tells us - just as feather nazis should not budge from what Yutyrannus tell us. What I really want to talk about is the face of Carnotaurus because, let's be real, that's what we really care about right? The question I ask is did Carnotaurus - and by extension other abelisaurids - actually have a scaly face?
Yeah I am actually serious when I say this. Carnotaurus may have not had a scaly face, it may have indeed had a croc face.
Skin sections (left) indicate that cracks correspond to epidermal bulges that reach the stiff underlying tissues. Immunohistochemnistry (right) indicates increased cell proliferation (green) within the skin grooves corresponding to cracks. Abbreviations: primary (pc) and secondary (sc) cracks (ep, epidermis, de, dermis, bo, bone tissue). from Milinkovitch 2013
Check out this nice summary piece, croccrack, showing that the assumed scales on the heads of crocs are actually cracked skin that forms embryologically from the underlying bone pressing against it.
Now, I am not one to lean to fully on the extant phylogenetic bracket to infer soft tissues, and I won't start now, but one of the interesting observations is that no living archosaurs have scaly faces. Birds don't although the wood stork comes close with a highly keratinized face. And crocs actually do not have scaly faces, they have a very tight fitting skin that cracks and creases along defined planes of stress embryologically creating the illusion of very irregular scales. Yep, that was something that really threw me for a loop when I first came across that info on Jaime Headden's blog. But it is true, crocs have scales everywhere else on the body but not on the head. Could this have something to do with better use of ISO's on the snout and head... possibly. Stay posted on that thought.
|CC3.0 credit A Stranger in the Alps|
It is one of those frustrating aspects of paleontology that the holotype of Carnotaurus supposedly contained remnants of the facial skin but they were prepared away... gripes!!
"Originally, the right side of the skull also was covered with large patches of skin - this was not recognized when the skull was prepared and these patches were accidentally destroyed." wiki citing Czerkas 1997
However there are at least some descriptions available of what the facial skin of Carnotaurus looked like which included bits from the lower left jaw and right side of the head:
"Scalation was similar across different body parts with the exception of the head, which apparently showed a different, irregular pattern of scales." wiki citing Donald F. Glut Carnotosaurus: Dinosaurs 3rd edition
Our first clue is that the "scales" of the head were described as "different" from the rest of the body. Hmmm why would that occur? Does the scalation on a monitor lizard head differ substantially from the rest of the body scales? Our second clue is that the scales of the head were described as "irregular"... where have we heard that descriptor before? Oh yeah, the "scales" on the heads of crocs (actually cracks in scaleless skin) are also described as "irregular". Our third clue is that the skull of both crocodiles and abelisaurids are highly rugose, bumpy, and just gnarly looking. The tight fitting skin that covered the skull was equally keratinized, rugose and just gnarly looking. Our fourth clue has to do with why - adaptationally - would crocodiles lose scales on the head? It might have something to do with allowing better transmission of data to those integumentary sensory organs on the head of crocs. Forgive me for lacking a reference but I thing I came across the suggestion of that idea on Mark Witton's blog once... need further info. Regardless I think it worth further investigation.
Remember when it was common sense that crocs had scaly heads? Why, if we for so long assumed that, is it not a definite possibility that the first impulse when confronted with the preserved facial integument of Carnotaurus could the researchers have not made a similar mistake?
|Fucking rad, right? Much props to J.W. Kirby for this Aucasaurus with a keratinized but non-scaly head|
J.W. Kirby was nice enough to lend his talents for this depiction of an Aucasaurus with a keratinized mug. I think it works exceptionally well and does a good job of splitting the difference between a wood stork and a croc. Visit his deviantart: KirbyniferousRegret
Am I suggesting that abelisaurids lost the scales on their face to enact ISO's? Nope, although the idea of subaquatic abelisaurids has been toyed with, the swarm of data points towards terrestrial. So I don't think that they had ISO's but I do think that they had especially tactile faces. They had tough faces for sure - highly keratinized - but they also had tactile and sensitive regions, especially along the oral margin i.e. the lips. And I would suggest that they had such tactile lips to better react and position themselves with regard to prolonged and dangerous engagements with prey. Their teeth, their jaws their whole predatory apparatus was set up to bite onto and hold onto prey - not slice through it like the vast majority of other theropods. Dispensing with scales on the head allowed for a more tactile and also tough and keratinized predatorial and cranial adaptation to flourish. And also face biting.
|Wood Stork. credit Sandy Sharkey/Great Backyard Bird Count|
So if you want to draw abelisaur heads a nice informed speculative venture would be to look at crocs and wood storks for inspiration and add some tough but tactile lips and thickened gums!! I am actually converging on a perception that the upper lips of abelisaurids were very keratinized and did not hang low - almost on the path to lipless crocs. I think, however that the lower lips did hang laterally and inferior with a nice big keratinized gummy mess leading up to the teeth. Go bold and add some flesh antlers and outgroths off the cranial crest as suggested by the vessels on the top of the skull of Rugops!!
While we are on the subject of cranial scale reduction, keratinized skin, croc heads, and rugose skulls it is also interesting to wonder if the rugose skulls of other theropods that lived a very facial - tactile existence such as spinosaurids, unenlagines, other theropods and even pachecephalosaurids and some ceratopsids - may have dispensed with scales and opted for keratinized facial halos. Lots of potential candidates there.
I have some other things to say about scales and the potential loss of scales on the heads of theropods but I'm gonna save than for upcoming posts. Just know that I am not done with you yet "lizard-lipped" theropods...
Scale Loyalists Should Not Simply Concede Kulindadromeus or Psittacosaurus
Another issue I take with scale loyalists is that they simply seem to concede animals like Kulindadromeus and to a lesser extent other quilly ornithischians and possibly Concavenator to the feather nazi hordes in their blitzkrieg conquest. Don't stand down scale loyalists these animals have important stories to tell about scales too!!
|credit Tomopteryx CC4.0 Kulindadromeus|
Kulindadromeus is the best example so I will talk about this little Siberian maverick the most. Somewhat lost in the excitement about this ornithischian with I think 3(?!) different types of filamentous coverings is that it also had a diversity of scaley type coverings. I think 2 or 3 types of scale coatings. That is an important point not to be conceded by the scale loyalists. Think about where these scales were located. On the tail, the hands, the feet. The areas most liable for temperature oscillations. Scales do have an important story to tell and it likely has a lot to do with thermoregulation and scales remained very important for the duration of dinosaur history.
|Psittacosaurus soft tissue reconstruction credit: - 3D Camouflage in an Ornithischian Dinosaur, Current Biology (2016), http://dx.doi.org/10.1016/j.cub.2016.06.065 CC4.0|
The quilly Psittacosaurus was pretty small, lived in a cool climate, could have probably grown a coat of insulating filaments. Certainly other similarly sized maniraptoran herbivores and omnivores in the same environment grew a full coat of feathers. So why not Psittacosaurus? We can rule out preservational bias as an argument so often posited for why feathers are "not preserved" as the Xixian beds preserve feathers/filaments well. This is a dilemma that feather nazis simply ignore by pretty much not addressing it or talking around it. It was scaled and those scales may have provided good enough thermoregulatory benefit that a more extensive filamentous coat was not needed.
An important thought to keep in mind with rear gut fermenting dinosaurian herbivores is that they walk around with big compost bins in their gut. Like compost does, rear gut fermentators release heat. This constant slow burn of heat could have allowed for a more reduced and basal insulatory apparel - scales dominating and perhaps in some instances supplanting filamentous coatings. If we look at scales as basically solar panels such animals could have further supplanted their heating costs by stealing and hoarding energy from the sun and environment. Muscular activity, solar panel scales, rearward heat creating fermentation, blood flow and perhaps a rather low and thrifty basal metabolic furnace could have all combined to create an especially energy thrifty and efficient design. An extensive plumage not really called for. This exceptionally thrifty design would have also begat exceptional ability to dedicate food resources to growth as opposed to heating the furnace, in line with the noted size of many dinosaurian herbivores.
Filaments and feathers may not so much as replaced scales but augmented them in some instances, especially in herbivorous dinosaurs.
Xenarthran Dinosaur: The Further Back You Go, the More Complex Things May Get
I want to end this piece by pointing out where I think we might be heading. We might be heading into a zone of less certainty than more certainty with regard to integumentary structures in not just theropods but dinosaurs as a whole as well as other beasties like various stem-mammals of the synapsid gestalt that have also been receiving interest as goes soft tissue reconstruction. Now I realize that some might not take much solace in this. But I think that we should really begin to expect the unexpected. Before I let you know why I suspect that the integumentary patterns we might infer based on extant relatives might be a little too conservative let me post this abstract to a rather practical and succinct paper.
Danielle Dhouailly, A new scenario for the evolutionary origin of hair, feather, and avian scale. Journal of Anatomy April 20, 2009
Another important read is this blog post from reptilis.net "feathers" on the big, "feathers" on the small but feathers for dinosaurs one and all? by Jura it is a little dated but still packed with some good info, as well as good comment section and further links.
Now, in several points in this piece I have referred to nekkid skin as the basal or germinal layer - that when scales or fur or feathers are lost what remains is pure skin. This study confirms that "inhibition (of the Wnt/beta-caratin pathway) leads to the formation of nude skin and is required for that of corneal epithelium." Which is pretty much what I have been suggesting happened quite a lot in dinosaurs that were formerly completely feathered but then - for a multiplicity of reasons - lost such feather coatings on parts of the head and potentially forequarters.
A more interesting bit of info from this abstract is the notion that - read modern - mammals and birds have ectoderm primarily programmed to grow feathers and fur respectively. Again, I want to reiterate that this is how modern bird and mammal skin appears to be programmed. But let us not assume that more basal (read extinct) forms were as hemmed in developmentally to form such structures. I first came across this notion in the above Jura post on the topic (feathers for one and all?) and there Jura referred to as a basic rule of thumb in the comments section: the regulatory pathways, the developmental constraints may have been a lot more lax and prone to slippage, reversions, and wholesale jumbled set ups than what the more modern derived and somewhat set in stone representatives would suggest. A lot more play. A lot more complexity. And a lot less certainty for anyone trying to arbitrate what is accepted and reasonable in paleoartistic representations.
|Pink Fairy Armadillo. credit cliff1066 C3.0|
*Update 1/22/17. I too hastily made the following comparison and it erroneous. I still think it worthwhile to consider more basal members of dinosaur lineages may have had a diversity of integumentary features - as Kulindadromeus suggests - but as several commentators pointed out the analogy is wrong. I will leave it up as is because I think it important to document things I get right and things I get wrong.
Consider xenarthrans - the strange and alluring mammals that include the extinct glyptodonts and giant grounds sloths, as well as armadillos, sloths, anteaters, probably some other weird ones too I am forgetting. I don't like to use the term "living fossil" but I think it fair to call these mammals the least derived or most basal among extant mammals. The most basal mammals but also the most diverse in terms of integumentary structures. Think about the weird scutes of armadillos and glyptodonts, the coarse hairs of anteaters, the osteoderm studded skin of giant ground sloths. Now in light of what was discussed in the last paragraph where I made the point that the most derived members of clades are "locked into" defined developmental pathways for respective integumentary structures. For extant birds this is feathers for mammals it is fur. But if you go further back into more basal forms in each group it is very probable that there was a lot more developmental wiggle room, a lot more play, and a lot less capacity to fully predict what was going on in terms of how much, where, and what type of integumentary structures were going on. Consider monotremes, also very basal and also diverse in terms of integumentary types. Pangolins and aardvarks as well.
Now, this is an area of needed study but in light of the basal nature of these animals and the breadth of integumentary structures that they evince I doubt it merely a coincidence that we see this congruence. That pangolins, monotremes, and xenarthrans showcase more diversity in integumentary structures than more derived and numerous ungulates, carnivorans, and rodentia combined is something to think about...
This notion that using highly derived extant relatives to ascribe dogmatic rules to their extinct relatives should not sit too well with both feather nazis or scale loyalists. And you know I like it when both sides of the divide get befuddled. That creates fertile living and creative space to speculate to truly bizarre and jarring amalgamations of quills, feathers, scales, scutes, keratinized skin, wattles, caruncles, frills, dewlaps, and other structures.
In fact we don't have to speculate. Kulindadromeus spells it right out for us being fairly basal and hosting a diversity of scutes, overlapping scales, and various types of filaments. Later ornithischians may have largely dispensed with pelage and just as derived modern birds and mammals are locked into a pathway of fur or hair more derived ornithischians became more and more locked into a developmental pathway of scales. You simply don't see derived ungulates with lumps of osteoderms in their skin do you? And while the maybe - maybe not ulnar quill nodes of Concavenator get most of the attention for that fantastic beast less widely reported is the unexpected diversity and size of large scales preserved with the body. The Mesozoic may have been rife with complex amalgamations of scaly, scutey, quilly, nekkid skinned, and feathery beasties. Enough to confound dogmatists on both sides of the aisle.
Its not about "the enfluffening" or the return of the retrosaur it is about the advent of the weirdening and we have been in it for a while now.
In summary both scale loyalists and feather nazis bring important points to the table but their rigidity sometimes veers into dogmatism.
Naked skin on many formerly fully feathered theropods is a thing, as evinced by Pelacinimimus. The reasons for clearing up naked skin on theropods (extinct and extant) is not fully understood, doubtless multifaceted, and as shown by species such as Kori Bustards and Siriemas not a 100% mandate or law. But it was a trend that was certainly common. The evolution of naked headed theropods depends in large part on how far back fully feathered dinosaurs goes. Whether or not tyrannosaurids sported naked skinned heads is up for grabs but a distinct possibility. Especially considering the tactile nature of their skulls, feathered pedigree, and several anecdotal records of scaleless or almost scaleless hide as remarked by several researchers (Phil Currie & Paul Sereno).
Because bird skin is so thin we should expect that when feathering is lost it should thicken up with wattles, caruncles, fleshy outgrowths, keratinized bits, and other diverse features we see in living naked faced birds. Expandable gular pouches, snoods, crests, and other features were also likely widespread. I would venture that these features are relatively more common in feathered non-avian dinosaurs than flight capable birds because 1) skin weighs more than feathers so flighted animals will tend to be a bit more thrifty with such features compared to grounded animals 2) the hyper competitive, hyper condensed mating politics of dinosaurs would stimulate outrageous, expensive, and elaborate features. Dinosaurs - unlike sea turtles, crocodilians, and long lived monogamous birds species - simply did not have multiple decades to make their reproductive mark. Instead the stakes were high to make their mark when they could and for many species had much less than a decade of good potential mating years. For large and combative males this opportunity may have been substantially less.
Abelisaurids - and by extension perhaps spinosaurids, unenlagines and several other rugose skulled dinosaurs - may have dispensed with scales on the head partially or completely and sported a tight fitting shellac of keratinized skin. Convergent in design with crocodilians and wood storks such a design would offer durability and protection while potentially allowing sensitivity for animals that explored their world largely through their heads.
Much to the consternation of both scale loyalists and feather nazis the more basal we go in many dinosaurian groups the more anarchic and varied might be the integumentary structures. In derived forms feathers or scales seem to predominate the ectoderm in various lineages. In maniraptorans it is feathers, while in ceratopsids, hadrosaurids, and abelisaurids it is scales. However in more basal forms of each group we might expect a veritable riot of weird, diverse, and unexpected integumentary coverings. This prediction is met by the rich and varied ectodermal topography of Kulindadromeus, the potential for both quills and diverse scales & scutes in Concavenator, quills and scales in many ornithischians, and the noted diversity of outgrowths in many basal mammal families today such as xenarthrans.
In short there is a lot to learn. Both scale loyalists and feather nazis have their role to play but I think we might be converging on a world where both sides are too dogmatic and that nekkid skin has a role to play in this as well. As well as keratinized skin. And funky jumbled up, mish-mashed piles of xenarthran like dinosaurs with piles of scutes, filaments, quills and other things galore. For those that like precise laws and set in stone didactic guidelines on how to depict dinosaurs - especially earlier more basal forms in which the developmental pathways were not yet established rigidily - probably not a pleasant time for you right now. And people of such mind sets will probably find greater and greater consternation at the sight of increasingly odd and bizarre dinosaur reconstructions. SUX2BU.
Ok, ok are you still with me? Have I done a good of enough job in triggering scale loyalists and feather nazis? Ok time to trigger myself a bit and it is the consistent critique I always get from some quarters regarding fleshy, hangy things or big lips jutting off the faces of theropods - especially toothy ones. triggrd
"But precious, such features would get snipped right off?!?"
And this is my final word on this.
1) Don't underestimate how tough and flexible skin can be. Chewing on the wattles of a theropod could have been like chewing on a flat tire. You won't really hurt it and you might compromise your position allowing for a more devastating counter strike to the back of the neck, throat, or belly/cloaca.
I really like the thickness, elasticity, and well toughness in J.W. Kirby's depiction of SANTAnaraptor below... they look like tough chew toys that would stretch out and cause frustration to anything that bit into them.
|SANTANAraptor credit J.W. Kirby|
Also don't be carnivore biased. If you can accept fleshy, hangy wattles, dewlaps on ornithischians, herbivorous theropods, and sauropodomorphs but not on predatory theropods you are being biased. The giant beaks of ornithischians, ceratopsids, the powerful beak and jaws of an oviraptor, a macronarian are not to be trifled. These animals could easily rip your limbs from their sockets and could do the same to any fleshy display device. Should we eschew such devices on all dinosaurs then? - clearly not. In fact Edmontosaurus and Pelacinimimus prove that they had them!!
3) Male bull elephant seals and hooded seals engage in epic biting battle yet maintain fleshy, vulnerable facial outgrowths. Elephant seals form huge colonies... at least some maniraptorans did too. Male elephant seals have a severely limited time to get to the top of the social heap and mate. Theropods - indeed probably all dinosaurs - did not live as long as crocodiles so they were likely under the gun to achieve mating success at any cost. Male southern elephant seals are the largest mammalian carnivoran of all time. Now that is not stated enough in my opinion. The largest mammalian carnivoran of all time is not from the Eocene, Miocene, or early Pleistocene but living with us on earth right now. And it engages in absolutely vicious face biting prolonged skirmishes. And it has the size, strength, and tusks to do some real damage. Just like theropods did. Yet this animal - colony forming, combative, multi-ton largest carnivore ever - it has big, fleshy pendulous display structures right there on the tip of it's face. And you know that sometimes it gets ripped into, indeed it is a bit of a liability in fights but to be a true signal, to evince handicap, to resonate intimidating sounds and visually stimulate opponents - evolution has eclipsed these liabilities and favored large, fleshy pendulous things on the face of an animal that should rightly have them yanked right off. But it has them anyways and most likely did toothy theropods.
And finally - inb4 criticism - highly perceptive and critical reader will have detected what I have done here. By highlighting the need of scale loyalists to look at insulatory and thermal potential in scaley dinosaurs and by suggesting nekkid skin as a kinda proxy for a formerly fully feathered condition, I have deftly played both sides of the aisle and bolstered my claims of 1) S.I.G.I.L. i.e. insulation without feathers and 2) a fairly common evolution of facial nekkid skin and nekkid skin structures in feathered theropods. Duane you self serving bastard!!
Bwa, Ha, Ha, Ha (evil cackle from bowels of paleo - dungeon) stroking my pet cat. "You don't know my paleo super - powers!!"
Scale Loyalists and Feather Nazis recant and end your evil ways and you may yet enter the SKINgdom of Heaven!!
"A Long habit of not thinking a thing wrong, gives it a superficial appearance of being right, and raises at first a formidable outcry in defense of custom". Thomas Paine