Dinosaur scales: some thoughts for artists

Turns out that Triceratops horridus had some of the coolest scales of any dinosaur: huge, interlocking tubercles with low bosses and spikes. No other dinosaur has skin like this - at least, not without supporting osteoderms. But what are dinosaur scales actually like, and are we depicting them accurately in our art?

The discovery that many Mesozoic dinosaurs were superfuzzyfilamentouspinyalidocious has been an major influence on contemporary Mesozoic palaeoart. This has affected more than just how we depict the gross appearance of dinosaurian subjects, but also our attitudes to their behaviour, demeanour and place in the Mesozoic world. I've written a fair bit about scientific and artistic attitudes to filamentous dinosaurs and joined choruses arguing that it's important to get these new depictions 'right': we want to see filaments of appropriate morphology, size and distribution in reconstructions of these animals.

In light of this, it's a little peculiar that we have slightly more lax attitudes to how we reconstruct scaly integuments in these animals. We have some truly spectacular skin impressions from scaly dinosaurs which provide a wealth of information about their detailed appearance, and yet many of our reconstructions incorporate little of this data. Instead, we often create 'generically' scaly or wholly speculative integuments. Common issues include rendering of scales of homogenous size and shape across an entire animal, showing little difference in scalation between species, and issues with the size, proportions and shape of individual tubercles. Other times, and most egregiously, some individuals understate just how good the records for scales in certain species are, this seemingly giving license to render a more speculative, but flamboyant body covering. It's not just amateurs making these mistakes and, in the interests of not being a hypocrite, I'll state early on that I'm guilty of some of these issues in my own work.

With this in mind, I want to see out 2015 with a fresh look at four exceptionally interesting samples of dinosaur scales, providing something of a refresher for myself and other about scaly dinosaur integument and food for thought on restoring these animals. The amount of scaly skin we have from dinosaurs means this list could easily comprise 10 or even 20 examples, but for the sake of brevity and detail I'm keeping the count low. The specimens here may be familiar to veterans of dinosaur literature, but I hope to cover them in sufficient detail that much of this information will be new to many readers.

The Carnotaurus holotype skin impressions

Outside of the feathered coelurosaurs, substantial remains of theropod dinosaur skin are pretty rare. There are lots of scraps, many of which are only cautiously referred to Theropoda, but large pieces of skin associated with specific skeletons are very thin on the ground. These circumstances make the extensive scaly skin impressions known from the Late Cretaceous Carnotaurus sasteri type specimen quite special. This specimen is already impressive: described in detail by Bonaparte et al. 1990, it comprises a near complete skeleton missing only parts of the legs and end of the tail. The fact this specimen also preserves a host of skin remains means Carnotaurus is an especially well represented large theropod. Many readers will know the skin remains associated with this specimen makes it quite integral to debates over the ancestral state of dinosaur and theropod skin. As one of the few relatively 'basal' theropods known with decent skin remains, Carnotaurus has quite a bit of sway in discussions about filament development in theropods.

Illustration of the tail base Carnotaurus skin impressions from Bonaparte et al. (1990). The deep grooves in the specimen represent topography of the associated axial skeleton, in this case the haemal arches. Scale bars represent 10 cm.

The skin remains of Carnotaurus are a little patchy, but represent many different parts of the body: the anterior neck, shoulder girdle, mid-torso, and the base of the tail. The skull also bore skin impressions before they were accidentally prepared away. The largest piece of skin covers the tail base, and is figured above. A huge amount of detail can be seen across the various skin pieces. They have a relatively uniform texture, each piece showing a mix of two scale types. The most obvious are the large, 4-5 cm diameter tubercles which protrude slightly from the rest of the skin. Instead of being randomly arranged, these are spaced regularly from each other at roughly 10 cm intervals, separated by large numbers of relatively tiny, 5 mm wide scales. The larger tubercles bear something of a keel, but the smaller structures are quite featureless. Parallel furrows with vertical orientation, perhaps representing creases, are impressed into the mosaic of smaller tubercles, but do not seem to leave an impact on the larger structures. Figures in Bonaparte et al.'s (1990) description suggest that this general skin texture extends right the way around the tail - the reduction in tubercle size and density on the ventral surface commonly seen in artwork is erroneous in this respect.

For artists, the Carnotaurus skin impressions enable us to 'connect the dots' as goes the appearance of this dinosaur's hide. It seems scales were present from skull to tail base, and it doesn't seem much of a stretch to assume most or all of the animal was scaly. There are a few reconstructions of extensively filamentous Carnotaurus out there but, sorry guys, this just doesn't jive with what we know of the skin of this animal. It also seems we shouldn't be drawing Carnotaurus with obvious differences in skin texture across the body - it looks pretty homogenous in the fossils. Also noteworthy is the size of most of the scales. It seems we'd only notice the larger, keeled tubercles and furrows on this animal unless we were standing very close. Those 5 mm tubercles might perhaps register as mottled colouration, but I doubt anyone without superhuman vision could distinguish each scale from afar. Note that Carnotaurus is not unusual in this respect - a lot of dinosaurs had much smaller scales than we show in our illustrations.

The Howe Quarry diplodocids

One of the most striking components of the 1999 Walking with Dinosaurs Diplodocus reconstruction was the tall dermal spines adorning the midline of the animal. These structures were not the idle fantasy of sculptors and artists, but actually based sauropod skin fossils from Howe Quarry, a famous Wyoming Jurassic locality. Described by the late palaeoartist Stephen Czerkas in 1992, these finds are frequently discussed by palaeoartists because sauropod skin impressions are extremely rare. The impressions are associated with incomplete skeletons representing animals from 2-3 to 14 m in length, with some skin pieces being exceptionally large at 25 x 75 cm. Unfortunately, Czerkas (1992) did not identify the remains of these animals. Howe Quarry yields at least one named diplodocid, the recently named Kaatedocus siberi, but it remains to be established if these scaled remains represent the same taxon.

The Howe Quarry diplodocid skin can be described as tessellating hexagonal scales with a rough surface, each about 3 cm across. There is no sign of these scales being divided by differently sized scales to form a pattern like those seen in Carnotaurus. The roughened texture of each scale is formed by small (2-3 mm) tubercles dotted across each large scale. As noted by several authors, this morphology is reminiscent of other examples of sauropod hide and seems common to at least Neosauropoda (e.g. Foster and Hunt-Foster 2011; Upchurch et al. 2015). As a rule, sauropods must've been quite rough to the touch.

Illustrations of the Howe Quarry diplodocid spines from Czerkas (1992). Top row, illustrations of specimens as preserved; bottom, interpretative drawings and reconstructed outlines. Scale bars equal 5 cm.

The truly exceptional part of the Howe Quarry diplodocid skin remains are the 14 subconical structures found dotted amongst the sauropod skeletons (above). Some were isolated, but several of these structures were found in connected rows. Perhaps the most significant of these were associated with a skin impressions wrapped around the tail base of one individual. It's from these remains that we can deduce that they were arranged in a row along back of the animal. This might seem like a minor feat, but - as anyone who's attempted to reconstruct stegosaur or titanosaur osteoderm arrangements might attest - being confident about the arrangement of extraneous pieces of dinosaur integument is nothing to be sniffed at. These cones vary quite a bit in size and shape. The largest, estimated at 18 cm tall when complete, seem to stem from the proximal end of the tail, but those of the distal end are smaller. Some cones are quite tall and straight, others blunter and recurved. The tips of all the cones are flattened laterally, but the bottoms more or less round in cross section. As with hexagonal scales on the body, these spines bear small tubercles across their surface. That these were purely comprised of the dermal tissues, and not osteoderms, is confirmed by the total absence of bone from any of the cones. Quite how far these conical structures extended across their owner's bodies cannot be said from the known remains, nor should we feel confident that we have the full spectrum of size or morphological variation of the spines (Czerkas 1992).

The detail and specificity of the Howe Quarry specimens give artists an atypically good insight into the appearance of these sauropods, and remain significant specimens or this reason. But as cool as this all is, the Howe Quarry skin specimens could be more useful. For instance, it is not clear how large each sauropod individual with associated skin remains was, and it's thus not clear how large those spines or scales were in comparison to each specific animal. The range of body lengths for the Howe Quarry specimens (2-3 -14 m) perhaps indicates that the scales of these animals (3 cm across) might be larger against body size than those of most other dinosaurs, but how visible they might be to observers is really dependent on knowing the sizes of the animals concerned. Likewise, the only published illustrations of these unique, interesting remains are pretty basic: it would be neat to get these specimens figured and described in a lot more detail. Hopefully, these details will be forthcoming soon.

The Sternberg/Osborn Edmontosaurus mummy

You can't discuss scaly dinosaurs without mentioning hadrosaurs. Research into hadrosaur skin is only second to that going into the fuzzballs at the other end of the dinosaur tree, there being so many skin impressions from these dinosaurs that we can gauge variation between species, see pathological skin tissues, and reconstruct virtually complete integuments for some taxa. This relative glut of data has spurned investigation into just why hadrosaur skin crops up so often. The exact cause remains elusive (it's seemingly unrelated to the rocks they occur in, nor their palaeoenvironmental or palaeoclimatic preferences), and it is suspected that there is something intrinsic to their skin anatomy which makes it more preservable (Davies 2012).

The amount of data we have for hadrosaur skin is really impressive. Here, in grey, you can see the skin impressions known for several hadrosaurid taxa: A, Brachylophosaurus canadensis; B, Edmontosaurus annectens; C, Gryposaurus notabilis; D, Maiasaura peeblesorum; E, Saurolophus angustirostris; F, Saurolophus osborni; G, Corythosaurus casuarius; H, Lambeosaurus lambei; I, Lambeosaurus magnicristatus; J, Parasaurolophus walkeri. From Bell (2014).

Even among hadrosaurids, Edmontosaurus annectens stands out as having particularly exemplar skin remains. Collectively, we have skin impressions from virtually its entire body (above). One of the most spectacular Edmontosaurus fossils with scaly remains has to be the "Trachodon mummy", discovered by George Sternberg (Charles Sternberg's son) in 1908 and described by Henry Fairfield Osborn in 1912. Osborn lavished attention on the integument of this near complete, fully articulated specimen, of which skin impressions covered the posterior jaws, neck, shoulders, chest, belly and forelimb. This specimen also revealed the presence of a low frill along at least the posterior part of the neck. Osborn's work on this animal stands out as a landmark document on extinct reptile integument, and interested parties really should download this article from the American Museum of Natural History here (NB. this is a 75 Mb download, it coming bundled with historic descriptions of the skulls of Tyrannosaurus and Allosaurus, whatever they are).

Pectoral (lower) and manual (upper) skin remains from the "Trachodon mummy" specimen. Notice the scales extending onto the unguals - these animals did not have nails or claws on their hands. From Osborn (1912).

Osborn's description revealed details of dinosaur skin which were, at that time, poorly known from other animals. He remarked on how thin the skin layer was and the remarkably small size of the scaly tubercles covering the body (1-5 mm). The fineness of the skin resulted in perhaps a third of it being accidentally destroyed during collection - 'dinosaur mummies' were an unknown quantity before this specimen, and collectors had no idea such data was at risk when skeletons were being uncovered. Edmontosaurus skin was a mosaic of larger and smaller tubercles, but their size variation is more continuous the obviously bimodal configurations of other species. The smaller (1-3 mm) tubercles were rounded structures located between larger (5-10 mm) hexagonal ones. Osborn called these 'pavement scales', and noted that they occurred in small (5-10 cm wide) clusters in some areas, such as the neck, inner surface of the arm and belly, but covered entire other parts of the body, such as the side of the chest, lateral surface of the arm and above the hips. The largest pavement scales, about 10 mm wide, occur on the lateral surface of the arm and tail. Both large and small scales occur on the frill (below). Folds, creases and smaller tubercles seem to correspond with intervertebral spaces, likely reflecting where these tissues flexed and creased with neck movement. The actual height of the frill is unknown from this specimen, the free margin being damaged during collection.

Osborn's illustration of the frill of Edmontosaurus. From Osborn (1912).

We could go on as there's so much detail on this specimen, but you're better off just checking out Osborn's description. He certainly provided lots of interesting details for artists: a visual summary of the distribution of larger and smaller scales in a cartoon hadrosaur (below), comments on his collaboration with Charles Knight to produce a 'trachodont' reconstruction in line with his new information on hadrosaur skin (also below), and even speculation on how pigmentation may pertain to the scale pattern. Of further interest is Osborn's comparison of the skin of Edmontosaurus with other hadrosaurs, this noting that the scales of his mummy specimen were a lot smaller than those of other, closely related animals. Other differences in hadrosaur skin texture has become even more apparent in subsequent years.

Left, Osborn's illustration of Edmontosaurus outlining the distribution of large scale clusters, with their size much enhanced for visibility; right, Charles Knight's iconic 1912 painting of the same taxon, an artwork produced in collaboration with Osborn and data from the "Trachodon mummy". From Osborn (1912) and The World of Charles R. Knight.

So, other than the obvious take-home - that we know a heck of a lot about the skin of Edmontosaurus -are there any obvious pointers for artists here? As noted for Carnotaurus above, it's doubtful that we'd be able to define individual scales or the patchy distribution of pavement scales on this large bodied (12-13 m long) species unless we were right next to it. Secondly, of all dinosaurs, surely this is one species to consider off limits to extensive filamentation. I suppose you could argue that filaments filled the few parts of this animal's hide left unrepresented in the fossil record, but that fuzz is going to look like weeds growing through a pavement if you're paying attention to where we know scales were. I also think it's worth paying attention to what Osborn meant by 'frill' along the back of this species: it does not appear to be a narrow, fibrous structure as commonly depicted, but a scaly continuation of adjacent dermal tissues.

The (unpublished) Triceratops superscales

I've saved what I consider to be one of the most interesting and impressive set of scale impressions for last, even though they are represented by specimens which have only currently received only very superficial publication through online news articles. These specimens belong to one of the most familiar and famous dinosaurs of all, the ceratopsid Triceratops horridus, and yet they demonstrate a scale topography completely unlike that of any other dinosaur. Their discovery is a particularly fun curve-ball because we have skin samples from a number of other ceratopsians, none of which are particularly like those now known for Triceratops. I'm reminded about earlier discussions of 'one skin fitting all': it seems ancient dinosaurs really could be just as varied in skin morphology as modern animals.

Huge patch of Triceratops skin, preserved as an internal mould - look at the size of the individual scales! Borrowed from the Rapid City Journal.

These extensive skin impressions were associated with one of the most complete Triceratops specimens ever found, a Wyoming individual known as 'Lane'. This specimen, including its skin, is now on display in the Houston Museum of Natural Science. Without a full description it's a little difficult to give much in the way of specifics about the skin, but published photographs reveal a network of very large (I'm estimating 50-60 mm wide based on the adjacent images) hexagonal tubercles dividing larger tubercles (perhaps c. 100 mm) with central, conical projections. These large scales are sometimes described being as 'nipple-like', for obvious reasons. Divisions between these tightly interlocked scales are marked, and we might have been able to distinguish individual scales on these animals from some distance away. The function of the larger tubercles with their prominences has been the source of much speculation in art - do these structures represent bosses and low spikes, or tubular supports for large, coarse filaments? I must admit to considering the latter unlikely as neither hair or scales in modern animals grow through scales, but instead around them. I'm happy to be wrong on this, though, and both interpretations could be easily tested by looking for apertures at the tip of each prominence. Hopefully these specimens will get a full write up soon, which might provide such details.

Detail of the large tubercles adorning the outside of Triceratops. Also borrowed from the Rapid City Journal.

Lane's skin impressions suggest that the scales of Triceratops were characteristically coarser, certainly a lot larger and perhaps more sculpted than those of most other dinosaurs. Their overall appearance is very different to the hadrosaur and theropod skin mentioned here, contrasts markedly from the scales known from other ceratopsians, and is rather unexpectedly most similar to the scales of sauropods. It's difficult not to intuitively equate Triceratops skin with that rhinos and armadillos: there's something almost armour-like about those heavy scales and low, projecting bosses. Perhaps this chimes with the unusually solid, reinforced cranial frill we find in this species - was Triceratops something of a horned dinosaur tank? I reckon there's a lot of fun to be had with depicting this animal as looking particularly tough and grizzled, with big skin creases and heavy folds - such a depiction can be seen at the top of this article. It's perhaps worth noting that the actual appearance of Triceratops is not a million miles off the Charles Knight's famous painting of 'Agathaumas' (probably = Triceratops) with its speculative heavy scaling.

Summary time

I hope what's becoming clear here is that we can obtain quite a lot of information from dinosaur skin impressions, and that they show scaly dinosaur species have their own characteristic integuments in the same way that filamentous ones do. There really doesn't seem to be a 'standard' type of dinosaur scale, and even closely related species show some significant variation between them. We have to conclude that those of us hoping to restore these animals accurately really need to pay close attention to these data, considering variation in tubercle size, texture and distribution. I particularly emphasise this for artists who draw every scale: if that's the route you're taking, make sure you're drawing them correctly! Moreover, the specimens outlined here are good reasons to be inventive when skin impressions are lacking. It seems most relatively extensive skin impressions of scaly dinosaurs reveal things like spines, keeled scales, armour-like structures, frilled projections and so on. Mesozoic dinosaur skin must've been as interesting as that of modern reptiles, and we might expect many species to have elaborate structures of some kind.

And that's it for 2015

OK folks, we're done here for this year, but there's plenty more to come in 2016. Weird archosauromorphs, stem mammals, some retropalaeoart and the publication of Recreating an Age of Reptiles will be covered early on. Huge thanks to everyone who's been reading and supporting this blog throughout 2015 - I hope you've enjoyed what I considered to be one of my best blogging years so far. All the best to you all for the festive period, and see you all in 2016!

References

• Bonaparte, J. F., Novas, F. E., & Coria, R. A. (1990). Carnotaurus sastrei Bonaparte, the horned, lightly built carnosaur from the Middle Cretaceous of Patagonia. Contributions in Science. Natural History Museum of Los Angeles County, 416, 1-42.
• Czerkas, S. A. (1992). Discovery of dermal spines reveals a new look for sauropod dinosaurs. Geology, 20(12), 1068-1070.
• Davis, M. (2012). Census of dinosaur skin reveals lithology may not be the most important factor in increased preservation of hadrosaurid skin. Acta Palaeontologica Polonica, 59(3), 601-605.
• Osborn, H. F. (1912). Integument of the iguanodont dinosaur Trachodon. Memoirs of the American Museum of Natural History v. 1