Can we predict the horn shapes of fossil animals? A thought experiment starring Triceratops

Triceratops horridus with some crazy long and curving brow horns. Just speculation, right? Surprisingly, maybe not...

For palaeoartists, animals with flamboyant headgear are among the most rewarding to render, but it's not only the bony aspects of their cranial ornaments that we have to pay attention too. Animal headgear is covered with various amounts of soft-tissue that, in extreme cases, can dramatically augment the shape of the underlying bony features. The headgear of living species has a spectrum of soft-tissue coverings from nothing at all (mature deer antlers), to relatively thin dermal tissues (giraffe ossicones), through to hard keratin sheaths that can add significant depth and length to a horn or crest (most other animal horns). This excellent breakdown of a bighorn sheep face by Aaron Drake of Colorado State University (uploaded by Simpleware Software Solutions) gives a pretty good idea of how much tissue extreme keratin sheaths can add to the underlying skull.

Not all horns are augmented to the extent seen in bighorn sheep, but even modestly proportioned keratin sheaths can add a lot of bulk, length and characteristic geometry to horn tissues. Thus, anyone hoping to accurately predict the appearance of ancient horned animals should want to predict the shape of their horn sheaths along with understanding the skull geometry. This isn't easy because, though incredibly tough and resistant, keratin sheaths are still prone to decay and rarely fossilise.

Researching horn growth for an upcoming book project has made me wonder if horn sheath shape might be more predictable than we've traditionally thought, however. Horn sheath growth mechanics are relatively simple, closely related to bone shape, and constrained by the properties of heavily keratinised tissues. They're also fairly universal across across tetrapods - the same processes that make a goat horn will make the enormous keratin sheath of a skimmer jaw, for instance. These properties might allow insights into sheath shape in fossil species even when the sheath is not preserved. So what aspects of horn sheath growth might allow this, and how could we transfer them to fossil animals?

Growing horn sheaths in living animals

Keratin sheaths are dead tissue with their only living components being the cells that synthesise the keratin at the horn core/sheath interface (e.g. at the inner surface of the horn soft-tissues, see diagram, below). Because no living tissue reaches the outer horn surface, they cannot grow by adding tissue to the tip. Rather, they grow by internal accumulation of keratin layers, each new deposit displacing the older sheath from the bony core. This creates a stack of keratin cones, with new cones growing at the base and causing the horn tissues to lengthen. Continuous internal deposition and displacement of old material is what creates the soft-tissue horn extension, as each new keratin layer shoves the older material a little further from the bony tip. This makes the tip of a keratin horn the oldest part of the sheath, and in many bovids the tips are many years old. Conversely, the youngest part of the horn tissues are located at the base. As we discussed in a recent post about the horns of Arsinoitherium, this growth mechanism binds the internal horn tissues in the overlying sheaths, limiting their ability to change size or shape. Changes in size or curvature can only be achieved by displacing the older horn layers, but complicating the horn shape - say, by branching the tip - is impossible unless the sheath is shed, pronghorn-style. The sheath itself can't be modified after deposition either, on account of no living tissue reaching it. Thus, old sheaths permanently maintain the size and geometry they were created with.

Stylised bovid horn growth, heavily modified from Goss (2012).

This growth mechanic presents three important points relevant to predicting the shape of fossil horn sheaths. The first is that sheath tissues are synthesised directly over the horn core, effectively making the internal sheath margin a cast of the bone at the time it grew. The second is that the shape of new keratin layers are constrained by the keratin sheaths that preceded them. They can't deviate too radically from the overlying horn shape and the horn core of the emerging layer should mostly nestle into margins of the older one. The third is that horn extensions are not simply exaggerations of their contemporary horn core, but a keratinous record of the horn history. Geometry exhibited by the earliest growth stages is maintained in the extending sheath regardless of later changes to the horn core morphology, and only periodic shedding or heavy abrasion are likely to alter this.

This being the case, could ontogenetic changes in horn cores provide insight into the sheath shape of fossil animals? If bone shape translates to keratin sheath shape, and sheath shape dictates the horn extension profile, then a growth series of bony horn anatomy may allow us to reconstruct horn keratin accumulations that are otherwise lost to decay. Horn core profiles give us a 'cast' of the inner sheath margin for that growth stage, and we can fit these into the margin of the preceding sheath layer (which, of course, can be deduced by the shape of a ontogenetically preceding horn core). Building a stack of nestled horn core profiles creates something akin the bovid horn diagram above and tells us something of how keratin layers were accumulated for that horn shape. The very tip of the horn sheath is lost to time because we cannot predict external appearance from horn core casts (they only represent the internal structure) but if the youngest animal in a growth series is suitably juvenile, we probably aren't missing much.

As proof of concept, I've taken the horncore outlines from the schematic bovid horn above and attempted to recreate the horn shape. Stacking them was achieved by simply eyeballing the margins, trying to fit the horn core outlines together as tightly as possible without their margins overlapping. Here's how it turned out...


I don't think that's too bad. It's not perfect, but it gives a pretty good idea what's going on with the actual horn. This method is very simple, but - as outlined earlier - keratin horns are simple, so we might not need a particularly complex method to predict their shape. But you're not here to talk about ram horns: what happens when we apply this idea to a fossil animal with a well-known growth series, and how do the results compare to our conventional means of reconstructing horn sheaths in fossil taxa?

Step forward, Triceratops

Triceratops growth series from Horner and Goodwin (2006). Both species of Triceratops are included here, but the generalities of this growth sequence are thought to apply to both. Say, that brow horn curvature looks pretty changeable - what would that mean for horn sheath shape?

The super-famous horned dinosaur Triceratops is a great animal to explore this idea with. It's known from dozens of specimens representing a range of ontogenetic stages, from small juveniles to giant adults (above, Horner and Goodwin 2006 - and no, the adults in question here are not Torosaurus). Like the horns of other ceratopsids, Triceratops brow horns have well-developed epidermal correlates for keratinous sheaths (oblique foramina and anastomosing neurovascular channels - Horner and Marshall 2002; Hieronymus et al. 2009) and these textures are present in the smallest known skulls, indicating that most or all their life was spent with sheathed brow horns (Goodwin et al. 2006). Confirmation of a horn sheath comes from poorly-preserved soft-tissues found on some Triceratops horns (Farke 2004; Happ 2010).

Triceratops skulls underwent pretty major changes as they grew, including complete reorientation and allometric scaling of the brow horns. In juveniles these curve backwards, but in big adults they arc forwards (Horner and Goodwin 2006). Typically, artists have assumed that the keratin sheaths covering these horns changed shape with them. Even pros, such as Greg Paul (2016), who have stressed that the keratin sheath should extend the horn shape, render the sheaths as more-or-less reflecting the underlying horn core of a given growth stage, without any hangovers from a previous iteration of horn shape. Whether intentional or not, the implication here is that the horn sheath was dynamic - capable of changing as the animal grew.

....just like this. Note how the brow horns of this Triceratops group are clearly changing shape as the animals increase in size, but that the keratin sheaths don't reflect any earlier horn history. Hmm. Say, do you know this image is on the front of my 2018 calendar?

The model outlined above conflicts with this traditional take, however. If we assume that the horn extension was composed of a series of retained keratin sheaths, and using Horner and Goodwin's (2006) ontogenetic sequence as a basis, the resultant horn shape is pretty surprising. Stacking horn cores in the juveniles sees those recurved shapes pushed off the horn core to extend and extenuate the curve strongly, to the point where the horn tip even points posteriorly at one stage (below). As the horn base tips forward on the approach to adulthood, these arcing tips rotate with them, creating a long, elaborate set of horns which curved twice: once at the tip, and again, but inversely, at the base. If the Triceratops in this model retained the full history of their horn sheaths into adulthood, the result would be pretty fantastic: very long horns where the tips pointed 90° away from the point of the horn core. Yowsers - that's quite different from our traditional 'just make it pointier' approach.

Stacking Triceratops horn cores, mimicking how living animal keratin sheaths grow, suggest the keratinous extension of the brow horns was strongly curved even in adult animals. As in the mock bovid horn above, the horn cores were stacked simply by trying to make them fit as neatly as possible.

Which is more likely: twirly horn sheaths or the more conventional, 'dynamic' sheaths? Where morphing horn sheaths immediately lose points is their requirement for the inert keratin horn tissues to react to each horn core shape, as well as for the horn sheath history to continuously disappear. Modern horn sheaths just don't grow like this: their extensions only exist because the old keratin tissues hang around, and we have to ask how the extending sheaths are created in our 'dynamic' sheath model. There are perhaps two ways we could attain morphing sheaths: the first is through continuous eradication of old sheath material, allowing new keratin to grow over the horn core without being obscured by previous sheath layers. This might have been achieved by Triceratops shedding and regrowing sheath extensions, or by abrading outer sheath tissues away. The second is that the horns weren't covered in one sheath but several interlocking plates, like the beaks of some birds, which might allow for jimmying and reconfiguration of the horn tissues through growth without adding lots of material to the end.

Let's consider shedding first. It's possible that at least some layers of Triceratops horns were shed because exfoliation is common on keratin sheaths in living species. For instance, puffins shed the outer layer of their beaks annually, and bovids exfoliate outer layers of their horns once or twice in their lifetimes (O'Gara and Matson 1975; Goss 2012). The fact that only a superficial layer of tissue is lost prevents the sheath being significantly altered however: exfoliation alone would probably not give us particularly 'dynamic' horn sheaths.

Constant reshaping of horn tissues might be plausible if Triceratops could regularly shed and regrow the horn sheath, as performed by pronghorns. Unfortunately, these mammals show us that detecting this growth mechanic in fossil species is challenging, however. Despite their unusual habit of regrowing an entirely new sheath each year, pronghorn horn cores have similar textures to those of animals with permanent sheathing (Janis et al. 1998). There are some differences, but they're subtle. O'Gara (1990) reported that pronghorn horn cores have annually variable properties, alternating between a spongy, relatively rounded horn core when the sheath is growing, and a smooth-textured, sharper horn core at peak sheath hardness (O'Gara 1990). It's pretty well established that dinosaur skeletons grade from spongy, rounded bones to smoother, sharpened bones as they aged, so perhaps variation in texture and shape of Triceratops horns that broke this pattern could indicate horn shedding - provided these differences could be distinguished from ontogenetic or intraspecific factors. I'm not aware of any evidence of this kind, despite the frequency in which Triceratops skull bone texture is commented, but I also don't know that anyone has specifically looked for this variation yet.

Lovely, lovely epidermal correlates on the skull of Triceratops prorsus illustrated in Hatcher (1907). Note that there's no divide between the correlates on the brow horn and surrounding skull - might we expect some sort of dividing sulcus if the horn sheath was routinely cast? From Wikimedia, uploaded by Biodiversity Heritage Library, CC BY 2.0.

A more illuminating insight may be that the correlates for Triceratops horn keratin are continuous with the epidermal correlates of the face (above). Horner and Marshall (2002) noted that the horn correlates for keratin sheathing extend over virtually the entire face - including the back of the frill (this is why so many Triceratops reconstructions have smooth 'face shields' nowadays). However, what's not seen on Triceratops horns is a boundary dividing the face sheath and a hypothetical temporary horn sheath, as might be expected where two keratinous sheaths meet (I'm assuming that the entire face shield wasn't shed annually either (palaeoartists: exfoliating/shedding Triceratops face - go!) - that's not a discussion I want to get into here).

A last, more arm-wavy point against horn shedding is that it is not at all common among living animals, possibly not even being present in some close pronghorn relatives (Janis et al. 1998). If Triceratops did shed its horns, it would be part of a club with very few members. This isn't a particularly scientific argument, but we have to concede that permanent horn sheaths are - by some way - far more common than ephemeral ones, and probably the 'default' condition for horned animals. Maybe we should assume permanence until there's good reason to think otherwise?

Could wear and abrasion create our morphing, dynamic horn sheaths in Triceratops? It's certainly true that keratin horns can be worn down, sometimes considerably. Bighorn sheep, for instance, can wear away years of horn growth in a behaviour known as 'brooming', but the results do not look like our palaeoart - in other words, they don't look like these sheep stuck their horns in a pencil sharpener. Nor do they echo the shape of the underlying skeleton. Instead, the ends are blunt, frayed and fractured (below). Any Triceratops that removed horn keratin through abrasion would presumably adopt a similarly 'sawn-off' appearance, and lack neat, pointed tips.

The broomed horns of a bighorn sheep (Ovis canadensis) - notice that they're heavily and deliberately worn at the tips, but they aren't shaped into fine points. From Wikimedia, uploaded by Lake Mead NRA Public Affairs, CC BY-SA 2.0.

Might a compound horn sheath be a route to horn sheath dynamism for Triceratops? Some readers may recall that we discussed compound keratin sheath covers last month and that they typically have deep grooves between abutting sheets. We don't see grooves of this nature on Triceratops skulls despite the very obvious rugosity profile created by the epidermal tissues, so I think we have to reject this hypothesis outright. The coverage of Triceratops horn core epidermal rugosities are pretty near identical to what we see on the horns of animals like cattle or goats, and I think we have to assume they indicate a similar, all-encompassing sheath morphology.

If Triceratops horns couldn't be renewed or take advantage of a more complex sheath arrangement, the likelihood of dynamic Triceratops horn sheaths is probably low. But does this idea of continuous sheath growth and twirly horns fare better under scrutiny? It seems to pass some basic tests, at least. The Triceratops brow horn outlines fit together pretty well with only a little displacement of the preceding horn layer, which is just what in see in modern horn growth, and the fact that their horn profiles don't change suddenly is consistent with them being perpetually constrained by layers of hard tissue. The predicted Triceratops sheath profile it is unexpected, but not beyond anything we see in living animals. And it scores points generally for being a simple model that is grounded in a well-understood aspect of living animal biology, in not needing to explain the loss of sheath tissue, and for factoring data we know is relevant to horn growth in living animals. I'm not saying this model is correct, but I am thinking that explains and fits our available data better than the dynamic sheath concept.

Of course, there are still lots of caveats. Remember that the model here is rough, being based on a generic Triceratops dataset and not the growth regime of a single species. The growth series outlined by Horner and Goodwin (2006) is a good general illustration of Triceratops growth, but results might vary if we restricted the data to a single species. My illustrations do not assume any exfoliation or tip abrasion, and we still don't have any idea what the external sheath morphology - including the presence of absence of ridges, spirals and bosses - might have been like. My attempt to stack the horn core profiles has also assumed minimal sheath thickness. If the sheath was thicker, the arcs of the horn could be stretched out over longer distances. So if you're buying this concept, remember that the horn shape proposed is only a general one - it's more in keeping with our understanding of sheath grow in modern animals, but it's still quite sketchy.

So...

Perhaps the take-home message here is not, however, that Triceratops might have had loopy horns, but that there might be more to consider about fossil horn sheaths than we've assumed. Our discussion of dynamic horn sheaths does not just apply to Triceratops: artists take this approach with most horns and spikes in palaeoart, and it's clearly at odds with how most animals grow keratin sheaths today. But maybe this isn't just a topic for artists to ponder. There's potentially scope for a real study here and, seeing as fossil horn shape has a lot of functional significance, predicting sheath morphology would be a useful aid to predicting ancient behaviour. This needn't be restricted to horned dinosaurs, or even just horns, either: keratin sheaths on plates, spikes and so on grow in a similar way, and there's not reason this technique couldn't be used on other body parts, if validated. Moving this from food-for-thought-blog post to genuine science would require testing on modern species, perhaps through reconstructing living animal horns, to see how well it holds up. Recreating a schematic, 2D goat horn sheath using this method is fine, but real-world tests - especially using 3D horn casts, not just 2D drawings - might be more challenging. In the meantime, I'm curious to know what others think of all this - the comment field is open below...

"Hello, I'm Triceratops. I'll be your odd-looking concluding dinosaur reconstruction for this evening."

Enjoy monthly insights into palaeoart and fossil animal biology? Support this blog for $1 a month and get free stuff!

This blog is sponsored through Patreon, the site where you can help online content creators make a living. If you enjoy my content, please consider donating $1 a month to help fund my work. $1 might seem a meaningless amount, but if every reader pitched that amount I could work on these articles and their artwork full time. In return, you'll get access to my exclusive Patreon content: regular updates on research papers, books and paintings, including numerous advance previews of two palaeoart-heavy books (one of which is the first ever comprehensive guide to palaeoart processes). Plus, you get free stuff - prints, high quality images for printing, books, competitions - as my way of thanking you for your support. As always, huge thanks to everyone who already sponsors my work!

References

• Farke, A. A. (2004). Horn use in Triceratops (Dinosauria: Ceratopsidae): testing behavioral hypotheses using scale models. Palaeontologia Electronica, 7(1), 1-10.
• Goodwin, M. B., Clemens, W. A., Horner, J. R., & Padian, K. (2006). The smallest known Triceratops skull: new observations on ceratopsid cranial anatomy and ontogeny. Journal of Vertebrate Paleontology, 26(1), 103-112.
• Goss, R. J. (2012). Deer antlers: regeneration, function and evolution. Academic Press.
• Happ, J. W. (2010). New evidence regarding the structure and function of the horns in Triceratops (Dinosauris: Ceratopsidae). In: Ryan, M. H., Chinnery-Allgeier, B. J. & Eberth, D. A. (Eds.) New Perspectives on Horned Dinosaurs: The Royal Tyrrell Museum Ceratopsian Symposium. Indiana University Press. pp. 271-281.
• 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.
• Horner, J. R., & Goodwin, M. B. (2006). Major cranial changes during Triceratops ontogeny. Proceedings of the Royal Society of London B: Biological Sciences, 273(1602), 2757-2761.
• Horner, J. R., & Marshall. C. (2002). Keratinous covered dinosaur skulls. Journal of Vertebrate Paleontology 22(3, Supplement):67A.
• Janis, C. M., Manning, E., & Ahearn, M. E. (1998). Antilocapridae. In: Janis, C. M., Scott, K. M., & Jacobs, L. L. (Eds.). Evolution of tertiary mammals of North America: Volume 1, terrestrial carnivores, ungulates, and ungulate like mammals (Vol. 1). Cambridge University Press
• O’Gara, B. W. (1990). The pronghorn (Antilocapra americana). In: Bubenik, G.A. & Bubenik, A. B. (Eds). Horns, pronghorns, and antlers: evolution, morphology, physiology, and social significance, Springer-Verlag. pp 231-264.
• O'Gara, B. W., & Matson, G. (1975). Growth and casting of horns by pronghorns and exfoliation of horns by bovids. Journal of Mammalogy, 56(4), 829-846.
• Paul, G. S. (2016). The Princeton field guide to dinosaurs. Princeton University Press.

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

Tyrannosaurus and Triceratops - friends at last?

Tyrannosaurus and Triceratops, not locked in mortal combat. Something must be wrong.  Cretaceous interspecies adoption concept, mimicking similar behaviours seen in modern mammals and birds, by Chidumebi Browne. Prints are available here.

Is there a more iconic palaeontological scene than Tyrannosaurus facing down Triceratops? The artistic association of these taxa has existed since at least 1906 when the very first, highly influential restoration of Tyrannosaurus (by Charles Knight, of course) pictured these animals alongside each other (Glut 2008). This idea flowed into aspects the first dinosaur movies - The Ghost of Slumber Mountain (1918) and The Lost World (1925) (unsurprisingly, given how much these films are indebted to Knight) - and, by 1928, the year Knight completed the famous Field Museum mural of Tyrannosaurus and Triceratops, their adversarial relationship was truly cemented. Book illustrations, TV shows and films have so perpetually shown encounters between these species that it's difficult to think of a new angle on this scene. At least, that's what I thought until being contacted by Chidumebi Browne, who asked me about working up a second Tyrannosaurus picture for him, this time co-starring Triceratops. Instead of combat however, he wondered about likelihood of a juvenile Triceratops being 'adopted' by the tyrant, as some animals make the headlines for doing so today (see below). Clearly I liked the idea enough to carry out the commission (I try to avoid things I feel are too unreasonable), but is this pure speculation, playing on gaps in our knowledge, or is there something credible to this idea? Could Triceratops and Tyrannosaurus, after more than a century of conflict, learn to be friends?

The literature on animal adoption is vast, with something like 270 species of mammal and bird known to adopt juveniles of their own species (via kidnapping, accidental inheritance or other means - Riedman 1982; Avital et al. 1998). Interspecific adoption is far rarer however, and most records pertain to animals housed in zoos or wildlife park. These adoptions can work both ways: juveniles can 'recruit' surrogate parents as readily as parents adopt surrogate offspring (for instance, the bond between Owen, a young hippo, and Mzee, a century-old giant Aldabran tortoise, seems to mostly reflect efforts of the hippo). There is relatively little documentation of interspecies adoption in wild animals, however. The example everyone knows is the Kenyan lioness Kamunyak, who has become something of a sensation for her habit of adopting young oryx. She adopted at least six calves before she was last sighted in 2004, defending them from others - including predators, humans seeking to intervene, and oyrx mothers - as if they were her own cubs. At least one species of monkey, as well as wading, raptorial and passerine birds have also adopted and reared the juveniles of other species (Izar et al. 2006; Literak and Mraz 2011; Oswald et al. 2013). Brood parasitism - the offloading of parental duties to other species - clearly exploits this behaviour (Riedman 1982), and famously occurs in cuckoos, certain ducks and geese, cowbirds, fish and bees.

The significance and evolutionary purpose of these interspecific relationships remains mysterious in many cases. Of course, the internet is awash with suggestions that these species have become 'friends', typically accompanied by heavily-edited video footage showing two different species at their squeeful snugglywugilinest. If they feature predators engaging in joyful play or nurturing behaviour with usual prey species, all the better. According to those sagest of human beings - internet commenters - these examples of natural harmony show us - spiteful, war-making human beings - to be the real animals. Truly, we are awful.

In the real world, the causes of these relationships are considerably less fluffy. The fact that most interspecies adoptions develop in captivity is not surprising, likely resulting from the close quarters contact between individuals and the deficient of conspecifics. Desires for parents, mates or group behaviours in some animals may be so strong in some species that they become blinded to the clear differences between themselves and the only other individuals they know. It's difficult to know whether these examples provide good models for interspecific adoption in natural circumstances.

Pictured: trouble in the neighbourhood.

The rarity of wild cases of interspecies adoption makes it hard to draw any firm conclusions about its adaptive significance, if it even has any (Izar et al. 2006). Although juvenile animals may receive some benefit from being adopted (especially if the alternative is not having parents at all), most biologists consider interspecific adoption a mistake - 'misdirected parenting' from confused adults. For some instances of bird adoption, this might reflect the similar appearance of chicks within certain lineages: adults simply can't tell them apart (Oswald et al. 2013). The circumstances surrounding some 'adoptions' are truly bizarre, where adoptees are ex-prey items which have become surrogate offspring. This has certainly happened with sea eagles where, having brought local buzzard chicks back to their own nest, presumably to eat, they started rearing them instead (Literak and Mraz 2011). It is assumed that the appearance of a raptor chick in their nest overrode any feeding impulses of these eagles, and they successfully reared several buzzards in this fashion (Literak and Mraz 2011). The idea that these parenting 'misfires' reflect recognition errors is supported by at least one instance where Caspian terns, rearing Ring-billed gulls, dropped their degree of parenting as juveniles outgrew resemblance to typical tern offspring (Oswald et al. 2013).

It is less easy to explain adoption across taxonomic and ecological boundaries so wide that even passing resemblance is unlikely. It must be said here that peer reviewed literature on these cases is hard to find, at least in my experience, so much of what is reported online is found in documentaries and news stories - not the most ideal venues for discussing complex, unusual animal behaviour (this is not a sleight against the experts featured in such outlets, just that these things are highly-edited and narrative-hungry, which often leads to embellishment and distortion of facts). As an example of how highly selective these reports can be, some stories of lions 'adopting' prey animals result from 45 minutes of observation, receiving justified scepticism from biologists. 45 minutes of coexistence does not equal a clear case of adoption, especially in species renowned for toying with easily overpowered prey.

Where these cases carry more reliability - such as the widely verified case of Kamunyak and her oryx calves - behavioural factors remain unclear. It seems unlikely that a lioness would visually confuse an oryx calf was her own, except for the possibility that her eyesight was very poor. I see explanations that possible recent, traumatic loss of her (genetic) offspring as premature on the available evidence, most likely spurred on by a desire to project human values into a simplified narrative. The fact that Kamunyak ended up eating the starved carcass of one of her adoptees, and became a serial adoptee suggests her condition might be more complex and deeper-seated than a response to one recent event. Moreover, if cub death is the catalyst for this behaviour, would it not be more common in other lions? As far as I'm aware, cub death is a pretty frequent occurrence. It also strikes me that lots of medical conditions - head trauma, brain tumours, organ malfunction leading to hormone imbalances, even certain diseases - can drastically alter animal behaviour. As far as I'm aware, no assessment of Kamunyak's health was made before she disappeared. I wonder if an illness of some kind is a more parsimonious explanation of Kamunyak's condition than complex, psychological trauma.

Let's bring all this back to Chidumebi's concept: could extinct dinosaurs have engaged in inter-species adoption, especially species as different as Triceratops and Tyrannosaurus? We certainly know that modern animals can establish these weird relationships even between animals as different as large predators and tiny prey. We also know that dinosaurs are capable of inter-species adoption, because modern birds engage in this behaviour. On these analogies, a Tyrannosaurus adopting a Triceratops is not too far fetched. We might assume that their morphological distinctions are so great that the tyrant is not misidentifying the ceratopsid for offspring of its own, and thus must be a 'behaviourally abnormal' tyrannosaur: a Cretaceous Kamunyak, if you like. The background tyrants are meant to be behaviourally 'normal', and have sighted the Triceratops calf - I expect, as is reported for many of Kamunyak's adoptions, that the Triceratops infant would not last long.

So... is this the first picture of an obviously slightly unhinged tyrannosaur?

This exercise is hampered ultimately by a lack of knowledge about the parentage of fossil dinosaurs however, and particularly that of tyrannosaurs. Despite the relative wealth of knowledge on tyrant dinosaur palaeobiology (they are extremely well studied compared to other fossil groups), we still know very little, if anything, about tyrannosaur parental behaviour. Parenting is so varied among reptiles and birds that even phylogenetic brackets are of questionable use here. Strong parental instincts seem like a prerequisite for interspecific adoption, and the evidence is equivocal for such instincts in Tyrannosaurus. Until we know more about this, the likelihood of the scene above remains questionable. Of course, that doesn't mean the image composition is without merit: there are scenarios where predators and baby prey individuals coexist peacefully, such as when adult prey animals have run off and juveniles, being slower, have hidden instead. Indeed, such scenarios likely explain some hastily dubbed predator-prey 'adoptions' reported in the media. At least that provides a partial answer to our question, then: could Tyrannosaurus and baby Triceratops get along? Probably - at least until the former got hungry.

References

• Avital, E., Jablonka, E., & Lachmann, M. (1998). Adopting adoption. Animal Behaviour, 55(6), 1451-1459.
• Glut, D. (2008). Tyrannosaurus rex: a century of celebrity. In: Larson, P. and Carpenter, K. (eds) Tyrannosaurus rex, the tyrant king. Indiana University Press. 398-427
• Izar, P., Verderane, M. P., Visalberghi, E., Ottoni, E. B., Gomes De Oliveira, M., Shirley, J., & Fragaszy, D. (2006). Cross‐genus adoption of a marmoset (Callithrix jacchus) by wild capuchin monkeys (Cebus libidinosus): case report. American Journal of Primatology, 68(7), 692-700.
• Literak I, & Mraz J. (2011). Adoptions of young Common Buzzards in White-tailed Sea Eagle nests. The Wilson Journal of Ornithology 123(1), 174-176.
• Oswald, S. A., Wails, C. N., Morey, B. E., & Arnold, J. M. (2013). Caspian Terns (Hydroprogne caspia) Fledge a Ring-billed Gull (Larus delawarensis) Chick: Successful Waterbird Adoption Across Taxonomic Families. Waterbirds, 36(3), 385-389.
• Riedman, M. L. (1982). The evolution of alloparental care and adoption in mammals and birds. Quarterly Review of Biology, 405-435.

Prints, custom palaeoart, and antisocial Triceratops

Hey now - look at this: there's a way to buy high-quality prints of my artwork, and it's dead simple. Drop me an email at wittonprints@gmail.com with your requirements, pay off the invoice, and wait for your prints to arrive - hopefully within a week or so of order confirmation. Prices, sizes and all are discussed over here. Added 02/2015: visit my print store!

To celebrate this occasion, I'm also offering a limited number of über-cheap palaeoart commissions for private clients:

Yep - your own palaeoartwork, a print and delivery for just £100, which is a stupidly cheap price for original artwork. Full details (including a few important conditions) are here (edit 02/2015: not any more!). As you may expect, I can't sustain working at that price for long. For that reason, there's only five of these deals being offered, and at time of writing, three of these deals have been taken. If you want in, don't delay. Don't despair if you miss this deal but would still like your own commission: drop me a line and we might be able to work something out.

Finally, because things have been a bit quiet about here for the last month or so, here's something to fill the void: a monochrome bristly Triceratops horridus, the dromaeosaur Acheroraptor temertyorum, and an interaction inspired by the wise, yellow philosophy of The Simpsons.

"...and like people, some [animals] are just jerks."

Apologies for yet another short post. Coming soon (when I'm allowed to publish them): Mammaliaformes! Heaps of pterosaurs! Deinonychus! Diminutive tyrannosaurines! And perhaps other subjects too!