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."

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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.

Why Protoceratops almost certainly wasn't the inspiration for the griffin legend

Protoceratops, the Late Cretaceous horned dinosaur widely suggested as being the inspiration for the griffin myth. This image shows the lesser seen P. hellenikorhinus, a larger, more ornamented species of Protoceratops than the familiar P. andrewsi.

One thing that everyone knows about the mid-sized, Late Cretaceous Asian horned dinosaur Protoceratops is that it's thought to be a fossil with historic, mythological significance. Specifically, it's said to be the origin for the griffin, the lion-bodied, bird-headed chimera that has appeared in art and folklore for thousands of years. You could be forgiven for thinking that this idea is quite old and established because it's mentioned frequently in books, TV shows, and online articles, but it's actually a relatively modern invention. What I'll be calling the 'Protoceratops-griffin hypothesis' was first proposed by Adrienne Mayor and Michael Heaney in the 1993 Folklore paper "Griffins and Arimaspeans" and then developed by Mayor across two editions of the book The First Fossil Hunters: Paleontology in Greek and Roman Times (2001, 2011). These authors were not the first to suggest that the griffin had a basis in ancient interpretations of fossil animals (Mayor and Heaney 1993), but they presented the first argument linking griffins to horned dinosaurs as well as a suite of historic evidence supporting their interpretation. The idea has been praised by several palaeontologists and is celebrated as one of the superior accounts of fossils influencing ancient mythology.

Bird-griffin statue, 7th century BCE. Was Protoceratops the inspiration for this creation? From Mayor and Heaney (1993).

The basic premise of the Protoceratops-griffin hypothesis is straightforward. Tales of Ancient Greek explorers of the 7th century BCE (but first written about in the fifth century BCE) include discussion of vicious, beaked, gold-guarding quadrupedal animals living in deserts to the northeast of Greece. These stories are said to have originated with the Scythians, nomadic peoples who mined gold from central Asia from localities close to the bonebeds of Protoceratops in Mongolia and China. It is reasoned that Scythian nomads saw the weathering skeletons of Protoceratops as they prospected for gold and told others of their existence. The Greeks interpreted these as real-life versions of the griffins they knew from history and the mythology as we know it was born. The hypothesis argues that specific aspects of griffin anatomy were based directly on these accounts of Protoceratops: the beaked jaws and quadrupedality are obvious, but griffin wings are argued to be Protoceratops neck frills or shoulder blades, taloned hands are thought to reflect Protoceratops claws and so on. As the Greeks continued to hear about these animals, eventually from direct trade with the Scythians in the 7th century BCE, their interest in griffins grew so that they became familiar components of Greek culture. For hundreds of years Greek scholars and artists would continue adding to griffin lore, always referencing the same touchstones of desert settings, powerful, beaked quadrupedal animals, and gold guarding. Their depictions and stories would be passed through to medieval times and, ultimately, the modern day.

I recently became genuinely interested in this interpretation as part of research into the earliest accounts of palaeoart. If griffin art is indeed of horned dinosaur origin, it might qualify as some of the oldest on record. But reading about the Protoceratops-griffin hypothesis (in Mayor and Heaney 1993; Mayer 2011) did not deliver the proverbial 'nugget of truth' behind the griffin myth I expected based on its fame. My impression was that evidence cited for this hypothesis was generalised to account for as much griffin lore as possible, that several major, obvious questions remained unanswered, and that there was not any attempt to refute other, non-fossiliferous takes on griffin origins. Digging into the primary literature on griffin iconography seemed to confirm my concerns, suggesting that the Protoceratops-griffin hypothesis is unfavourable among archaeologists (e.g. Frankfort 1937; Goldman 1960; Wyatt 2009; Tartaron 2014). Moreover, there are far more parsimonious and well substantiated takes on these creatures which do not rely on fossil data. In the interests of providing a counter-argument to all the 'pro'-Protoceratops-griffin hypothesis media out there, I'm sharing the products of my research here.

The griffin timeline

Perhaps the largest issue with the Protoceratops-griffin hypothesis is the fact it largely ignores griffin lore before the 7th century BCE. Griffin iconography extends deep into human history with one of their best early appearances dating to 4th millennium BCE Susa - an ancient city in what is now Iran (below, Frankfort 1937). Similarly aged or older artefacts from Egypt also show griffin-like forms (Wyatt 2009), and by the 3rd millennium BCE griffins were a regular component of art in many Near Eastern countries. The role of griffins in these communities remains a matter of controversy because we have little or no written explanation of their significance. Nevertheless, they are abundant enough to suggest some importance in these cultures, and modern scholars have attempted to interpret griffin imagery based on religious and cultural practises of these times (e.g. Wyatt 2009).

Line drawing of perhaps the oldest known image of a griffin, from Susa, 4th millennium BCE. From Frankfort (1937).

As noted above, the Protoceratops-griffin hypothesis relies on Greek and central Asian evidence no older than the seventh century BCE, picking up the griffin story thousands of years after it begins in the Near East. How does it account for this older period of griffin history? Mayor and Heaney (1993) simply write "...we have no way of knowing what kind of folklore, if any, was attached to these creatures" (p. 41), and a similarly brief discussion is presented by Mayor (2011). What we need to valiate the Protoceratops-griffin hypothesis is a link between Protoceratops and the oldest Near Eastern griffin art, especially if these fossils were meant to have directly inspired griffin appearance. To my knowledge, no such link has been presented, and this is a problem: whether we understand them fully or not, these early griffins still provide basic information on where and when griffins entered ancient cultures, and they must therefore be the focus of any attempt to explain griffin origins. As it is, the fact that Near Eastern griffins substantially pre-date any from central Asia is a clear argument against the Protoceratops-griffin hypothesis.

Taking this point further, overlooking the early history of griffin art also means that the Protoceratops-griffin hypothesis does not engage with current, mainstream interpretations of the spread of griffin culture to Ancient Greece. Griffins are thought to have become popular in Greece during the 'Orientalizing Period', a cultural event occurring around the 7th century BCE when Greek art, technology and literature became heavily influenced by Near Eastern civilisations (Tartaron 2014). Put simply, the uptake of griffins into Greek culture coincides exactly with their sudden interest in the guys who'd been drawing and sculpting griffins for thousands of years. It's easy to understand why this is the preferred explanation for the rise of Grecian interest in griffin imagery. It involves the civilisations known to have depicted these animals before anyone else, fits the dates attributed to Greek and Near Eastern griffin art perfectly, and is easily explained as part of a well-established period of cultural exchange between these peoples. To accept the Protoceratops-griffin hypothesis we need to explain why it better accounts for griffin history than the consensus view. The lack of attempt to do this by proponents of the Protoceratops-griffin link is a weakness in their argument. 

Griffin appearance, variation and the 'need' for exotic fossil anatomy

The Protoceratops-griffin hypothesis also presents a simplified interpretation of griffin iconography. Numerous variants on griffins are found in the ancient world, reflecting differences in anatomy, pose and behaviour. The 'bird-griffin' - the winged lion with an avian head (see images, above and below)- is the type Protoceratops is thought to have inspired, but is just one of many griffin chimeras identified by researchers. Reflecting taxonomy on real animals, the identification of distinctive griffin 'species' varies between researchers, but they are generally thought to include wingless sphinxes (human head on a recumbent lion), bipedally standing winged lions with human heads, winged humans with avian heads, winged lions, long necked 'lion-griffins' (sometimes called 'lion-dragons'), and lions with avian heads, wings and forelimbs (Frankfort 1937; Goldman 1960; Wyatt 2009; Gane 2012). Within these forms are more variation: they may or may not include wings, tails, ears, 'crests' or horns on the snout, manes of hair or feathers, and teeth, as well as differences in neck length, mouth gape and claw size. The animal species used in these chimeras differ too. For instance, there are bird-griffins with eagle, peafowl and falcon heads, as well as a variety of big cat species reflected in their bodies and limbs. Tails may be of either avian or felid identity.

A selection of griffins forms from Goldman (1960). Note variation in tails, faces, neck length and ears.

Both Mayor and Heaney (1993) and Mayor (2011) use different griffin types from a variety of cultures in their argument for the Protoceratops-griffin hypothesis, including wingless forms, lion-griffins/dragons, 'classic' bird griffins, as well as toothed and long necked variants. It's argued that these can be distilled to common elements reminiscent of Protoceratops in size and form despite their (sometimes major) anatomical differences, and that this implies a common origin. Variable interpretation of broken fossils are said to explain features which differ from genuine Protoceratops anatomy. For instance, the horns and ears of some griffins might reflect misinterpreted broken skulls and neck frills, and wings could be damaged frills or misidentified shoulder blades. Embellishment of stories passed on from distant lands might explain other variations.

This homogeneous treatment of griffin imagery is troublesome for two reasons. Firstly, the disregarding of griffin form shows a selective approach to evidence gathering, cherry picking elements that suit the Protoceratops origin while ignoring those which are problematic. The fact is most griffin artworks do not look like Protoceratops beyond the superficial similarity of being beaked quadrupeds (see below). Furthermore, griffin art remains differentiated even after Greek and Scythian cultures were known to have been communicative and - theoretically - Protoceratops begin could influence griffin depictions. If there was a solid, real-world basis for griffins once the Greeks and Scythians began talking, why didn't griffin appearance crystalise into something more definitively Protoceratops-like?

Homogenising griffin forms also contradicts modern interpretations of griffin art. Many researchers stress the unique histories, origins and cultural significance of different griffin forms, and some even directly caution about treating these chimeras as interchangeable for fear of obscuring their true meaning and history (e.g. Goldman 1960; Wyatt 2009; Gane 2012). Most scholars simply see griffins as chimeras - creatures invented from components of animals and human individuals for symbolic or literary intent (Wyatt 2009; Gane 2012). It's assumed that differences between griffins reflects efforts to convey information about these creatures or the scenarios they were depicted in. For example, the addition of wings may indicate swiftness or divinity; large, erect ears might suggest alertness; claws imply ferocity, and so on. Studies show that these features were not added randomly to griffin art, and the development of distinctive griffin types can be traced over time (e.g. Goldman 1960). The message from mainstream archaeology seems to be that griffin iconography had complex origins and development within the framework of chimera creation common to ancient cultures, and that generalising their form is probably not the best way to understand them.

Superficial musculature of a lion, illustrated in Goldfinger 2004. The torsos and limbs of detailed griffin art shows the same characteristic muscle groups, specific anatomies and proportions as these cats, suggesting they are not generic quadrupeds but true chimeras of large felids and birds. This muscle plan can easily be seen in some of the imagery posted below and above.

But is it correct to interpret the griffin as a traditional chimera of familiar extant animals or do we need the exotic, extinct form of a Protoceratops to explain their anatomy? I'm not going to compare this dinosaur with all variants on griffin composition here, but will suggest that the 'classic' bird-griffin does not need Protoceratops. To the contrary, it's obviously composed of a bird head mounted on a lion torso, limbs and tail, and topped off with bird wings mounted on the shoulders. There are no especially weird or exotic anatomies that cannot be explained without reference to modern species, and even the oldest renditions of griffins show closely observed details of lion and bird anatomy that leave little doubt as to their source. This is particularly true for the lion elements, where the forefeet often have lion-like thumbs, and large, padded, clawed digits. When griffin tails are not just clumps of feathers, they are long, slender and curve upwards in a very lion-like fashion, and their necks are often adorned with manes. I'm struck at how lion-like the proportions and musculature of the torso and limbs are in most griffin depictions: they are not just generic quadrupeds, but really obviously and specifically referencing big cats (above).

Sketch of a juvenile Protoceratops andrewsi skull, right lateral view.

We can also observe that, on the whole, griffin anatomy often strongly contrasts with the anatomy of Protoceratops. I don't want to set up a straw man here - after all, it's likely we know far more about Protoceratops than anyone who lived thousands of years ago, and the hypothetical passing of tales about Protoceratops from central Asia to eastern Europe is an incredibly long game of Chinese whispers. However, if the Protoceratops-griffin hypothesis is to be accepted it needs to pass some basic anatomical tests.

Let's start with the head. It's immediately obvious that there is nothing projecting rearwards from the posterior head region of most griffins, whereas all Protoceratops (even very small juveniles) have some sort of frill extending posterodorsally from the back of the skull (above). The ears and crests of griffins, explained as being the broken frills of Protoceratops fossils, are structures which project upwards from the head, not backwards. If we must give these structures a basis in reality, we can look to the ornamental head feathers of birds for the crests (remember that the heads of some elaborate birds, like peacocks, are used in some griffin art) and any number of common mammal species for the ears. These are surely simpler alternatives than the broken skull bones of dinosaur fossils occurring thousands of miles away from the source of griffin origins. It is often suggested that griffin wings might be mistaken interpretations of the Protoceratops frill, but the wings are clearly set on the shoulders in most griffin art, often behind lion-like neck manes. Moreover, as noted above, not all griffins have wings - how do these versions account for the Protoceratops frill? Protoceratops is also not toothless, its densely packed cheek teeth being obvious in even weathered skulls. The majority of griffin images show a fully toothless beak far more like that of a bird than a ceratopsian dinosaur.

Scott Hartmans's skeletal reconstruction of Protoceratops andrewsi. Borrowed from the excellent Scott Hartman's Skeletal Drawing.com.

Protoceratops also does not have lion-like hands or feet, nor any raptorial claws (above). Ceratopsians had relatively stout, blunt claws, and the hands of early taxa like Protoceratops are not especially big. I'm not sure anyone - even folks living thousands of years ago - has ever looked at Protoceratops and been amazed by its powerful limbs or ferocious talons, whereas these are striking characteristics of big cats. Finally, the tail of Protoceratops is proportionally deep, seemingly incapable of significant dorsal curvature, and not at all like that of a lion.

So beyond being beaked animals with four legs, there's no striking similarity between Protoceratops and bird-griffins. Once we start considering the variance in griffin art - the long necks, manes, feathers and so forth - even more differences become apparent. In light of this, and the fact that living animal anatomies can easily account for all elements of ancient griffin depictions, there seems no need to invoke Protoceratops as a part of griffin anatomy. The mainstream view of griffins being simple chimeras of living animals has to be considered a far simpler, and thus more likely, interpretation of their form.

Written accounts of griffin behaviour, and the development of griffin lore

Even if Protoceratops did not inform the raw appearance of griffins, could it be referenced in written accounts of griffin appearance and behaviour, such as their desert-living, parental care and gold-guarding habits? It's perhaps these accounts which provide the best evidence for the Protoceratops-griffin hypothesis as they imply the gold-strewn deserts of central Asia as the griffin's home. It's worth summarising some details of the first griffin accounts here as their nature and propagation is important. Please check out Phillips (1955), Bowra (1956), Mayor and Heaney (1993) and Mayor (2011) for more details.

Much of Greek griffin lore is derived from stories of the Greek poet Aristeas, who travelled through Asia in c. 675 BCE. His adventures and travels are first recorded in texts from 460-450 BCE (Mayor and Heaney 1993) and were so influential that they continued to be referenced well into the Common Era. However, it's worth stressing that these stories are semi-mythical tales of a semi-mythical man: Aristeas was a real person, but he is described as seeing and doing things which are combinations of real and fantastic phenomena. Scholars still debate the realities behind the locations, events, creatures, and peoples Aristeas encountered, and even ancient Greek authors, such as Herodotus, did not believe everything Aristeas was said to have seen and done (Phillips 1955, Bowra 1956). Among the earliest accounts of Aristeas' travels is the tragedy Prometheus Bound, a tale involving gods, titans, gorgons and other monsters. Here, griffins and other creatures were suggested to live to the far north-east of Greece in a desolate desert setting where nomadic barbarians (the Scythians) also hunted for gold. Other documents from the fifth century BCE, also influenced by tales of Aristeas, tell of griffins guarding the gold sought by men and other beasts. Griffin burrows were mentioned by Pliny the Elder's Naturalis Historia, written in 77 CE, as well as by Pausanias in 170 CE. These authors, again citing Aristeas, described how griffins were engaged in a constant war with a race of one-eyed men, the Arimaspi (Bowra 1956). Later accounts, penned in 200 CE, provide specifics of griffin anatomy and behaviour. They include the familiar accounts of their far eastern habitation of mountains and deserts, as well as new information: their membranous wings (considered useless for flight), the extent of their feathering, the colouration of different body parts, the fiery look in their eyes, the fact that men cannot best adult individuals but can capture their offspring, their nesting behaviour and parental nature, and how miners prospect for gold at night to avoid upsetting them.

Line drawing of a bird-griffin with offspring from Mayor (2011). The original hammered bronze relief dates to 7th Century BCE, Greece. Note the extremely lion-like torso, including strands of hair dangling from the mane. The original has texturing around the neck to further demonstrate the presence of long, shaggy hair.

These stories are the start of griffin lore as we know it today thanks to medieval scholars carrying these basic elements into later versions of griffin legends. But do these stories strengthen the idea that Protoceratops is the 'real' griffin? Again, there are problems. For starters, the major early accounts of griffins are - at best - semi-mythical stories containing numerous imagined beasts and supernatural phenomena. Why we should consider griffins to have any more basis in reality than the gods, monsters or strange human races also mentioned in these stories? If griffins are based on actual phenomena, do we need to seek rationales for these other creatures, too? Secondly, these texts echo griffin art in providing no anatomical details specifically reminiscent of Protoceratops. Indeed, many of their embellishments (feathers, colours, wing membranes etc.) are clearly not based on anything to do with horned dinosaur fossils. These accounts also blatantly refer to living animals, not fossil or skeletonised ones, and their descriptions of griffin wars with one eyed men, the vulnerability of their offspring to human capture and so on fit better with fantastical yarns than accounts of fossil creatures. Mayor (2011) suggests that the some griffin behaviour identified in these texts supports Protoceratops as the griffin source, such as their parenting skills (see image, above). These might marry up nicely with the well-known occurrence of nests and juvenile Protoceratops alongside older individuals, but parental care is easily observable for many animals, including the mammals and birds that comprise the griffin chimera. There is no need to invoke a 'third party' fossil species to explain this behaviour in griffins when thousands of modern species could have provided the same inspiration. This trait is just not specific enough to implicate Protoceratops as being referenced in griffin lore, not to mention that there's no evidence whatsoever of ancient peoples discovering dinosaur eggs or nests.

Protoceratops localities (red) superimposed onto the map of ancient central Asian trade routes and alluvial gold sites presented in Mayor and Haeney (1993). Note the scale bar, bottom right, which represents 200 miles, and the distance between Protoceratops sites and gold deposits (black stars). Protoceratops locality information from Fastovsky et al. (1997) and Lambert et al. (2001).

What of the gold guarding, behaviour, though? This is a specific trait that cannot be casually dismissed for being common among living animals. Mayor and Heaney (1993) and Mayor (2011) identify a wealth of alluvial gold deposits that may well be the real inspirations of the gold described in griffin tales and found that some ancient trade routes do bisect central Asian Cretaceous dinosaur beds (see map, above). An argument for Scythian people encountering Protoceratops is starting to look compelling, but, again, closer scrutiny reveals complications. Mayor and Heaney (1993) and Mayor (2011) show maps with Cretaceous fossil sites right the way across central Asia, giving the impression that Scythian miners and traders were falling over fossils wherever they went. But we're not just after any old Cretaceous fossils: we're specifically after Protoceratops. Both species of this dinosaur only occur in a few select localities in the southernmost region of Mongolia and adjacent to the China/Mongolia border (Fastovsky et al. 1997; Lambert et al. 2001). Those ancient trade routes and mining sites need to approach these specific sites if we're to bring Protoceratops into this story. Comparing modern Protoceratops localities with the maps in Mayor and Heaney (1993) and Mayor (2011) shows that these dinosaurs occur several hundred kilometres east from the nearest alluvial gold deposits, and even further away from the most productive regions (above). The identified ancient gold sites are mostly west or southwest of the Altai Mountains, suggesting ancient folks would only encounter Protoceratops fossils if they travelled hundreds of kilometres away from the core mining sites.

This also present a further complication to the Protoceratops-griffin hypothesis: are Protoceratops localities likely to contain gold when they're so far away from the alluvial gold sites? Both Mayor and Heaney (1993) and Mayor (2011) argue that desert storms may have transported nuggets of gold to Protoceratops localities, and that seeing these transported nuggets alongside Protoceratops fossils may account for the gold-guarding element of the griffin mythos. This is something we can test because the geology of Protoceratops sites is well documented and understood. Assuming the same basic meteorological processes occur today as thousands of years ago, we should see evidence of windswept gold in the Protoceratops bonebeds. But as far as I'm aware, no gold has been reported from these sites, either as surface debris or as buried elements. Moreover, although the possibility of wind transportation is not excluded entirely, no gold is mentioned by the palaeontologists with Mongolian field experience interviewed by Mayor and Heaney (1993) or Mayor (2011). All this considered, the link between Protoceratops and gold deposits is not compelling.

Finally, it's worth noting that the Greek accounts of griffins may no longer be the only texts on these creatures from the first century BCE. Gane (2012) discusses Babylonian and Neo-Assyrian literature which is tentatively thought to describe another take on griffin lore. This provides a very different interpretation of griffins as divine guardians against evil spirits, possibly associated with funerary rites. This sounds little like the idea that they were desert-dwelling, gold-hoarding wild animals, and of course suggests no obvious link to fossil animals of China and Mongolia. If correct, this find shows that our Greek legends are only one set of griffin lore: they are more familiar to us because of their retention in the post-classical period, but they might not be the only, or even the original interpretation of these creatures. Thus, even if Protoceratops is something to do with the griffin - which is far from a done deal - it is likely only involved in one component of griffin folklore. This seems to echo points made above about the griffin as a very old and complex concept, and how interpretations of its origins are blurred by multiculturalism.

So... is Protoceratops the basis of the griffin myth?

Before we answer that, here's a quick summary of the main issues outlined here:

• Near Eastern griffin culture seems to occur thousands of years before we have evidence for it in central Asia, suggesting Protoceratops anatomy could not be referenced in any way in the conception of the original griffin.
• Griffin anatomies, in all their variants, are entirely and best explained as chimeras of extant animals. There is no need to invoke any exotic fossil anatomies in their design.
• Griffin iconography, and perhaps written legends, are sufficiently varied to suggest a complex set of origins and legends for these creatures.
• Ancient Greek writings seem to lack compelling references to Protoceratops, and the aspects of appearance and behaviour they discuss clearly indicate they were not informed by fossilised animals. Several details of these accounts suggest they must be talking about imaginary creatures.
• Protoceratops fossils are found hundreds of kilometres from ancient Scythian gold mines, undermining the suggestion they might be the source of griffin gold guarding lore. There is no indication - historic or geological - that fossils of this dinosaur species have ever been associated with gold.

With all this said, it seems invoking Protoceratops to the griffin myth is nothing but a complication for griffin origins. Data has to be selected to fit this model and then worked around, rather than with, existing ideas on griffin origins that better account for its history, cultural diversity and spread among ancient peoples.

So, no, I can't see any reason to think Protoceratops has anything to do with griffin lore, and entirely understand the mainstream view of it as a chimeric animal cooked up by ancient cultures of the Near East. Interestingly, none of the recent papers on griffin lore and imagery I looked at in preparation for this article mention the Protoceratops-griffin hypothesis, and it's surprisingly challenging to find much mention of it in any peer-reviewed literature. This is despite its 23 year vintage and wide popularity among educators, media outlets and some palaeontologists. It clearly has not been adopted as readily by archaeologists as by those of us interested in dinosaur science. I suspect this idea has found greater mileage among the palaeontologically minded because it presents an interesting and seemingly reasonable story, but it also straddles disciplines and knowledge bases to discourage further research from people mainly interested in extinct animals. Given the lack of commentary on this idea from archaeological quarters, I'm curious to know what folks with a greater understanding of ancient cultures and histoy make of this idea.

This Protoceratops article and painting has origins at Patreon

The artwork and words you see here are supported by folks who back me on Patreon, the service which allows you to directly support artists and authors with monthly payments. This long, detailed article is exactly the sort of thing I can produce because of this support. If you enjoyed it and would like to see more, you can back my blog from $1 a month. In exchange, you get access to bonus art, discussion and rewards - the more you pledge, you more bonuses you receive! For this post, my patrons were privy to in-progress versions of the painting at the top of the article, discussions of Protoceratops anatomy, and narrowly avoided lots of swearing about rendering of complicated frill geometry. As usual, thanks to everyone who already supports me!

References

• Bowra, C. M. (1956). A Fragment of the Arimaspea. The Classical Quarterly, 6(1/2), 1-10.
• Fastovsky, D. E., Badamgarav, D., Ishimoto, H., Watabe, M., & Weishampel, D. B. (1997). The paleoenvironments of Tugrikin-Shireh (Gobi Desert, Mongolia) and aspects of the taphonomy and paleoecology of Protoceratops (Dinosauria: Ornithishichia). Palaios, 59-70.
• Frankfort, H. (1937). Notes on the Cretan griffin. The Annual of the British School at Athens, 37, 106-122.
• Gane, C. E. (2012). Composite Beings in Neo-Babylonian Art (Doctoral dissertation, University of California, Berkeley).
• Goldfinger, E. (2004). Animal Anatomy for Artists: The Elements of Form: The Elements of Form. Oxford University Press, USA.
• Goldman, B. (1960). The development of the lion-griffin. American Journal of Archaeology, 64(4), 319-328.
• Lambert, O., Godefroit, P., Li, H., Shang, C. Y., & Dong, Z. M. (2001). A new species of Protoceratops (Dinosauria, Neoceratopsia) from the Late Cretaceous of Inner Mongolia (PR China). Bulletin-Institut royal des sciences naturelles de Belgique. Sciences de la Terre, 71, 5-28.
• Mayor, A. (2001). The first fossil hunters: paleontology in Greek and Roman times. Princeton University Press. (First edition)
• Mayor, A. (2011). The first fossil hunters: paleontology in Greek and Roman times. Princeton University Press. (Second edition)
• Mayor, A., & Heaney, M. (1993). Griffins and Arimaspeans. Folklore, 104(1-2), 40-66.
• Phillips, E. D. (1955). The legend of Aristeas: fact and fancy in early Greek notions of East Russia, Siberia, and Inner Asia. Artibus Asiae, 18(2), 161-177.
• Tartaron, T. F. (2014). Cross-Cultural Interaction in the Greek World: Culture Contact Issues and Theories. In Encyclopedia of Global Archaeology (pp. 1804-1821). Springer New York.
• Wyatt, N. (2009). Grasping the Griffin: Identifying and Characterizing the Griffin in Egyptian and West Semitic Tradition. Journal of Ancient Egyptian Interconnections, 1(1), 29-39.

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