Gallery and print store

Friday, 23 March 2018

Dinosaurs in the Wild: a review

Dinosaurs in the Wild's Quetzalcoatlus. OK, it's not a dinosaur, but it is in the wild.
If you travel to London's Greenwich Peninsula before the end of July 2018 you might find Dinosaurs in the Wild, a unique dinosaur experience that's been touring the UK since 2017. Created by the same team that brought us the original Walking with Dinosaurs, it continues the apparent mission statement of director Tim Haines to bring realistic, lifelike dinosaurs from cinema screens into everyday life. Walking with Dinosaurs allowed us to see realistic, movie-grade dinosaurs in our own living rooms, and DITW takes us one step further: what if we - the general public - could be among extinct dinosaurs ourselves?

DITW defies easy categorisation, taking inspiration from education centres, theatre, film and theme park rides. At the core of this blend of media is a simple idea: DITW visitors are transported 67 million years back in time to Maastrichtian North America, the final stage of the Cretaceous and the home of some very famous dinosaurs, including Tyrannosaurus, Triceratops and Ankylosaurus. Once there, visitors are guided through the labs of 'TimeBase 67', a research base dedicated to the study of Late Cretaceous life. Note that isn't a sit-down VR experience but a tour through a real physical environment with actual rooms, simulated vehicle rides, lab stations, 3D video displays acting as windows, animatronics and trained humans creating a convincing illusion of DITW's setting and narrative.

Vehicle rides are part of the DITW experience, as are traffic jams caused by dinosaurs with little in the way of road awareness.
The tour we took included people of many different ages and, so far as I could tell, everyone was having a lot of fun. Children in particular seemed completely sold by the setting and only the most jaded adults won't be pulled into the experience somehow. Even if older visitors aren't completely able to suspend disbelief for the 70 minute run time, there's huge amounts of detail to appreciate in the lab environments, the back story to the TimeBase to unravel, some terrific sequences with the animals, and a lot of genuine science to find behind the 'edutainment' exterior. Tour guides are on hand to answer questions along the way and keep guests moving on time. There is a narrative to the journey through TimeBase 67, which I won't spoil here, but parents with young kids be warned that Tyrannosaurus is an appropriately big, scary motherhubbard in DITW, and some bonus parenting* might be required at times.

*I don't have kids. I assume this is the right terminology.

Alamosaurus, Dakotaraptor and a collection of tourists approach TimeBase 67. Note that the necks of Alamosaurus are not hugely oversized, but augmented with a long skin flap along the underside.
While many will see DITW as a great activity for kids, I have no doubt that the people who'll get most out of it are genuine palaeontology enthusiasts, especially those who pay close attention to the TimeBase 67 interior, know a little about dinosaur palaeobiology, and have some experience in real labs and wildlife hides. There are Easter eggs galore for the experienced palaeo or wildlife nerd, and it's clear that great attention has been paid to the interior design to evoke the feeling of real-world research labs and wildlife observation posts. Though guides present information in each room, eyes are encouraged to wander to video footage of nesting dinosaurs, instructional posters on animal handling, open notebooks, specimens awaiting cataloguing, tissue samples being processed and - most sciencey of all - weird things in jars. The observation dome - an obvious highlight of the tour - bears animal spotting guides much like those you'll find in nature reserve hides, and they cleverly include a number of animals that (I think) are not featured in the show, tricking us into looking at the animations as we would a real landscape. I can't have been the only one looking for small mammals, birds and lizards among the more obvious dinosaurs. The impression from such details is of a rich, detailed world, and it's convincing enough that you might have to occasionally remind yourself that you're in 21st century east London and not, actually, in Cretaceous Montana looking at freshly caught extinct insects. 

Visitors are given time to wander around rooms to take in these details, but not much. The clear intention is to deliberately overwhelm us in the same way that comparable real world settings might - if you've ever taken a tour through an unfamiliar lab or museum, you'll know the frustrations of barely glimpsed curiosities and quickly glimpsed specimens. It's a risky strategy: pull people through DITW too quick and they'll feel rushed and unsatisfied, but let visitors linger and they might get bored, or notice the proverbial wiring under the board. For the most part, I think DITW gets the timing right. I felt I had sufficient time to satisfy myself with the main details of each setting, but left knowing that a future visit would reveal more. This said, I'm aware that palaeo enthusiasts might be able to experience rooms a little quicker than the average visitor. If, say, you're familiar with sclerotic rings and Tyrannosaurus brains you'll immediately recognise these objects when you see them, experience a quick nerdy thrill, and then move on. Other visitors might need a little more time to read labels and work out unfamiliar objects, and I wonder if the tight schedule could be a little more frustrating for those not so familiar with dinosaur theory.

Fully-lipped Tyrannosaurus surveys the TimeBase 67 floodplain. Note the feathers - they shouldn't be over the pelvic region, right? DITW has an obvious solution to this - though you'll have to visit to see what it is.
Of course, most sensible people won't visit DITW to look at notepads and specimen trays: they want dinosaurs, preferably in the wild. These also do not disappoint, with the digital versions being especially well produced. 3D glasses UV-protection goggles need to be worn whenever you're next to a window, allowing us to appreciate a great sense of depth when we look out over the Cretaceous floodplain surrounding TimeBase 67. We get a number of opportunities to see the animals in their full digital glory, and they're refreshingly animalistic instead of Hollywoodised monsters. Half the fun of the experience is not knowing what the animals will do and I won't spoil anything here, but you can get a good sense of DITW ethos from the snippets released by the DITW Twitter feed. I won't pretend I wasn't super-chuffed to see terrestrially-stalking azhdarchids...
...and this sequence of Alamosaurus irritating a flock of Dakotaraptor is terrific. No Jurassic World-style tag-teaming to dispatch a giant dinosaur here, just lots of irritated feather poofing. Shake harder, boy!

It's not all yawning dinosaurs and preening pterosaurs, though: fans wanting dicier threat displays and hunting behaviours won't be disappointed**. Happily, the quality of the reconstructions matches the depicted behaviours. The animals are thoroughly modern takes on familiar species and seem to have received refreshingly little, if any, embellishment to make them more ferocious or marketable. Extra-oral tissues (lips and expanded rictal plates) are standard, bold but credible decisions have been taken with their integument, and the volume of muscle and other soft-tissues is substantial, but within reason. Their animations are pretty good too, with larger species having an appreciable sense of mass and inertia instead of pirouetting around like creatures half their size. This is especially noticeable when the animals are close to viewing windows, these being large enough to appreciate their real-life size. A lousy sense of mass in the animation would have ruined the illusion, but they move with a weight and heft comparable to large living animals. This might not be something that we appreciate consciously, but is one of narrowest precipices over the uncanny valley and the downfall of many dinosaur animations. Hats off to the animators for taking time to get it right.

**Apparently. I, er, was basically watching the pterosaurs most of the time.

Variation in animal proportions, integument and colouration gives a sense of looking at real populations and not cloned digital models - it's subtle, but makes all the difference. I suspect deliberate efforts were made to avoid the uniform greys and browns that still characterises many popular dinosaur reconstructions with most species sporting elaborate colouration or patterning somewhere. These are not garish carnival monsters though, and look consistent with our knowledge of pigment mechanics and evolution, as well as appropriate for the habitat and lifestyle of the creatures concerned. Ultraviolet colouration and iridescence features too. Further points are awarded for the animals not being dressed up in the colours of living species: there's no cassowary-inspired maniraptorans or other obvious real-world colour schemes to jar the illusion. I'm fairly certain the facial colours of Triceratops (below) owe something to Darth Maul, though...

DITW's Triceratops takes a dip. Sadly, this great image isn't a still from DITW proper, but the attention to detail and nuanced behaviour shown here - including the birds on the Triceratops face - is typical of the show in general.
Several aspects of the reconstructions recall All Yesterdays for their boldness, such as the pterosaur dewlaps, display flaps on the sauropod necks, and some inflatable nasal tissues - this isn't surprising when you realise that an All Yesterdays author - Darren Naish - was DITW's scientific consultant. I'm sure these additions will startle some folks who aren't familiar with modern palaeoart conventions, especially those used to dinosaurs depicted as shrink-wrapped walking musculoskeletal systems, but, simultaneously, none of the animals look 'over speculated': their appearance acknowledges our limits to predict extinct animal anatomy without losing sight of what real animals look like. This isn't to say there aren't some aspects of the reconstructions that won't be quibbled by experts, but we're talking nitpicks here, not glaring problems. In terms of broad-brush strokes, and most of the finer ones, DITW hits home in all critical aspects of its reconstructions and animations. It's rare to see big-budget, mass-audience palaeoart achieve this sort of credibility and is especially surprising given how much animation was needed to create the sense being in a real 3D environment, sometimes with multiple views over the same landscape and animals transitioning between viewing stations. The sensation is believable enough that, upon leaving the event, I felt a strong urge to head off to the countryside for genuine wildlife watching.

Back in the real world, away from the TimeBase, is where some minor criticism of DITW might be found. Having been thoroughly impressed with DITW I was a little disappointed to find that there was no book or other media (behind the scenes DVD? Blu-ray with the animations?) allowing us to preserve the experience at home. There's some great work gone into this show and it's a shame to think that, when DITW eventually ends its run, there'll be no way to truly appreciate the designs and ingenuity that went into it. There are DITW toys, posters and clothes, but they only go part way to capturing the experience itself. A book or 'behind the scenes' film could reinforce the science behind the spectacle, too - a lot of visitors surely enjoy DITW, but do they know how saturated in science the event is? I'm aware that this might change in the future - I hope it does.

Another look at the DITW Tyrannosaurus. There are other, non-tyrannical choices of PR art, but I really like the composition of this piece. Half-obscured dinosaurs have an almost classical vibe, I suspect Zdenek Burian would have approved. The artworks you're seeing in this post are promotional renderings by Damir Martin - check out his site for more cool stuff.
The ticket cost of DITW has drawn some comment on social media, some of which may be unwarranted. Tickets are upwards of £20 each so, yes, DITW is undeniably a more expensive dinosaur experience than, say, visiting a museum or watching the next Jurassic World movie at the cinema. Such cost, however, is comparable to that of gigs, theatre shows and travelling exhibitions - not perfect analogues for DITW, but similar in terms of running expenditure and event duration. I'd argue that the novelty, ambition and execution of DITW trumps most of these experiences too: there really isn't much else out there like it, let alone something of such quality and educational potential. I appreciate this doesn't diminish what will be steep door prices for some, but a little online research revealed a number of family passes, promotional codes and other means of trimming the ticket cost down, sometimes quite considerably. Group rates are available too, if you're looking to attend with a suitably sized clan. If cost is an issue I heartily recommend checking these offers out - dinosaur fans young and old(er) will not want to miss this.

In sum, Dinosaurs in the Wild is a terrific blend of cutting edge science, technology and entertainment that dinosaurophiles - or anyone with a general interest in extinct life - will enjoy immensely. Whether you visit just for the spectacle, to nerd-out over the palaeontological Easter eggs, or to see what next-generation science outreach could be like, you're sure to enjoy it. As far as I know, the next location for DITW has not been announced yet. It leaves London on the 31st of July, so I recommend exploiting London's accessibility to visit as soon you can in case the next venue is less reachable. Hopefully, if DITW does well, we can look forward to sequel experiences set in alternative times or locations: DITW-style experiences in Mesozoic seas, Permian Russia or Pleistocene Europe, anyone?

Exclusive, unused promotional still of the DITW Alamosaurus, AKA Spods Maclean, finished acting for the day and out of character, relaxing in a pub close to the venue. It's funny how performers are often smaller in life than they appear onscreen.

Enjoy monthly insights into palaeoart, fossil animal biology and occasional reviews of palaeo media? 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!

Sunday, 25 February 2018

A mural for Dippy: restoring a celebrity Diplodocus in art

My mural of a Diplodocus carnegii herd, currently keeping Dippy, the Natural History Museum's Diplodocus cast, company in Dorset County Museum. At 4 x 2 m, it's the third biggest picture I've ever done, and - as positioned at the museum - the most visible.
If you head to Dorset County Museum at some point before May 9th 2018 you'll be able to see a genuine dinosaur celebrity: the Natural History Museum's 'Dippy' Diplodocus skeleton, on the first stint of its 'Dippy On Tour' campaign of UK museums. The trip is well worth the visit even if you're familiar with the specimen from the NHM's Hintze Hall. A mezzanine around the skeleton, and the smaller size of the exhibition space, allows visitors to get closer to Dippy than ever before, and you can see the specimen from elevated positions unavailable at the NHM. If you're a sauropod fan in the UK, this might be your best chance to see this specimen up close and personal. It's free to see the skeleton, but you do need to book in advance - the tickets are flying off the shelves, so don't expect to just walk in.

A discerning audience checks out my prints at Naturally Curious. Say, some of those images look a little Life-through-the-Ages II-y...
Alongside Dippy is a collection of art entitled Naturally Curious, works by four different artists inspired by fossils and the natural world. My work is among them (above) and includes a 4 m wide mural based on the Dippy specimen and its palaeoenvironment - the same image that welcomed you to the post. It's not placed with the rest of my work but hanging right next to the Dippy skeleton itself - the first time a detailed artistic restoration has been associated with the specimen since the 1980s when a scale model stood next to its tail. This mural, commissioned by the Dorset County Council, was a great opportunity to bring Dippy's visitors up to speed on the latest ideas on sauropod dinosaur life appearance (as well as very flattering for me - it's not every day you're asked to display art next to one of the most famous dinosaurs in the world). The process involved learning a lot about the Dippy specimen, applying some new ideas about dinosaur anatomy to Diplodocus, and looking into the specifics of Dippy's palaeoenvironment. If that's not fodder for a blog post, I don't know what is.

Because production time on the mural was short, we decided to augment an existing picture rather than start from scratch. The image in question is below, and was created in 2009 to publicise work by Mike Taylor, Mathew Wedel and Darren Naish on sauropod neck posture (Taylor et al. 2009). The Dorset team liked the image and, though quite dated now, it gave an anatomical and compositional framework that had been approved by several sauropod experts, shaving a lot of time off the production schedule. The final artwork is different to the original in many respects but much of the 2009 DNA remains obvious, including our nod to Rudolph Zallinger's Age of Reptiles mural.

PR art for Taylor et al. (2009), showing D. carnegii with its neck held aloft rather than - as was fashionable at the time - held horizontally. 2009 was a long time ago for me, artistically speaking.

Working with Dippy, and establishing the scene

It's important to any palaeoartwork to know the nature of the actual fossil material behind a reconstruction, and it might come as a surprise to know that 'Dippy the NHM Diplodocus'* is a different entity to the specimen it's cast from. The 'real' Dippy is Carnegie Museum specimen 84 (CM 84 for short), the holotype of Diplodocus carnegii, unearthed from Jurassic sediments of Wyoming in 1899. It's a mostly-complete skeleton missing elements of the limbs, the end of the tail and the skull, and these elements were sculpted or casted from other animals to create the mounted Dippy skeletons in museums around the world. This makes Dippy mostly representative of a single individual, but still a composite of several Diplodocus. CM 84 has been extensively documented - especially in Hatcher's 1901 monograph - and this makes it an excellent specimen to base a palaeoartwork on. Scott 'Master of Dinosaur Bones' Hartman's 2013 Diplodocus skeletal restoration was used to fill in the proportional gaps, and Tschopp et. al (2015) provided some very useful data on diplodocid osteology, often up close and in clear detail.

*The NHM's CM 84 cast is not the only Diplodocus to bear this nickname: the actual CM 84 specimen was also christened 'Dippy' when discovered in 1899, and several museums around the world use this name for their casts. In this article, my use of 'Dippy' consistently refers to the NHM cast.

Where the Diplodocus roam: depositional settings of the Morrison Formation at the time when Dippy lived. The Dippy site itself is in southeast Wyoming, among the series of wetlands that line the eastern side of the Morrison basin. From Turner and Peterson (2004).
CM 84 stems from the centre of the Morrison Formation, a famous Late Jurassic unit that yields, in addition to Diplodocus, many famous dinosaurs: Allosaurus, Stegosaurus, Brontosaurus, Ceratosaurus and Camarasaurus, among others. The Morrison Formation is geographically extensive with major outcrops in Colorado and Wyoming, and additional exposures in 11 other states (above). Palaeoenvironmental studies show variation in climate and habitats across that range. We know that southern regions were drier, that a number of water bodies existed across the basin, and that water and sediment influxes were received from highlands to the west and, possibly, the east (Turner and Peterson 2004). The Wyoming quarry where CM 84 was recovered represents an ancient lake, part of a broader series of wetlands in the east of the Morrison depositional basin (Turner and Peterson 2004; Brezinski and Kollar 2008). It's been suggested that these relatively well-watered settings may have been important habitats for dinosaurs of all kinds, offering abundant plant material compared to the surrounding arid environments (Turner and Peterson 2004). I took these details on board for the mural, changing the backdrop of the 2009 image from a sparse lake margin to a well-vegetated, westward-facing gateway with distant hills. The result is hopefully something not too far off the environment that CM 84 was buried in, and maybe lived in.

Proportions, poses and pedes

Although some tweaks were made to the proportions of the animals from my 2009 image, the basic poses of each was maintained. Readers may question why the necks of the animals have remained aloft when some researchers and artists still use the horizontal neck poses popularised in the late 1990s. The primary basis for horizontal sauropod necks are the famous Dinomorph digital models (Stevens and Parish 1999 and subsequent works) and, though debates on these matters continue, a number of papers have found issues with these models, to the extent that I'm not sure they're reliable at present. Rather than summarise these issues here, I suggest you simply read Mike Taylor and Matt Wedel's blog series on sauropod neck posture over at SV:POW! - all the citations and discussion you need are there, and in much greater detail than I could cram into one paragraph.

This means that the postures used in my mural are still, almost 10 years on, based on the conclusions of Taylor et al. (2009). If you missed this paper (which you need miss no more, seeing as it's open access), it used x-ray data to show that all extant terrestrial amniotes habitually hold their necks with an elevated base during idle but alert behaviour. As summarised by Mike and colleagues in their abstract:
"Unless sauropods behaved differently from all extant amniote groups, they must have habitually held their necks extended and their heads flexed."
In other words, if sauropods didn't carry their necks at an upward angle, they would differ from all terrestrial tetrapods alive today, and there's really no compelling reason to think that was the case. I like this argument because it's based on a broad dataset of real, live animals, not a series of assumptions about how we think they work - when reconstructing animal poses, that's an important distinction. Articulated sauropod fossils show that such poses were attainable, and biomechanical studies suggest that strung-out, horizontal poses would be energetically demanding compared to more vertical poses, and that the necks of sauropods are frankly maladaptive if the neck was not capable of reaching up to gather food (e.g. Taylor et al. 2009; Christian 2010). More work needs to be done here, and it remains difficult to say exactly how sauropods carried their necks for a number of reasons, but data arguing for elevated neck postures seems more compelling than the alternative for the time being. With all this in mind, I am still happy with the neck poses from 2009, and only added some slight curvature to give a sense of motion.

Of course, no-one is saying that sauropods could only carry their necks aloft: we're talking about their default, habitual pose, not those employed during other behaviours like foraging or drinking. Here's artistic proof.
But while my sauropod necks remained mostly unchanged, tweaks were made to other anatomies. I missed papers regarding sauropod foot posture in my original work and gave my Diplodocus elephant-like feet, as if they were walking on the tips of their toes. It turns out that this was wrong: their feet were semi-plantigrade and we need to be restoring all sauropods with longer, flatter feet (Bonnan 2005). With sauropod hands having an unusual horseshoe-shaped profile (Paul 1987), it's long past time to bin elephant hands and feet as a model for sauropod appendages: any artists out there still using elephant legs as a model for sauropod limbs, take note. The overall proportions of the animals were modified too, with more muscle added to the neck base, torso and tail base; the cranial proportions corrected, and the torsos given more bulk. I didn't add too much, though: diplodocids were relatively slender as sauropods go, with deep, but not especially wide bodies. They're a world away from the likes of titanosaurs, which were much heftier throughout the trunk (below)

My PR art for the description of the titanosaur Shingopana songwensis, with another titanosaur - Rukwatitan bisepultus - in the distance. Notice the bulk in their torsos - the chests of diplodocids were a world away from these chunkers.

A very Dippy face-lift

Ideas about the facial anatomy of sauropods have been undergoing something of a quiet revolution in recent years (as explored in blog posts by Matt Wedel and Darren Naish), and good skull material of Diplodocus allows for artists to consider their craniofacial tissues in detail. Many readers will know that the long-held notion of sauropod nostrils being placed at the top of their skulls has been challenged through careful analysis of their bony nasal anatomy (Witmer 2001). It seems that the obvious nasal openings atop sauropod skulls are only the 'internal' apertures of a larger nasal complex which covered most of their snouts. These are especially obvious in some taxa, like Giraffatitan (below), but are also evident in diplodocids. Knowing this, we can move the position of the nostrils to the front of the snout, at the anterior limit of the nasal region. This isn't an arbitrary decision: virtually all reptile nostrils are located at the front of their nasal skeleton, so sauropods would be weird if they didn't do this (Witmer 2001).

Giraffatitan brancai shows us how extensive sauropod nasal skeletons really are - they actually extend right the way down the face (Witmer 2001). Illustration from Witton (in press).
But other than probable nostril placement, we don't know much about the soft-tissues inhabiting these expanded nasal regions. Were they relatively slender, only slightly modifying the shape of the skull contours, or where they expanded, drastically altering the shape of the face? We don't know, but the unusual noses of monitor lizards give one model for artists to follow. As with sauropods, monitor nasal cavities are large, complex basins occupying much of the snout. Within them sit bulbous cartilaginous nasal capsules, and it's these, rather than the bones of the skull, which create the swollen, sometimes 'boxy' appearance of monitor snouts. If the same was true for sauropods, their facial contours might have deviated markedly from the underlying skull. I used this model in my Dippy mural, adding a healthy bulge of tissue to the face over the entire nasal region. It changes the shape of the craniuim quite considerably, contrasting with the horse-like face so familiar to us in other Diplodocus restorations, but still - hopefully - being within the realm of scientifically credibility.

A tiny eye, big nose and Jaggeresque lips. This is not the Diplodocus I grew up with, but all three of these anatomies have a grounding in sound science. 
Regular readers will not be shocked to see covered teeth on my Diplodocus. The conversation about dinosaur lips and other extra-oral tissues is ongoing, but the presence of covered teeth in virtually all tetrapods suggests we should assume this condition for dinosaurs too, unless we have good reason to remove them (I've blogged on this a lot - see this, this and this). Sauropods meet most of our current, provisional criteria for having covered teeth: their snouts have low foramina counts, which seems to superficially correlate with lips in living species (Morhardt 2009); they lack evidence of sculpting typical of tight facial tissue around their their jaw margins (or anywhere else on their skulls, for that matter - sauropod skulls in general seem to lack obvious epidermal correlates), and their teeth are small enough that they would be easily covered by lips. New data on Camarasaurus teeth further supports the assertion of generous oral tissues in sauropods (Wiersma and Sander 2017) and, collectively, these lines of evidence suggest a set of (perhaps lizard-like?) lips around the mouth of Diplodocus is a reasonable inference, without providing any supporting evidence for a perpetual toothy grin.

Diplodocus sp. skull CM 11161 - note the well-preserved sclerotic ring in the orbit. It's quite large, but the internal aperture - which the eye peeps though - is pretty small. From Tschopp et al. (2015).
I was happy to find that we have some good data on eye size in Diplodocus. Many readers will know that sclerotic rings - small bony plates arranged in a ring that line the front of the eyes of many tetrapods - are great indicators of eye size in fossils. The diameter of the ring itself gives a minimum size for the eyeball, and the internal opening approximates the extent of the visible eye tissue. Tschopp et al. (2015) figure a terrific, only slightly distorted sclerotic ring in a Diplodocus skull which suggests a reasonably large eye considering the size of the animal, but the ring plates are quite thick, creating a relatively small internal opening. Thus, while the eyeball was large (perhaps indicating good eyesight?), the visible eye area was not huge. No giant eyes for my Diplodocus in the mural, then, and especially with the additional nasal and oral tissue on the face, they ended up looking quite beady-eyed.

Spines, skin and colour

We don't have any data on the skin for Diplodocus, but skin impressions from other sauropods - including other diplodocids - suggest non-overlapping scales are their most likely covering. I used the extensive skin impressions from the Howe Quarry diplodocid (possibly Kaatedocus?) as my main reference point for the mural: these show not only details of diplodocid scales (polygonal, each about 3 cm across) but also that a line of subconical spines was present along the top of the tail (Czerkas 1992). Some of these were relatively large - up to 18 cm tall - so would be conspicuous even from a distance. These structures were included in the 2009 work and I saw little reason to remove them for the mural, as they remain based on best insight into Diplodocus skin. We don't know how extensive the spine row was in the Howe Quarry animal, so I arbitrarily extended it along almost the entire animal, creating a look consistent the spiny backs of many lizards. The skin was topped of with a number of deep folds: these seem prominent in many living reptiles, but we don't often include them in dinosaur art.

Colour scheme for my Diplodocus. If you're a carnivorous dinosaur, the body says 'all you can eat', but the tail says 'you can't afford it'.
Colour and patterning remains a complete unknown for sauropods, so our only mechanism for restoring colour their colour involves looking for modern analogues and considering their likely pigmentation mechanisms. Very generally speaking, larger tetrapods show less striking patterning and duller colouration than smaller ones, and this trend seems common enough to assume it might have been true in fossil tetrapods too (and yes, I know there are plenty of exceptions, but we're looking for the wood here, not the trees). This may reflect, at least in part, the availability of carotenoids - pigments which create bright colours - in terrestrial settings. Animals cannot create carotenoids directly so must ingest them, and the bigger they are, the more they need to generate large patches of brilliant colouration. We know that many birds struggle to attain their maximum degree of pigmentation because terrestrial habitats offer variable, often limited carotenoid availability. If many of these relatively tiny animals struggle to find enough of pigment to colour themselves, it's hard to imagine the biggest terrestrial animals of all time faring any better. If so, sauropods would be reliant on melanin, which animals can synthesise, but only produces dull shades of grey, red, brown and black, and layers of structural colour on their scales. Reptiles employ structural colour frequently to create vivid colours, but mostly in concert with other pigments - green lizards, for instance, have scales with yellow pigment overlain by blue structural colour.

Pigmentation mechanics is not our only consideration, of course: we must also consider colour function. Colour has important roles in animal homeostasis and behaviour, and we have to give our reconstructions colour schemes which are appropriate to their lifestyle and biology. At such large size we might assume that camouflage was not important for Diplodocus, and we might also infer that too much dark pigment would be detrimental to its heat exchange. Dark pigments attract heat, and given that sauropods almost needed to lose heat more than gain it, darker skin have been disadvantageous in hot climates like those of the ancient Morrison. Putting all this together, I chose a fairly dull mottled pattern of browns, creams and greys, with some lighter ornamental scales and spines to break up the monotony. One area that I did elaborate was the tail: if, as long suspected, Diplodocus employed its whip-like tail defensively, it could have drawn attention to its weaponry with colouration and patterning. Eagle-eyed viewers might also note that the smaller Diplodocus has some more vivid patterning, echoing a common condition of reptiles where juveniles are more brilliantly coloured than their parents. I toyed with adding a strikingly coloured juvenile, but decided not to on grounds that tiny, precocial baby sauropods probably didn't hang out with adults, and because less can be more when it comes to composing paintings. Hopefully, the colour scheme is believable and consistent with our understanding of animal colouration, which - 99.9% of the time - is the best we can hope for in the palaeoart game.

The mural in situ, mere metres from Dippy's tail. This photo was taken at the opening night of the exhibition, hence the funky lighting. The museum is also entirely horizontal, not at a slight angle as shown here, but I'd been at the opening night wine by this point. Note the small panels next to the mural - they explain the science that went into it, effectively being a condensed version of this article.
That covers the majority of the major decisions that went into the mural, so I'll leave our discussion here. Remember that you have until May 9th to see the mural and other artworks in Dorset. They aren't an 'official' part of Dippy on Tour show so, when Dippy leaves Dorset, the mural and other art won't be following - book those tickets now if you want to see them. I'll be talking about palaeoart at the museum on March 14th - book tickets for that here and, if you're reading this and come along, please say hello.

Finally, if you'd like a copy of this mural for yourself, you can grab a good quality print from my online store, where it's available in a range of sizes. Alternatively, you can access a high quality printable file of the mural if you sponsor my work at Patreon - details below.

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!


  • Bonnan, M. F. (2005). Pes anatomy in sauropod dinosaurs: implications for functional morphology, evolution, and phylogeny. In: Tidwell, V. & Carpenter, K (eds) Thunder-Lizards: The Sauropodomorph Dinosaurs. Indiana University Press, Bloomington, 346-380.
  • Brezinski, D. K., & Kollar, A. D. (2008). Geology of the Carnegie Museum dinosaur quarry site of Diplodocus carnegii, Sheep Creek, Wyoming. Annals of Carnegie Museum, 77(2), 243-252.
  • Christian, A. (2010). Some sauropods raised their necks—evidence for high browsing in Euhelopus zdanskyi. Biology Letters, 6(6), 823-825.
  • Czerkas, S. A. (1992). Discovery of dermal spines reveals a new look for sauropod dinosaurs. Geology, 20(12), 1068-1070.
  • Hatcher, J. B. (1901). Diplodocus (Marsh): its osteology, taxonomy, and probable habits, with a restoration of the skeleton (Vol. 1, No. 1-4). Carnegie institute.
  • Morhardt, A. C. (2009). Dinosaur smiles: Do the texture and morphology of the premaxilla, maxilla, and dentary bones of sauropsids provide osteological correlates for inferring extra-oral structures reliably in dinosaurs? Western Illinois University.
  • Paul, G. S. 1987. The science and art of restoring the life appearance of dinosaurs and their relatives - a rigorous how-to guide. In Czerkas, S. J. & Olson, E. C. (eds) Dinosaurs Past and Present Vol. II. Natural History Museum of Los Angeles County/University of Washington Press (Seattle and London), pp. 4-49.
  • Stevens, K. A., & Parrish, J. M. (1999). Neck posture and feeding habits of two Jurassic sauropod dinosaurs. Science, 284(5415), 798-800.
  • Taylor, M. P., Wedel, M. J., & Naish, D. (2009). Head and neck posture in sauropod dinosaurs inferred from extant animals. Acta Palaeontologica Polonica, 54(2), 213-220.
  • Tschopp, E., Mateus, O., & Benson, R. B. (2015). A specimen-level phylogenetic analysis and taxonomic revision of Diplodocidae (Dinosauria, Sauropoda). PeerJ, 3, e857.
  • Turner, C. E., & Peterson, F. (2004). Reconstruction of the Upper Jurassic Morrison Formation extinct ecosystem—a synthesis. Sedimentary Geology, 167(3-4), 309-355.
  • Wiersma, K., & Sander, P. M. (2017). The dentition of a well-preserved specimen of Camarasaurus sp.: implications for function, tooth replacement, soft part reconstruction, and food intake. PalZ, 91(1), 145-161.
  • Witmer, L. M. (2001). Nostril position in dinosaurs and other vertebrates and its significance for nasal function. Science, 293(5531), 850-853.
  • Witton, M. P. In Press. The Palaeoartist's Handbook. Crowood Press.

Friday, 26 January 2018

Did tyrannosaurs smile like crocodiles? A discussion of cranial epidermal correlates in tyrannosaurid dinosaurs

Brain 1: "Right, you need an image for your tyrannosaurid facial tissue post."
Brain 2: "OK, here're some Tyrannosaurus rex in a really dark and back-lit scene. Their faces are in shadow, and you can't really see the features."
Brain 1: "This is perfect. After all, only losers want to see the faces of animals in posts about facial tissues."
Brain 2: "Exactly. Hey, since when did I have two brains?"
Brain 3: "Beats me."
Discussing the craniofacial tissues of tyrannosaurid dinosaurs is the palaeointernet equivalent of lighting a match in a straw-filled barn - the slightest spark of opinion spawns a 100-strong comment field about extra-oral tissues, tooth exposure, rictal tissues, facial skin depth and a number of other topics. But despite this keen popular interest, there's been relatively little academic study into tyrannosaurid facial tissues, perhaps because their soft-tissues mostly remain unrepresented in the fossil record. Happily, close examination of tyrant skulls reveals a number of textures and rugosity profiles which were almost certainly created by bone-skin interaction, so we can form some idea of their life appearance even without soft-tissue specimens. The first detailed attempt at interpreting tyrant cranial rugosities was published last year by tyrannosaur expert Thomas Carr and colleagues (Carr et al. 2017 - you might also know Thomas by his super-comprehensive blog Tyrannosauroidea Central). This widely publicised paper proposed a number of hypotheses about the face of Daspletosaurus horneri: that the sides of the jaw were adorned with crocodile-like 'facial scales'; that various scales, dermal armour and cornified sheaths adorned the nasal and orbital region; and that it lacked lips (not explicitly stated in the paper, but restored as such in an illustration and touted in the paper's PR). The idea that tyrannosaurids may have had crocodylian-like facial tissues has since generated a lot of discussion online, some in favour, some against, and as someone increasingly looking at epidermal correlates for palaeoartistic purposes, I thought this topic was worthy of a blog article: are tyrannosaurid jaws really croc-like enough to assume comparable skin types?

(An important caveat before we start this discussion is that the following is based on tyrannosaurids generally, not D. horneri specifically, because the horneri study does not include photographs of its alleged epidermal correlates. The D. horneri paper describes them very well (see Carr et al. 2017, supplementary data), but it's difficult to evaluate them without images of the bone surfaces themselves. Dave "Tyrannosaur Chronicles" Hone needs a shout out here for sharing his expertise and extensive image library of tyrant fossils as I prepared this post - though I have some experience with tyrannosaur bones and their interpretation, this article has been considerably improved by his involvement.)

Tyrannosaurids and crocodylians: face off

An obvious place to begin this discussion is crocodylian facial structure. Crocodylian skulls are so familiar that it's easy to forget how distinctive they are among modern animals, and I don't think it's widely known that their skin plays a significant role in shaping their skull tissues. Crocodylian jaw bones have incredibly high numbers of foramina, with averages of 100 in each major jaw bone (premaxilla, maxilla and dentary) and over 1000 in each bone in some specimens (Morhardt 2009). These openings are the loci around which gnarly ridges and tubercles grow by a process of dermal ossification: tissues from the skin are turned to bone and build up the sculpting on the skull surface (Grigg and Kirshner 2015; de Buffrénil et al. 2015). Simultaneously, the bone immediately surrounding the foramina is resorbed, enhancing the rugosity pattern further and creating that highly distinctive, deeply pitted and grooved crocodylian skull texture (de Buffrénil et al. 2015). This restructuring can be extensive and, over ontogeny, crocodylian snout surface area can increase by as much as 20% (de Buffrénil et al. 2015). That's a major reworking of the superficial bone of the skull, and their skin has a major role in its development.

Skull of a mature American crocodile, Crocodylus acutus, demonstrating that classic crocodylian skull texture. Cropped from public domain Wikimedia image by Daderot.
Among living tetrapods, only some turtles and a couple of geckos show a comparable degree of sharply-defined cranial sculpting (Evans 2008; de Buffrénil et al. 2015) but, among extinct taxa, stem-tetrapods, temnospondyls, parareptiles and many crocodylomorphs present analogous cranial conditions (Witzmann et al. 2010; de Buffrénil et al. 2015). Studies show that temnospondyl skulls developed their sculpting via a similar mechanism of ossifying dermal tissues (Witzmann et al. 2010), perhaps indicating croc-like skin properties in these animals, too. Until recently it was thought that crocodylian facial skin was scaly, but new research shows that it is actually a sheet of toughened skin which cracks through growth, creating a scaly appearance, but not true epidermal scales akin to those seen in lizards (Milinkovitch et al. 2013). Regardless of whatever other conclusions are drawn here, this has to be a minor amendment to Carr et al.'s (2017) interpretation: if tyrannosaurids (or any other extinct animal) have croc-like textures on their jaw bones, we should be visualising tight, tough skin, not epidermal scales.

Juvenile alligator, Alligator mississippiensis, showing virtually crack-free facial skin - it's only adults that develop the extensively cracked, superficially 'scaly' faces. Photo by Joxerra Aihartza, from Wikimedia, FAL 1.3.
Whether tyrannosaurid jaws are truly crocodylian-like is open to question, however. Carr et al, (2017) are clear that they consider tyrants and crocodylians jaws as identical in superficial appearance ("The texture in crocodylians is identical to that of tyrannosaurids, except that the entire face of crocodylians is coarse in texture" - p. 21; Supplementary information to Carr et al. 2017) but I disagree: there are a number of ways in which they differ and, given the link between crocodylian skull development and dermal tissues, these differences may be critical to our considerations of facial anatomy. Many of these contrasts pertain to jaw foramina, which we know are important in defining crocodylian cranial sculpting (de Buffrénil et al. 2015) and may have a deeper relationship with jaw tissue properties (Morhardt et al. 2009; Hieronymus et al. 2009).

Firstly, although tyrannosaurids have elevated numbers of jaw foramina compared to other dinosaurs, their numbers are, on average, significantly lower than those of crocodylians (Morhardt 2009). No tyrannosaurid jaw bone reported by Morhardt (2009) exceeds 81 foramina, which is high for a dinosaur, but still short of the crocodylian average, and well below the 1000+ figure reported for some croc jaws. Interestingly, data in Morhardt (2009) suggests that foramina numbers weakly correlate to jaw size: the longer a jaw is, the more foramina it generally has. This trend is particularly well shown in her tyrannosaurid sample but seems true of other fossil and extant animal groups as well, and might also be reflected in ontogeny (smaller Tyrannosaurus have fewer foramina, on average, than large ones). The cause behind this trend seems to be elusive at present - might it reflect a change in tissue type with age (Morhardt 2009)? does it reflect demands of supplying an absolutely larger jaw with nervous and vascular tissues? - but whatever the reason, it implies that we should consider foramina frequency proportionate to jaw size when analysing rugosity profiles. Under this metric, foramina values in crocodylian jaws are even more impressive as, compared to some extinct animals, their skulls are of middling size. By contrast, the slightly above-average foramina counts of even the largest tyrannosaurines seem less significant because, even with extreme jaw size, they don't attain a value comparable to a much smaller alligator. If we remove size from our consideration by comparing similarly-sized tyrant and croc jaws, we find they are worlds apart in terms of jaw perforation. Indeed, the foramina values of smaller tyrants are nothing special - they are comparable to most other similarly-sized tetrapods (Morhardt 2009). Presumably, this explains why - as many internet conversations have pointed out - tyrannosaurid jaws simply don't have that same obvious, pitted surface as those of crocodylians.

Further differences might be noted in relative foramina sizes. Those foramina occurring high on tyrant snouts - such as at the top of the maxilla - are much smaller than the broader, obviously deep labial foramina paralleling the jaws (Brochu 2003; Carr et al. 2017). In crocodylians however, jaw foramina seem to have a lower size range. Foramina shape and size is an important consideration for facial tissues (Hieronymus et al. 2009) and this might imply different tyrant facial tissues over the side of the snout vs. those at the jawline, whereas the more uniform foramina sizes of crocodylians are entirely consistent with their homogeneous jaw skin.

Schematic drawing of Tyrannosaurus skull FMNH PR2081 (the specimen better known as 'Sue') showing the distribution and (somewhat conservatively) size differences in jaw foramina. This huge skull is said to be one of the most rugose Tyrannosaurus skulls known (Brochu 2003), but it fails to meet the high foramina numbers, sculpting extent and uniform foramina size of mature crocs. Image from Brochu (2003).
A related issue concerns a possible link between extra-oral tissues and foramina counts. Morhardt (2009) noted that, as a general rule, extant animals with average foramina counts below 50 in each jaw bone have tooth-covering extra-oral tissues; that those above 50 but below 100 have immobile facial tissues; and only those with 100 or more are reliably excluded from having lips or other means of tooth coverage. Average tyrant jaw foramina counts are well below that upper threshold for exposed teeth so, by this metric, they should have lips, and would not look like bipedal crocodiles. This might match what we're seeing with tyrant foramina size: perhaps those large labial foramina are something to do with nourishing and innervating extra-oral tissues, while those on the side of the snout need only access the overlying skin. There are some complications to Morhardt's data (if anyone is looking for a PhD project, a more extensive follow up would be terrific) but, at face value, her research does not support crocodile-like facial tissues for tyrannosaurids.

Finally, we can observe that the ontogeny of tyrannosaurid skull textures is not at all crocodylian-like. Tyrants do have some sculpting on their jaw bones and, as with most reptiles, these become better defined with maturity (e.g. Evans 2008; de Buffrénil et al. 2015). However, even the most rugose tyrannosaurid skulls do not match the complex and sharply pitted rugosity patterns of mature crocodylians (e.g. Osborn 1912; Carr et al. 1999; 2017; Brochu 2003; Hone et al. 2011). Given that ossifying facial skin is a direct factor in jaw bone sculpting in crocodylians, the lack of comparable development in tyrannosaurids is a blow to the idea that their faces bore the same dermal regime. Histological examination of tyrannosaurid jaw bones for might have further insight here, as the resorption/remodelling pattern might reveal details about bone/dermal interactions (Witzmann et al. 2010; de Buffrénil et al. 2015) but, for now, this inconsistency seems to be a big hole in the idea that tyrannosaurids had crocodylian-grade facial tissues.

Does the tyrannosaurid EPB help here?

Collectively, these points seem to suggest that tyrant jaws are not as croc-like as argued, and that it's not a given that the two groups had similar facial tissues. A counterargument to this is that crocodylians are the best tyrant analogues in their extant phylogenetic bracket (EPB), and thus give us our best, most phylogenetically informed insight into tyrannosaurid faces. Indeed, the croc-snouted tyrant hypothesis was informed primarily by comparisons with taxa from the tyrannosaurid EPB - specifically the skulls of birds and alligators (Carr et al. 2017) and, sure, crocs and tyrannosaurid jaws may not be exactly alike, but they're undeniably more similar to each other than either is to a bird. Might we concede that the comparisons aren't perfect, but that this is simply the best we can do without violating the tyrannosaurid EPB?

Our issue is that, while the EPB is a terrific method for predicting ancient anatomies, it really struggles with the complexity of archosaur facial tissue evolution, perhaps to the extent of being redundant. One major issue is that we can be near certain early archosaurs had neither croc- or bird-like facial tissues because no species representing the earliest phases of archosaur evolution have comparable skin-influenced jaw textures (see Nesbitt et al. 2013, and papers therein). Rather, we only see these features developing in relatively crownward archosaur groups, implying independent development of their respective facial anatomies well after the croc-bird split. This being the case, the common archosaur ancestor must have had a different set of facial tissues, and the facial anatomy of extant archosaurs may tell us little about the faces of Mesozoic dinosaurs.

Like crocodylians, birds have jaws with surface textures shaped by their overlying skin: networks of branching neurovascular canals and oblique foramina underlie cornified sheaths (their beaks). The prominence of such jaw rugosities in living archosaurs allows us to predict the facial condition of fossil archosaurs and stress test the tyrannosaur EPB, and it doesn't seem to hold up well. This skull is a marabou stork (Leptoptilos crumenifer), photo by me.
A second major issue is evidence that living archosaur faces don't reflect tissues known from their fossil cousins. In addition to tight facial dermis and cornified sheaths, a plethora of fossil evidence show that fossil archosaurs had faces with epidermal scales, projecting skin tissues (e.g. pterosaur crests) armoured dermis, and cornified pads (Frey et al. 2003; Hieronymus 2009; Hieronymus et al. 2009; Carr et al. 2017). These go well beyond the anatomical range implied by the EPB and show that fossil archosaur faces sometimes had more in common with non-archosaurs than their closest extant relatives. We must remember that the EPB is a predictive method which should be applied where no other data is forthcoming: in this case, we have enough fossil data to show that our EPB predictions are problematic, and that we can't rely on it for insight into tyrannosaurid faces. I'm hardly the first to suggest EPB approaches don't help discussion of non-avian dinosaur faces (e.g. Vickaryous et al. 2001; Knoll 2008), but these points are worth repeating in this context: I don't think the EPB is a compelling supporting argument for a croc-faced tyrannosaurid.

So, if not croc-like, what might be happening here?

If croc-skinned tyrant snouts are problematic, what are our other options? Our discussion above really only pertains to the maxillary region of tyrannosaurid snouts and, for the rest of the skull, I think Carr et al. (2017) nailed it: what I've seen of tyrannosaurid skulls suggests the orbital region and skull roofs were covered in cornified sheaths, armoured dermis and large scales. There seems to be quite a bit of variation in these tissues, with some taxa having more defined scale correlates over the nasals than others, as well as differences in elaborations of the hornlets above the eye. In all likelihood, different tyrant species would be highly recognisable in life by the development of scales, armour and horn across the top of their faces. These armoured tissues are entirely consistent with what we understand of tyrannosaurid behaviour: if you were being routinely bitten about the face by another tyrannosaur, you'd want some protection too (see opening image).

Dorsal view of the snout of a red river hog (Potamochoerus porcus). These pitted, grooved bone textures are fairly widespread across tetrapod skulls and don't seem to correlate to any one skin type, but might indicate the presence of tough, well-cornified skin (these hogs wrestle with their faces, so need protected snouts). Note the projecting rugosities on the side of the snout and on the ascending maxillary projection - these anchor vast skin projections in life. Red river hog skulls are awesome. Photo by me.
But what of that maxillary portion of the snout - the lateral region suggested as being crocodile-like? The surfaces of tyrannosaurid maxillae are pretty complex with a hierarchy of rugosity profiles (Carr et al. 2017). Very obvious features include many pits and short, branching neurovascular grooves: these might not necessarily indicate of particular tissue type in themselves, but are often associated with a well-cornified, tough epidermis (above). The high number of foramina in tyrant maxillae implies immobile facial tissues (Morhardt 2009), which I guess we probably expected in a reptile anyway.

Holotype maxilla of Zhuchengtyrannus magnus: check out that network of elliptical depressions bordered by raised regions. Note how they terminate about a few centimetres above the line of labial foramina - we'll come back to this in a moment. From Hone et al. (2011).
Underlying these pits and grooves are a series of large, elliptical shallow depressions surrounded by low ridges (above). These vertically-aligned structures are found in many tryannosaurids and are especially obvious in large tyrannosaurines like Tyrannosaurus, Tarbosaurus and Zhuchengtyrannus. You can see them easily in museum mounts, even from across the room. Some taxa have single rows of these structures below the antorbital fenestra (Tyrannosaurus), but others have tessellating networks of depressions and ridges that extend to the top of the maxilla (Tarbosaurus), terminating beneath the scaly region overlying the nasal bone. They're unusual structures which are almost certainly epidermal in origin: they're in a place where epidermal correlates often form; are more pronounced in mature individuals; are regularly and consistently arranged across the surface of the skull; and are not associated with any pneumatic or neurovascular openings. They broadly recall the 'hummocky' rugosity profile seen under epidermal scales (Hieronymus et al. 2009) and, if so, the convex, ridged areas probably underlay vertically aligned scales, or rows of scales. Some tyrant skulls, such as the especially rugose Tyrannosaurus skull AMNH 5027, have especially sharp and rugose ridges which, to me, recall the facial ridges of certain iguanine lizards: specifically, anoles, chameleons and basilisks. These are often quite rugose and sculpted, but smoother, more tyrannosaurid-like conditions exist in a number of species (I'm thinking of things like helmeted basilisks and smooth chameleons). Prominent, ornamental rows of relatively large and often colourful scales overly these structures in these iguanines and I wonder if the same was true for tyrants. Alternative hypotheses, such as scales sitting in the depressions between the ridges, aren't consistent with the relationship between scales and bone in living species, and there's no indication that other tissue types (e.g. cornified sheaths, armoured dermis) were present in these areas, so I think the ornamental ridge hypothesis is sensible (or, at least, not outrageously daft given the available data). I must admit to liking this hypothetical juxtaposition of fancy ornamental scales around the mouth and tough, reinforced tissues over the snout: perhaps tyrannosaurs weren't just big biting machines, but liked to look nice, too.

AMNH 5027 is just riddled with interesting surface textures that probably relate to epidermal features. To my reckoning, in addition to those depression/ridge pairings on the maxilla, the dorsal region of the lacrimal bears coarse projecting rugosity (armoured dermis); the top of the premaxilla and postorbital has a series of coarse hummocks (probable scales); and the ascending processes of the postorbital, lacrimal and maxilla are covered in a dense, anastomising network of neurovascular foramina (cornified sheath). What a neat looking animal Tyrannosaurus must've been - my take on this data is seen in the paintings accompanying this post. Image from Osborn (1912), in public domain.
Significantly, I can't find any tyrannosaurid skulls where these possible scale correlates extend right to the base of the maxilla (see photos, above). Rather, they terminate a few centimetres above the line of labial foramina, and this might have bearing on ongoing discussions about dinosaur lips. Scleroglossan lizards (the group that includes geckos, skinks, varanoids and amphisbaenians) frequently have osteoderms on their faces which cover their snouts (including the maxillae) except for a region around the labial foramina, which is smooth. This seems to relate to the presence of lip tissues displacing the scales from the skull and prohibiting formation of a epidermal correlate adjacent to the toothrow. Their maxillary juxtaposition of epidermal correlates is the same configuration that we see in tyrannosaurids as well as a number of other non-avian dinosaurs with maxillary epidermal correlates (e,g, pachycephalosaurids, ankylosaurids, some ceratopsids) and this has to be regarded consistent with hypotheses of extra-oral 'lips' in tyrannosaurids and other dinosaurs. If we add this to the evidence from foramina counts (Morhardt 2009, also see above) as well as other arguments for extra-oral tissues the case for crocodylian-like exposed teeth is looking increasingly doubtful. I must admit to thinking that proponents of exposed dinosaur teeth really need to start making better cases for this idea: most ways we can slice this particular debate suggests that extra-oral tissues are looking likely (and no, the common argument that their teeth were too big to be sheathed isn't valid: it's simply a speculation based on incredulity, not actual data from dinosaur skulls).


To sum up this long, detail-heavy post:
  1. Crocodylian skull textures are basically built by their skin, and we should expect any prehistoric animal with croc-like facial tissues to have a croc-like cranial rugosity profile. What we see in tyrannosaurs is a little croc-like, but only superficially. Differences between croc and tyrant skull tissues may be more significant than their similarities and seem to contradict the notion of croc-like facial tissues in tyrannosaurids.
  2. Attempts to ground discussions of dinosaur facial tissue in the EPB are problematic: a great deal of what we know about archosaur facial tissue refutes what the EPB predicts. Basic comparative anatomy, framed by a wide phylogenetic bracket, might be the way forward for understanding dinosaur faces.
  3. Tyrant faces - as largely predicted by Carr et al. (2017) - seem to have been adorned with scales, cornified sheaths and armoured dermis, but their jaw regions may have been covered in vertical (perhaps ornamental?) bands of epidermal scales, not croc-like skin. Distribution of epidermal correlates around the jaws of tyrannosaurids (and other dinosaurs) is suspiciously reminiscent of many lizard skulls, and may favour a lipped condition.
Tyrannosaurus rex portrait, based on my take of epidermal correlates of the AMNH 5027 skull. No, you tell it that its ornamental ridges look a bit silly.
Perhaps unsurprisingly, I couldn't research and write all this without wanting to draw my take on tyrannosaur facial anatomy. I'll leave you with my take on the face of AMNH 5027 (above): I'm sure it'll need modifications as more details on tyrannosaurid faces come to light, but I won't pretend it wasn't neat to draw a Tyrannosaurus based on relatively objective reading of available data. Palaeoart is at it's most exciting when we join dots between data rather than, as is so often the case, largely imagine huge swathes of our subject species. The duelling Tyrannosaurus that welcomed you to the post are based on the same model.

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!


  • Brochu, C. A. (2003). Osteology of Tyrannosaurus rex: insights from a nearly complete skeleton and high-resolution computed tomographic analysis of the skull. Journal of Vertebrate Paleontology, 22, 1-138.
  • Carr, T. D. (1999). Craniofacial ontogeny in Tyrannosauridae (Dinosauria, Coelurosauria). Journal of vertebrate Paleontology, 19(3), 497-520.
  • Carr, T. D., Varricchio, D. J., Sedlmayr, J. C., Roberts, E. M., & Moore, J. R. (2017). A new tyrannosaur with evidence for anagenesis and crocodile-like facial sensory system. Scientific reports, 7, 44942.
  • De Buffrénil, V., Clarac, F., Fau, M., Martin, S., Martin, B., Pellé, E., & Laurin, M. (2015). Differentiation and growth of bone ornamentation in vertebrates: a comparative histological study among the Crocodylomorpha. Journal of morphology, 276(4), 425-445.
  • Evans, S. E. (2008). The skull of lizards and tuatara. Biology of the Reptilia, 20, 1-347.
  • Grigg, G. (2015). Biology and evolution of crocodylians. Csiro Publishing.
  • Hieronymus, T. L. (2009). Osteological correlates of cephalic skin structures in amniota: Documenting the evolution of display and feeding structures with fossil data. Ohio University.
  • Hieronymus, T. L., Witmer, L. M., Tanke, D. H., & Currie, P. J. (2009). The facial integument of centrosaurine ceratopsids: morphological and histological correlates of novel skin structures. The Anatomical Record, 292(9), 1370-1396.
  • Hone, D. W., Wang, K., Sullivan, C., Zhao, X., Chen, S., Li, D., ... & Xu, X. (2011). A new, large tyrannosaurine theropod from the Upper Cretaceous of China. Cretaceous Research, 32(4), 495-503.
  • Frey, E., Tischlinger, H., Buchy, M. C., & Martill, D. M. (2003). New specimens of Pterosauria (Reptilia) with soft parts with implications for pterosaurian anatomy and locomotion. Geological Society, London, Special Publications, 217(1), 233-266.
  • Knoll, F. (2008). Buccal soft anatomy in Lesothosaurus (Dinosauria: Ornithischia). Neues Jahrbuch für Geologie und Paläontologie-Abhandlungen, 248(3), 355-364.
  • Milinkovitch, M. C., Manukyan, L., Debry, A., Di-Poï, N., Martin, S., Singh, D., ... & Zwicker, M. (2013). Crocodile head scales are not developmental units but emerge from physical cracking. Science, 339(6115), 78-81.
  • Morhardt, A. C. (2009). Dinosaur smiles: Do the texture and morphology of the premaxilla, maxilla, and dentary bones of sauropsids provide osteological correlates for inferring extra-oral structures reliably in dinosaurs?. Western Illinois University.
  • Nesbitt, S. J., Desojo, J. B., & Irmis, R. B. (2013). Anatomy, phylogeny and palaeobiology of early archosaurs and their kin. Geological Society, London, Special Publications, 379(1).
  • Osborn, H. F. (1912). Crania of Tyrannosaurus and Allosaurus; Integument of the iguanodont dinosaur Trachodon. Memoirs of the AMNH; new ser., v. 1, pt. 1-2.
  • Vickaryous, M. K., A. P. Russell, and P. J. Currie. (2001). Cranial ornamentation of ankylosaurs (Ornithischia: Thyreophora): reappraisal of developmental hypotheses. In K. Carpenter (ed). The Armored Dinosaurs. 318–340. Indiana University Press.
  • Witzmann, F., Scholz, H., Mueller, J., & Kardjilov, N. (2010). Sculpture and vascularization of dermal bones, and the implications for the physiology of basal tetrapods. Zoological Journal of the Linnean Society, 160(2), 302-340.

Friday, 17 November 2017

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.

File:Desert Bighorn Sheep (8981484583).jpg
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.


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!


  • 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.