Friday 26 May 2023

Book review: Prehistoric World of Zdeněk Burian, Volume 1: From the Creation of the Earth to the Extinction of the Dinosaurs

Finally: a modern book devoted to everyone's favourite mid-20th Century Czech palaeoartist, Zdeněk Burian. It's big, it's expensive, and it's amazing. For palaeoart fans, it's an essential purchase.

As much as I’m a fan of 19th century palaeoart, the 20th Century is where our efforts to reconstruct fossil organisms really take off and stretch their wings. In its early decades, Charles R. Knight led a charge to reinvigorate and re-legitimise palaeoart after enthusiasm for illustrating the life appearance of fossil organisms cooled in the late 19th century. Later, the likes of Gregory S. Paul and Jay Matternes transformed how palaeoart would be executed and valued by scientists. And between these trailblazers lies the career of another palaeoartistic giant, an individual who took Knight’s mantle as the most important palaeoartist of his day and carried it almost until his death in 1981: Zdeněk Burian.

Born of Czech descent in Austria-Hungary in 1905, Burian began his palaeoartistic career in the mid-1930s. Much has been said about Burian over the years and his status as a master of palaeoart is unquestioned. Whether we see him as tag-teaming with Knight to define the second half of a “classic palaeoart” era (Witton 2018) or as the lead of another phase, termed “modern palaeoart" by Manucci and Romano (2023), Burian defined the look of prehistoric animals for many of us born in the 20th century. Already a well-established illustrator before being headhunted for high-profile palaeoart projects, Burian took on his first prehistoric subjects just as Knight's career began winding down, and his prodigious artistic ability and eye for natural history made the young Burian a great and intuitive palaeoartist from the start. Heavy promotion from his collaborators pushed his work into the same space of international recognition once held by his American forerunner. Influencing or simply outright copied by countless other illustrators, Burian remained a leading palaeoartist until the final days of his career. The torch was only passed during the late 1970s when a new generation, armed with new science and palaeoartistic philosophy, changed the rules of restoring extinct animals and — for better or worse, depending on your perspective — introduced new aesthetics and values into the discipline (Lescaze 2017; Witton 2018; Manucci and Romano 2023; Müller et al. 2023).

There are lots of famous images of sauropods from the mid-20th century, but few are as iconic as Burian's 1941 Brachiosaurus. Yes, this scene is impossible but no, that doesn't affect the quality or influence of this image one bit. Borrowed from Albatros Media.

Given this pedigree, it’s mysterious that Burian’s prehistoric artworks have not been written about as much as those by Knight, Waterhouse Hawkins, or even recent artists like Jay Matternes, at least in the English-speaking world (e.g. Czerkas and Glut 1982; Paul 2000a; Bramwell and Peck 2008; Milner 2012; Carrano and Johnson 2015; Witton and Michel 2022). Take a look at any palaeoart bookshelf and the lack of a decent Burian-themed collection is an obvious and major hole. Until recently, filling this space was only possible by collecting second-hand copies of Burian’s collaborations with Josef Augusta, Zdeněk Spinar and Vratislav Mazák. But with the vintage of these texts reaching or well surpassing 50 years, they are increasingly difficult to find or are prohibitively expensive — rarer Burian texts retail for hundreds of pounds in the UK.

It’s into this void that Albatros Media is injecting their new tome Prehistoric World of Zdeněk Burian Volume 1: From the Creation of the Earth to the Extinction of the Dinosaurs (or, in its original Czech, Pravěký svět Zdeňka Buriana - Kniha 1: Od vzniku Země po zánik dinosaurů). Written and compiled by Ondřej Müller, Bořivoj Záruba, Martin Košťák and Rostislav Walica, it represents the first of an ambitious three-part series compiling the totality of Burian’s palaeontological paintings, illustrations and sketches, both familiar and obscure, and many never published before. Available through the Albatros Media website and also select retailers (see below), this first volume may prove to be the most popular as, along with his Precambrian and Palaeozoic artworks, it contains all of Burian’s pre-Cenozoic dinosaur pieces. These, of course, include many unquestionably iconic pieces of palaeoart: water-bound Brachiosaurus (above), blue-green Archaeopteryx, rearing Tarbosaurus and so on. The book is written in Czech and there are reportedly no plans for a full English translation, but English speakers are catered for with some in-book caption details as well as an introductory essay on the Albatros website derived from two important chapters.

Undoubtedly, splitting Prehistoric World of Zdeněk Burian into three books is the only way to document Burian’s hyper-prolific output. Despite containing just a subset of his prehistoric-themed artwork, this first volume is enormous. Covers measuring 24 x 33 cm sandwich 600 pages of heavy, high-quality paper into a spine almost 6 cm thick. Containing over 400 images, many of which are displayed at large size, the whole package is just a few grams shy of 4 kg. I’ve been tempted to try a patented Tetrapod Zoology Podcast-style table drop to hear the noise of the thud, but I worry about what might happen… to the table. It’s remarkable that another two books, presumably of similar size, will be necessary to document Burian’s palaeontological artwork given that this was not the main focus of his career: he continued to provide illustrations for other books and these, not his extinct animals, form the bulk of his portfolio. I look forward to seeing all three of the palaeo volumes together, even if I am a little concerned about what 12 kg of new books might do to my already creaking bookshelves.

I'm not kidding about that nigh-on 4 kg weight. In future, we'll be able to identify Burian fans by their especially large, well-toned biceps.

Such size carries a cost: €99 for European buyers (excluding VAT and shipping) and $130 in the US. That aims Prehistoric World of Zdeněk Burian Volume 1 squarely at palaeoart diehards and suggests a price tag north of €/$300 for all three volumes. As steep as that might initially seem, it’s actually a bargain compared to the cost of collecting Burian’s art through secondhand book markets. Moreover, with the majority of artworks being rescanned or re-photographed for this project, as well as the inclusion of 135 pieces from private collections, this will be the definitive Burian palaeoart archive for many years to come. Price-conscious buyers may be pleased to know that the illustrations are grouped by geological age rather than by date of production (see below) so you can selectively buy the volume with your favourite time periods or taxa if desired. I suspect there is scope for a coffee table book that compiles the most famous Burian pieces into a more affordable product, but that’s another project, for another time. For now, palaeoart and Burian fans are in for a treat: every penny of that cover cost has gone into making Prehistoric World of Zdeněk Burian Volume 1 an exceptional piece of work.

Given that I can’t read Czech, I can only comment briefly on the text via the online introduction and two translated chapters. These cover the outlay of the book, Burian’s palaeoartistic career and his artistic process. Müller et al. (2023) explain their overarching (but not religiously followed) geochronological and systematic arrangement as the best manner to catalogue Burian’s work because he returned to the same subjects, and sometimes even the same compositions, again and again. Displaying his art in order of production may have given a greater perception of Burian’s developing talent and career, but would probably have been confusing and repetitive to read. Instead, we are presented with everything Burian created for different time periods — the complete Burian Archaen, the complete Burian Cambrian etc. — within which artworks are ordered taxonomically. Only here, at this granular level, does Burian’s professional history affect image order, with his oldest takes on a given topic presented first.

The iconic Burian 1970 Tarbosaurus in Müller et al. (2023, left) compared to its appearance in Z. V. Spinar’s 1972 Life Before Man (right). Even in this wobbly phone photo, the difference in quality is obvious.

This approach makes for an intuitive way to tour Burian’s portfolio and, with a turn of the page, we can compare different eras of his illustrations of the same subjects. We see that his style somewhat tightened over time, the looser qualities of his earliest work turning into more exacting brushstrokes and finer detail by the 1950s. He also revised and repainted many illustrations in light of new science. The latter is an overlooked aspect of Burian’s palaeoartistry because it can, owing to its volume, ubiquitousness and the sometimes subtle alterations he made to older paintings, seem homogenous and largely unchanging when viewed in isolation. But when presented together, we see that Burian’s portfolio is arguably one of the most scientifically transformative of any palaeoartist. In spanning the 1930s to the earliest 1980s, his career captured wholly different, often strongly contrasting palaeontological perspectives. Sometimes viewed as following the “modernist consensus” of dinosaur science (Paul 2000b), Müller et al. (2023) argue that Burian pushed boundaries in dinosaur reconstruction where he could, such as by depicting mobile-wristed and charging ceratopsids as early as the 1940s and 1950s, as well as embracing Bakkerian dinosaur form in the 1970s. I wonder what Burian, who would have been in his 70s when working on these “new look” dinosaurs, thought of producing such radically different takes on subjects after 40 years of creating “traditional”, now old-school reconstructions.

Burian's 1976 Barosaurus, an example of the "giraffoid Barosaurus" meme started by Bob Bakker. We can criticise this painting for being derivative of Bakker's original but, for 1976, this was still cutting-edge stuff: compare this to the Brachiosaurus above, drawn decades earlier. As noted by Müller et al. (2023), Burian's paintings sometimes borrowed elements from other artists because he lacked access to specimens himself, and had to work pretty much exclusively from secondary sources of paleontological information. Image from Albatros Media.

Müller et al. (2023) give some fascinating insights and observations on Burian’s career and artwork, both of scientific and cultural nature. Among the most interesting and, to my knowledge, novel, concerns how Burian’s palaeoart served as propaganda. The prominence of his art in the West makes it easy to forget that Burian lived and worked on the eastern side of the Iron Curtain, and Müller et al. (2023) argue that he ultimately benefitted from socialist rule despite the hard times faced by artists in Czechoslovakia during the mid-20th century. A communist coup saw private publishers nationalised in the late 1940s and this put many illustrators out of work, potentially including Burian. But his palaeoartistic collaborations with Josef Augusta were officially supported and encouraged by the new regime because of their embodiment of Darwinism, a philosophy embraced by Marxist-Leninist outlooks. Somewhat ironically, Burian’s own conviction on evolution was not so firm; his take on life's development was open-minded and pantheistic in nature. Regardless, he profited from the capacity of palaeoart to promote ideals important to a socialist government and his illustrator career was not only rescued from ruin, but destined for opportunities among lauded authorities and individuals. Müller et al. (2023) do not go so far as to compare Burian's career with those of other famous palaeoartists (at least, not the in the texts I can read) but it’s hard not to see his professional life as a political mirror of Knight’s, who found his own fame via support from the independently wealthy, thoroughly capitalist benefactors of American museums.

Müller et al. (2023) further comment on Burian’s approach and attitude to art, and their view is a little more complex and nuanced than that offered recently by Zoe Lescaze (2017). Lescaze posits that a troubled upbringing, a strained relationship with his father, a penniless independence at the age of 15 and the near-constant threat of war and political upheaval contributed to what she perceives as a savage, brutal art style. She sees his paintings as being populated by “sinister dinosaurs”, “thuggish Neanderthals” and other “monsters charged with an almost carnal corporeality”. Müller et al. (2023) agree with this to a point but caution against “foreign theorists” stereotyping Burian by interpreting his work as being the product of a politically troubled state. While also finding melancholy in Burian’s lonely, sad-looking dinosaurs and his fragmented hominid family groups, they marry the tinge of sadness and isolation in Burian’s paintings with the more pragmatic, aspirational and upbeat sides of his character. It's noted that his compositions were often shaped by his forensic attention to scientific detail, his desire to teach through his paintings, and an “uncomplicated and playful” worldview that was captivated by nature. This multifaceted interpretation of Burian’s seems more likely to me than the barely-contained savagery suggested by Lescaze, especially because, for all his dozens of scenes of animal confrontations, physical violence is not a feature characterising his artwork. You can thumb all the way through Prehistoric World of Zdeněk Burian Volume 1 and find virtually no violence or gore.

Burian's moody Monoclonius. If you asked me to place this in palaeoart history I would have suggested the 1970s or 1980s for its somewhat Stoutian qualities. But no, this is from 1948: Burian was taking palaeoart to strange new places well before the modern push for greater artistic styles and approaches. Another swipe from Albatros Media.

Of course, one does not really buy a compilation of Zdeněk Burian palaeoart just for the text: what about the presentation of the artwork itself? On this most critical matter, Prehistoric World of Zdeněk Burian Volume 1 truly excels. It seems that no expense has been spared in documenting everything Burian created on prehistoric subjects, from preparatory sketches and drawings to paintings and illustrations in all sorts of styles and media. All are reproduced at good sizes and in excellent detail. Short of looking at the actual artworks themselves, it’s hard to imagine getting a better view of his creations. Unsurprisingly, the visualisation is far superior to that of older books and yes, you definitely want these versions for their improved colour balances and detail even if you already have a copy of something like Spinar’s Life Before Man or Augusta’s Prehistoric Animals on your shelf (see direct comparison, above). Look closely and you’ll note that even the surface texture of Burian’s oil pieces is evident. The printed page can’t replicate the actual three-dimensional quality of his paintings with their thick paint ridges or their picked and threadbare, canvas-exposed regions, but you certainly get a sense of their presence. Special mention should be made of the multitude of plans, notes, roughouts and sketches that feature alongside finished pieces. We rarely or only tokenistically document these peripheral but enlightening pieces in our palaeoart literature, so their inclusion is very welcome here.

My only slight gripe with the images concerns layout, as many are stretched across two pages in a way that loses details in the page binding. This is a concern for any book but, in this 6 cm deep monster, the page gutter is a deep chasm, especially if the book is not laid flat on a table. But to mention this is nit-picking of the highest order and I’m ultimately moaning about a design no-win scenario: if seam issues were avoided by consigning images to one page only, I’d probably be asking for bigger pictures.

Preview pages of Prehistoric World of Zdeněk Burian Volume 1 compiled from Albatros Media's website. Note the diversity of images: ink drawings, sketches, annotated illustrations and full paintings.

Indeed, there’s only one real issue with Prehistoric World of Zdeněk Burian Volume 1: it’s too good. In this first book alone, Müller et al. (2023) have set the standard for documenting our palaeoart masters so high that other efforts look underwhelming by comparison. If volumes 2 and 3 follow suit, the Prehistoric World of Zdeněk Burian series will be the gold standard for documenting any palaeoartist, and the fact Ondřej Müller, Bořivoj Záruba, Martin Košťák and Rostislav Walica have achieved this for someone as prolific as Burian is nothing short of a triumph. The amount of care and investment that’s gone into this project is extraordinary and it will take an especially dedicated team to create a palaeoart compilation for another artist that can stand shoulder-to-shoulder with it. And that, for the record, is the new goal: I’m not sure any collection will ever surpass it. Yes, it really is that good.

Prehistoric World of Zdeněk Burian Volume 1: From the Creation of the Earth to the Extinction of the Dinosaurs (Pravěký svět Zdeňka Buriana - Kniha 1: Od vzniku Země po zánik dinosaurů) is out now. It is internationally available from Albatros Media as well as select retailers and distributors, including Donald M. Grant in the US and NHBS in the UK. European buyers can also email for more information.

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  • Bramwell, V., & Peck, R. M. (2008). All in the Bones: a biography of Benjamin Waterhouse Hawkins. Academy of Natural Sciences
  • Carrano, M. T. & Johnson, K. R. (2015). Visions of Lost Worlds: The Paleo Art of Jay Matternes. Smithsonian Books.
  • Czerkas, S. M., & Glut, D. F. (1982). Dinosaurs, mammoths, and cavemen: the art of Charles R. Knight. Dutton Adult.
  • Lescaze, Z. (2017). Paleoart: Visions of the prehistoric past.
  • Manucci, F., & Romano, M. (2023). Reviewing the iconography and the central role of ‘paleoart’: four centuries of geo-palaeontological art. Historical Biology, 35(1), 1-48.
  • Milner, R. (2012). Charles R. Knight: the artist who saw through time. Abrams.
  • Müller, O, Záruba, B, Košťák, M & Walica, R. Rostislav. (2023). Pravěký svět Zdeňka Buriana - Kniha 1: Od vzniku Země po zánik dinosaurů. Albatros Media.
  • Paul, G. S. (2000a). The art of Charles R. Knight. In Paul, G. S. (ed). The Scientific American Book of Dinosaurs. Macmillan. 113-118 pp.
  • Paul, G. S. (2000b). A quick history of dinosaur art. In Paul, G. S. (ed). The Scientific American Book of Dinosaurs. Macmillan. 107-112 pp.
  • Witton, M. P. (2018). The Palaeoartist’s Handbook: Reconstructing Extinct Animals in Art. Crowood Press.
  • Witton, M. P. & Michel, E. (2022). The art and science of the Crystal Palace Dinosaurs. Crowood Press.

Thursday 30 March 2023

New paper: Fresh evidence and novel analyses strongly suggest that theropod dinosaurs were lipped

A juvenile Edmontosaurus disappears into the enormous, lipped and gummy mouth of Tyrannosaurus. Those of us in the palaeoart community are used to seeing lips on dinosaurs now, but neither the lipped or lipless hypothesis has been given a thorough seeing-to in peer-reviewed literature yet. Until, that is, today. This is the PR art for our new paper, Cullen et al. 2023, that dives into the question of lips for theropod dinosaurs.
If you follow developments in palaeoart to any level of detail, you can’t have missed “the Lips Debate”: the controversy surrounding the application of extra-oral tissues (essentially various kinds of lips and cheeks) to extinct animals. This discussion has touched on virtually all fossil vertebrates at some time or another, but the presence or absence of lips on dinosaurs has, predictably, been the major focus for most palaeoartists. Owing to their general popularity, the lipped-or-not status of the predatory theropod dinosaurs has drawn a particularly large amount of attention. This debate is now so well covered online that its basic tenets will be familiar to many: the question of whether extant dinosaur relatives offer misleading insights for facial reconstruction; the importance of tooth size and angle to dental “sheathability”; the similarities and differences of jaw bone morphology between crocodylians, lizards and theropods, and so on.

But for all of this online visibility, the question of dinosaur lips and cheeks has received only a little attention from dinosaur researchers. A number of conference abstracts have been presented in this area, but only a handful of these studies have been pushed through peer review to become fully published scientific papers (e.g. Galton 1973; Ford 1997; Knoll 2008; Keillor 2013; Nabavizadeh 2020). It’s largely been dinosaur artists, mostly writing for blogs (such as this, this and this), social media posts or specialist books and magazines (e.g. Witton 2018; Paul 2019), that have provided the bulk of recent conversation on this issue. Whatever merits these discussions have (and there are some very fine, commendable assessments out there), the lack of detailed, authoritative scientific studies has allowed trends in dinosaur facial reconstruction to be shaped by popular culture, palaeoart memes and the opinions of influential palaeoartists more than conventional science. This means that, however comfortable we are with our opinions on dinosaur mouth appearance, this question would benefit from more study, more data and more insight from experts in reptile facial anatomy.

A visual review of where we are with restoring theropod mouths, from Cullen et al. 2023. Do you prefer your Tyrannosaurus without lips (B and C), or with lips (D and E)? And far more importantly, which of these is better supported by fossil data?

To that end, today is a good day. I’m part of an international team of researchers publishing a major new paper in Science dedicated to the question of whether theropod dinosaurs possessed lips. Led by Thomas M. Cullen and receiving contributions from Derek W. Larson, Diane Scott, Tea Maho, Kirstin S. Brink, David C. Evans and Robert Reisz, this is the end product of a long-running investigation into theropod faces that was first initiated by Robert 11 years ago. I was invited onto the project at the end of 2020 to create some artwork of lipped and lipless Tyrannosaurus (above) and, once onboard, I’m pleased to say I was able to help contextualise and interpret our data alongside providing some pretty pictures. But the real hard graft of the research was performed by others on the authorship team, so they deserve full credit for the nitty-gritty science and methodological concepts. They also taught me a lot about reptile jaws, teeth and oral soft tissues, so I’m in their debt for this experience.

As you’ll have guessed from the PR artwork that greeted you above, we conclude that yes, theropods almost certainly had lips. As evidence of this, we present multiple lines of evidence that all point to lizard-like scaly tissues covering predatory dinosaur teeth, and perhaps even other types of lizard-like oral tissues as well. Our work can be divided into four independent investigations that collectively support our assertions.

Jaw and tooth form

Firstly, we make some broad-brush comparisons between the tooth orientation and jaw bone morphology of lizards, crocodylians and theropods, some of which will be familiar to those who’ve followed the Lips Debate so far. We note that theropods, early croc-line archosaurs and lizards are similar in having low numbers of jaw bone foramina* distributed along their oral margins, as well as vertically-aligned teeth. Extant crocodylians, in contrast, have splaying teeth and hundreds of evenly-distributed foramina across their skull bones, the functions of which are more specialised than the lip-nourishing jaw openings of lizards.

*If you're new to all this, foramina are small holes in bones that typically house nerve tissues or blood vessels, but can also record other structures, like outgrowths of air sacs.

Reptile jawbone surface textures and foramina distribution compared, from Cullen et al. (2023). I think this image speaks for itself: the arrangement of theropod jaw foramina (those holes along the jawline) is far more lizard-like than croc-like.

Some of these observations are not novel as the significance of jaw bone foramina to the theropod lip question has long been recognised (e.g. Bakker 1986; Morhardt 2009; Keilor 2013; Barker et al. 2017; Carr et al. 2017). Our resurrection of this point is, in part, a response to Thomas Carr and colleagues' 2017 paper on Daspletosaurus horneri, which favourably compared tyrannosaur jaw surfaces to those of crocodylians. We don’t think they’re actually much alike at all, especially in foramina distribution, so disagree with that assessment. Carr et al. (2017) also assumed that the thick, immobile facial anatomy of living archosaurs — crocs and birds — was ancestral to their entire group, including non-bird theropods. We question this too. Like theropods, the jaw surface properties and tooth orientations of early croc-line archosaurs recall those of lizards more than modern crocodylians, probably reflecting a different soft-tissue configuration. We thus agree with the increasingly evidenced view that the faces of living archosaurs are specialisations suited to very particular lifestyles and that the dinosaur extant phylogenetic bracket is of limited use for inferring their facial anatomy.

Enamel, hydration, and tooth wear

Secondly, we discuss the damage and wear inflicted on permanently exposed teeth, a conversation that is mostly about enamel hydration. Enamel is one of the hardest tissues that animals can synthesise and is thus highly resistant to damage, but its resilience is dependent on moisture. Hydrated enamel is more plastic, and thus more resistant to abrasion, than dehydrated enamel, which is brittle and prone to cracking and breaking under strain. To that end, teeth emerging from oral margins tend to be more damaged and worn than those kept within a moist, sealed mouth. This difference can be seen with the naked eye but is particularly obvious under microscopic examination. In our paper, we show that the tips of alligator teeth are shorn off on their outward-facing, exposed surfaces, with both the enamel and several layers of dentine worn to a flattened edge. Tooth dehydration almost certainly factors into crocodylians frequently suffering from broken and cracked dentition, and they have to replace their teeth regularly (something like 45-50 times in a lifetime — Grigg and Kirchner 2015) to maintain a set of fully functional jaws.

These observations give us a clear hypothesis regarding theropod oral tissues. If they were permanently exposed, theropod teeth should show, at minimum, similarly obliterated enamel and dentine layers at their tips. The poster children of exposed dinosaur teeth, adult tyrannosaurids, are especially relevant here as they replaced their teeth at a very slow, sometimes even biennial rate (Erickson 1996). What’s more, they engaged in particularly violent, tooth-on-bone feeding strategies. So, if any theropods are going to have knackered, abraded teeth, it’s tyrannosaurids.

Detailed comparisons of tyrannosaurid (upper row) and crocodylian (lower row) tooth wear. Note how the Daspletosaurus tooth, despite being over 500 days old, is intact despite dinosaurs teeth having particularly thin enamel layers. The alligator tooth tip, by contrast, has not only lost the enamel coating on its outer surface, but also several layers of underlying dentine. A fully intact, enamel-covered erupting alligator tooth is shown in panel H to show that these are features of wear, not the original tooth condition. From Cullen et al. (2023).

But extracting a fully grown, c. 510-day-old tooth from a Daspletosaurus maxilla revealed a tooth tip in great condition. Both the inner and outer surface retained their relatively thin enamel covering and the only minor damage found was on its medial, inward-facing surface, possibly as a result of occasional tooth-on-tooth contact. This confirms what has generally been observed for theropod dental wear in other studies and conflicts with what we'd expect from a dehydrated, perpetually exposed tooth. We take this as evidence of theropod dentition being maintained in a moist, well-hydrated setting, and being located within a lipped mouth is realistically the only way this might be achieved.

Too big to sheath?

We also address the possibility that some theropods had teeth that were simply too big to cover with lips (e.g. Ford 1997). Our focus here is not on the widely known, but still surprisingly prevalent issue of artists and researchers not accounting for tooth slippage in fossil specimens**. Rather, we focus on realistic estimates of tooth crown height when they are fully socketed in theropod mouths. We calculated a ratio of tooth crown height to skull length for 37 theropod specimens and compared them with the same metric in 40 varanids, the monitor lizards. Varanids are, of course, well-known for possessing large, theropod-like teeth, as well as copious amounts of lip and gum tissue.

**This is the decay of tooth-anchoring ligaments resulting in teeth sliding somewhat from their sockets, preserving them at longer lengths than they held in life.

Tooth size: theropods vs varanids. It turns out that both groups have similarly-sized teeth relative to skull length and, while this doesn't directly tell us if theropods had lips, it shows that their dentition was of a size that we know can be sheathed by extra-oral tissues today. From Cullen et al. (2023).

Plotting these data showed that theropod and monitor teeth are about the same size for their skull lengths and even increase in proportion at the same approximate rate. But the winner of the biggest tooth contest wasn’t something like T. rex: it was the varanids. Some monitor species, like the crocodile monitor Varanus salvadorii, have almost cartoonishly-large dentition. From this, we suggest that theropods did not need unprecedentedly big lips to cover their mouths and the largest theropods wouldn't look, in terms of lip proportions, very different to something like a komodo dragon. Indeed, we note that monitors are able to cover their teeth with the same basic configuration of labial and gingival tissues across a 12-fold size difference. The discrepancy between the largest monitor skull and our largest theropods is only half that: 6-fold. So if lizard lips and gums can scale 12 times over without substantial anatomical deviation, perhaps they could stretch to cover the teeth of much bigger animals without much change, too? Whether we've realised it or not, a lot of us already evidently believe this is possible, given the abundance of lippy, monitor-esque mosasaur reconstructions.

Sealing the deal

These are the points we cover in the main paper but, this being an article in Science, it’s essential to also check out the supplementary files for additional discussion and context. Therein we raise another point that resulted from our efforts at reconstructing a scientifically-informed illustration of a lipless Tyrannosaurus: it’s really, really hard, maybe even impossible, to seal at least some theropod mouths without lips. Forming an oral seal, even if it’s just by pressing lipless jaws against one another, is important to avoiding dehydration as well as maintaining basic oral health and hygiene. We could not, however, find a way to reconstruct T. rex jaws without leaving a gap behind their maxillary teeth. I actually pushed our lipless reconstruction (Fig. 1B in the paper) a little beyond what I think is reasonable and we're still left with a small opening.

We are not the first people to ponder this issue, and dinosaur literature has contrasting views on how far theropods could close their jaws. Some authors propose that theropod mandibles could be pulled way up into the cavity of the upper jaw and have even identified landmarks for the resting position of the lower teeth (below). These include depressions in the walls and roof of the upper oral chamber that seem suited to act as socket-like structures for receipt of the lower dentition (e.g. Molnar 1991; Ford 1997; Currie 2003; Hendrickx et al. 2014). Others, most notably Tyler Keillor (2013) in his excellent book chapter on restoring the face of the “Jane” Tyrannosaurus, have questioned this idea on grounds that theropod mandibles can’t close so tightly without literally bashing into problems.

Examples of landmarks suggested to record the resting poses of theropod lower jaws. Maxillary wall sockets are depressions in the internal wall of theropod oral cavities, and some theropods are also preserved with round depressions in the roofs of their mouths. Neither are universal features of all theropods, however. Images from Osborn (1912), Lü et al. (2014) and Cullen et al. (2023).

Reconstructing the face of T. rex for our paper saw us agreeing with Tyler's conclusions. At a certain point of mouth closure, theropod lower jaws collide with bones under the eye socket (specifically, the ectopterygoid) so that further adduction either requires the jaws to literally crush themselves shut, or else the bones of the posterior skull act as a hinge, swinging the jaw tip into the mouth but dislocating the jaw joint. Ford (1997) proposed that a notch in the ectopterygoid accommodated the closed lower jaw during mouth closure but we don’t think this is plausible. Theropod ectopterygoids can be complex shapes and yes, some have regions that superficially look like they could nestle the lower jaw, but these were almost certainly filled by deep jaw muscles in life (e.g. Gignac and Erickson 2017). There are, of course, theropod skulls preserved with their jaws tight shut in the fossil record but we have to be careful assuming these represent in vivo conditions, given how routine processes of decay and fossilisation can pull and crush carcasses into unnatural configurations.

Inspired by this, we devote some discussion to how theropods posed their closed jaws in life. Beyond ruling out impossible, jaw-busting configurations, it's difficult to know exactly how tightly theropods held their mouths, but this is something for artists to consider. X-rays and scans of lizard carcasses show that their jaws are far from clenched shut when their mouths are closed and, in some species, their upper and lower dentition barely overlaps. If we go "full lizard" with our theropod reconstructions, where we apply minimal overlap of the upper and lower toothrows, their lips would have been deep and their snouts much taller than we’re used to. We play about a little with this visually in the paper and I was struck at the blocky, chunky cranial profile of our lizard-like, loose-mouthed T. rex (below).

Some of our experimentation with theropod mouth postures. The "crush closed" pose is almost certainly impossible, but it's hard to say how relaxed theropod resting gapes may have been held. If modern lipped reptiles are anything to go by, they may have been held far more "open" than we're used to. Modified from Cullen et al. (2023).

Conclusion: theropod jaws only make sense if they had lips

Putting all this together, our investigations of jaw structure, tooth size, tooth wear and jaw closing all point to the same inference: theropod jaws don’t make much anatomical or functional sense without lips of some kind. To validate the alternative lipless model, we have to engage in a lot of special pleading and scientific weaselling. Exposed theropod teeth would have to be unprecedentedly resistant to wear; all our understanding of jaw structure and foramina distribution correlating with oral soft tissues would have to be wrong, and theropods would need to be unique in not bothering to create oral seals. If we're being good scientists, we can’t currently say that theropods definitely had lips, drop the microphone and walk off stage, but I think we've made it far more challenging for anyone to legitimately object to the lipped theropod hypothesis. Time will tell on that front.

In addition to substantiating the lipped hypothesis of theropod appearance, our hope is that our paper may establish some lines of inquiry for the oral tissues of other extinct animals. Many of the most extreme dentitions to ever evolve belong to fossil taxa, after all, and theropods are far from the only species with uncertain facial appearances. What of nimravids, gorgonopsians, uintatheres, or Thylacosmilus? And what, for that matter, of the superficially crocodile-like spinosaurids and other weird theropods — were they lipped or not? If our ideas hold water, they provide a relatively straightforward way of deducing whether the teeth of these animals were held within oral tissues.

We can't, of course, finish without some brief notes on the life appearance of lipped theropods. We address this a little in our supplementary information and conclude that lepidosaurs, the lizards and tuataras, are the best modern analogue for theropod lips and gums. This is, admittedly, a “best of a bad situation” recommendation because there are plenty of differences between theropod and lepidosaur jaws that preclude total confidence in their comparison, but we only have so many extant reptile groups to choose from and lepidosaurs are, on the whole, morphologically closer to theropods in areas we think are influential on labial and gingival tissues.

What did lipped theropods actually look like? Lizard mouths have a lot more soft-tissue surrounding their teeth (jaw cross sections show a komodo dragon, B, and alligator, C), and this might be something we need to bring into our theropod artwork. The gaping T. rex shown here is outfitted not only with large lips, but also a conservative 25% of its tooth height covered with gums. From Cullen et al. (2023, supplementary data).

This being the case, our model for theropod mouths is that they were sealed by non-muscular*** lips covered with scales or — to hedge our bets a little more — whatever epidermal covering was present on the side of the snout. Lepidosaurs show variation in lip size, with most having generous upper lips but some having thinner lower lips than others. This variation continues to their gums. Lepdiosaur gingivae are more voluminous than those of mammals and crocodylians and generally cover at least 20-25% or so of tooth crown height. This is why lizard teeth aren’t always that conspicuous in their open mouths. Varanids take these enlarged gingivae to an extreme, hiding almost all of their formidable teeth with enormous gums. We currently don’t have much insight into where theropods sat within this range. Paul (2019), independently of our study, advocates for full monitor-like conditions for theropods, and this might be possible, but we can't rule out smaller gums or, indeed, a unique theropodan take on oral soft-tissues at this time. We propose, however, that since extant lipped reptiles have at least 25% of their tooth crown heights covered with gums, we should apply that to theropods, too. We’ve explored this in our paper and PR art with that gummy, lippy T. rex shown above.

***It’s not strictly true that lizards have no muscles around their mouths. Some agamids have muscles that move their lips or flaps of mouth-adjacent skin for communicative purposes. I'm no expert on these structures, but I think it’s fair to assume that they are specialisations of their respective lineages, not the remnant of a once ubiquitous, clade-wide lepidosaur ability.

And I think we’ll leave it there for now. There’s more to say on the tyrannosaur restorations we created for the paper with their small eyes and some of their skin details, but at least some of that discussion needs to wait for developments in other research I’m involved in (I'm so, so sorry, Dave). For now, I’ll thank my coauthors once again for inviting me onto such a great project, and I’ll leave you with this handy infographic summary of our research, which you can share around the internet to instigate discussion of dinosaur life appearance wherever you may be. The QR code in the corner will take you directly to the paper so, if you or anyone you encounter wants to know exactly what we have to say about theropod lips, you can always find it from this image.

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  • Barker, C. T., Naish, D., Newham, E., Katsamenis, O. L., & Dyke, G. (2017). Complex neuroanatomy in the rostrum of the Isle of Wight theropod Neovenator salerii. Scientific Reports, 7(1), 3749.
  • 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(1), 1-11.
  • Cullen, T. M., Larson, D. W., Witton, M. P., Scott, D. Maho, T. Brink, K. S., Evans, D. C. and Reisz, R. (2023). Theropod dinosaur facial reconstruction and the importance of soft tissues in paleobiology. Science, 379, 1348-1352.
  • Currie, P. J. (2003). Cranial anatomy of tyrannosaurid dinosaurs from the Late Cretaceous of Alberta, Canada.
  • Ford, T. L., 1997, Did Theropods have Lizard Lips? Southwest Paleontological Symposium – Proceedings, 1997, 65-78.
  • Galton, P. M. (1973). The cheeks of ornithischian dinosaurs. Lethaia, 6(1), 67-89.
  • Gignac, P. M., & Erickson, G. M. (2017). The biomechanics behind extreme osteophagy in Tyrannosaurus rex. Scientific Reports, 7(1), 2012.
  • Grigg, G., & Kirshner, D. (2015). Biology and Evolution of Crocodylians. Csiro Publishing.
  • Hendrickx, C., & Mateus, O. (2014). Torvosaurus gurneyi n. sp., the largest terrestrial predator from Europe, and a proposed terminology of the maxilla anatomy in nonavian theropods. PloS one, 9(3), e88905.
  • Keillor, T. M. (2013). Jane, in the flesh. In: J. M. Parrish, R. E. Molnar, P. J. Currie, E. B. Koppelhus, (eds.). Tyrannosaurid Paleobiology. Indiana University Press.
  • Knoll, F. (2008). Buccal soft anatomy in Lesothosaurus (Dinosauria: Ornithischia). Neues Jahrbuch fur Geologie und Palaontologie-Abhandlungen, 248(3), 355-364.
  • Lü, J., Yi, L., Brusatte, S. L., Yang, L., Li, H., & Chen, L. (2014). A new clade of Asian Late Cretaceous long-snouted tyrannosaurids. Nature communications, 5(1), 3788.
  • Molnar, R. E. (1991). The cranial morpholgy of Tyrannosaurus rex. Palaeontographica. Abteilung A, Paläozoologie, Stratigraphie, 217, 137-176.
  • 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.
  • Nabavizadeh, A. (2020). New reconstruction of cranial musculature in ornithischian dinosaurs: implications for feeding mechanisms and buccal anatomy. The Anatomical Record, 303(2), 347-362.
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  • Witton, M. P. (2018). The Palaeoartists' Handbook. Crowood Press.

Tuesday 28 February 2023

Horned dinosaurs vs. theropods: how much did horns matter?

The hero of the hour, Triceratops horridus. But how often were those long horns stuck into predatory dinosaurs in defensive action? I feel a long discussion coming on...

A persistent idea around dinosaur biology is that the ceratopsids, or horned dinosaurs, were among the most formidable prey for predatory theropods. Though lacking armour, the large cranial horns and frills of these dinosaurs have been widely interpreted as having anti-predator potential, functioning like a knight’s lance and shield in their capacity to stab and parry attacking carnivores. Such notions are well over a century old and have taken on a life of their own in palaeontological media, especially thanks to widespread romanticising of the relationship between Triceratops and Tyrannosaurus, genera that we’ve decided represent the ultimate expression of dinosaurian predator and prey. Henry Fairfield Osborn wrote on such matters as early as 1917:

“The first of these [dinosaurs with anti-predator anatomy] are the aggressively and defensively horned Ceratopsia, in which two or three front horns evolved step by step, with a great bony frill protecting the neck. This evolution took place stage by stage with the evolution of the predatory mechanism of the carnivorous dinosaurs, so that the climax of ceratopsian defense (Triceratops) was reached simultaneously with the climax of Tyrannosaurus offense. This is an example of the counteracting evolution of offensive and defensive adaptations, analogous to that which we observe today in the evolution of the lions, tigers, and leopards, which counteracts with that of the horned cattle and antelopes of Africa, and again in the evolution of the wolves simultaneously with the horned bison and deer in the northern hemisphere.”

Osborn 1917, p. 224-225.

At this early stage in dinosaur research, the likes of Triceratops and Tyrannosaurus weren't viewed as laudable champions of evolution, but as animals so stupid and instinct-driven that their predatory and anti-predatory strategies had to be as simple and idiot-proof as possible. Tempting as it is to reduce this passage from William Matthew’s 1915 American Museum of Natural History book Dinosaurs to some choice soundbites, it’s such an amazing window into old-school concepts of dinosaurian stupidity that I present it here in its full glory.

“[Tyrannosaurus] probably reached the maximum of size and of development of teeth and claws of which its type of animal mechanism was capable. Its bulk precluded quickness and agility. It must have been designed to attack and prey upon the ponderous and slow moving Horned and Armored Dinosaurs with which its remains are found, and whose massive cuirass and weapons of defense are well matched with its teeth and claws. The momentum of its huge body involved a seemingly slow and lumbering action, an inertia of its movements, difficult to start and difficult to shift or to stop. Such movements are widely different from the agile swiftness which we naturally associate with a beast of prey. But an animal which exceeds an average elephant in bulk, no matter what its habits, is compelled by the laws of mechanics to the ponderous movements appropriate to its gigantic size. These movements, directed and controlled by a reptilian brain, must needs be largely automatic and instinctive. We cannot doubt indeed that the Carnivorous Dinosaurs developed, along with their elaborately perfected mechanism for attack, an equally elaborate series of instincts guiding their action to effective purpose; and a complex series of automatic responses to the stimulus afforded by the sight and action of their prey might very well mimic intelligent pursuit and attack, always with certain limits set by the inflexible character of such automatic adjustments. But no animal as large as Tyrannosaurus could leap or spring upon another, and its slow stride quickening into a swift resistless rush, might well end in unavoidable impalement upon the great horns of Triceratops, futile weapons against a small and active enemy, but designed no doubt to meet just such attacks as these. A true picture of these combats of titans of the ancient world we cannot draw; perhaps we will never be able to reconstruct it. But the above considerations may serve to show how widely it would differ from the pictures based upon any modern analogies.”

Matthew 1915, p. 52-53.

The image that launched a thousand Cretaceous daydreams... but not the version you know. This is the rarely-seen, 96 cm wide (presumably preparatory) version of Charles Knight's classic 1928 Triceratops vs. Tyrannosaurus mural, held today at Princeton University Art Museum. Knight's near-blindness meant that he could only execute the mural at scale; other artists then painted the better-known, full-size version. I don't need to explain why this image is included in this post.

Over the last century, our views on dinosaur physiology and intelligence have (perhaps thankfully) changed, but the concept of horned dinosaurs protecting themselves with their facial ornaments has not. Some authors (e.g. Colbert 1948) have seen anti-predator functions as the primary role of ceratopsid horns, and even those who view these structures as evolving under different selection regimes (e.g. Hone et al. 2011) assume some predator defence was possible. Seminal figures like Robert Bakker have worked sweeping hypotheses from concepts of long-standing predator-prey interactions between dinosaur species, rephrasing Osborn’s “counteracting evolution of offensive and defensive adaptations” into the catchier “Mesozoic arms race” (Bakker 1986). Of course, Triceratops and Tyrannosaurus are considered the final, ultimate example of this era-spanning feud. On their relationship, Bakker wrote:

“No matchup between predator and prey has ever been more dramatic. It’s somehow fitting that these two massive antagonists lived out their co-evolutionary belligerence through the very last days of the very last epoch in the Age of Dinosaurs.”

Bakker 1986, p. 240.

Gregory S. Paul, also a fan of the idea that Triceratops was the apex challenger to theropod aggressors (Paul 1988), has further worked the concept of ceratopsid predatory combat into other hypotheses. Specifically, in 2008 he proposed that such dangerous prey items were a factor in the short (c. 30 year) lifespans of tyrannosaurids, while also echoing Bakker’s concept of an “arms race” between these clades (“the upgrading the weaponry in tyrannosaurids and ceratopsids… may represent a Red Queen arms race”; Paul 2008, p. 344.).

Today, a large body of evidence has challenged the idea that ceratopsid evolution was driven by developments in predatory dinosaurs. Instead, it points to ceratopsid skulls being primarily shaped by their own intraspecific behaviour. First proposed in the 1970s, this concept arose after researchers noted the many similarities between the sexually-selected horns of living animals and the ornaments of horned dinosaurs. These shared features include their exaggerated and often complex shapes, their functional peculiarity (i.e. that many seem maladapted for other activities, including predator defence), their positive allometry (that they grow faster than the rest of the skull), their high amount of intraspecific variation, and their high morphological diversity between species (e.g. Farlow and Dodson 1975; Spassov 1979; Sampson et al. 1997; Horner and Goodwin 2006, 2008; Hone et al. 2011, 2016; Knell et al. 2012). Evidence that horned dinosaurs injured each other in ways consistent with modelled ritualised combat (e.g. Farke 2004; Farke et al. 2009; D’Anastasio et al. 2022) supports the hypothesis that their horns were employed against one another, not necessarily other dinosaurs, and, along with their fossilisation in huge monospecific bonebeds, we can readily rationalise horned dinosaurs as boisterous animals with somewhat bovid-like behaviours.

Centrosaurus aperatus, a ceratopsid that's so familiar nowadays as to seem unremarkable. But look at that face anew, dear reader: what a crazy animal. Skulls like those of ceratopsids are ripe contenders for dinosaur anatomy shaped by sexual, or at least intraspecific, selection pressures.

The concept of ceratopsid horns serving primarily as anti-predator devices idea has not, however, entirely been set aside despite these data. Nowhere is this more obvious than in popular culture, where horned dinosaurs frequently employ their ornament in life-or-death struggles against predators, and we routinely discuss “armed” dinosaurs as being more dangerous prey than their "unarmed" relatives. But are these action-packed scenarios really a defensible alternative to their horns being used in intraspecific display and aggression? How realistic, really, are these predatory scenarios? Rather than taking the tried and tested route to address this by looking into ceratopsid skull form and function, we’re going to look at the use and evolution of horn-like structures (horns, antlers, ossicones etc.) in living vertebrates. Our view on extinct animal behaviour, after all, is seen through the lens offered by living species and it’s generally our assessment of modern taxa that dictates behavioural models for extinct animals, not the other way around. The anti-predator behaviour of living animals, even just those with horns, is a huge topic that is far too broad and multi-faceted to cover in detail here — especially as I need this to be a relatively short article* — but even in this brief visit, I hope we can hit a few key points that may give food for thought on horn function in Mesozoic animals.

*Yeah, nice try, Past-Mark.

An obvious place to begin is with well-known examples of horn-like structures being used as predator deterrents. It’s absolutely true that some species, like muskox, African buffalo, various rhinos and red deer use their cranial ornament aggressively against predators (Geist 1966, 1999; Schaller 1972; Kruuk 1972) and it is assumed that predator deterrence may explain the presence of horns in a great number of bovids (e.g. Packer 1983; Bro-Jørgensen 2007; Stankowich and Caro 2009; Metz et al. 2018). However, the idea of widespread horn use against predators has been challenged because field observations show such behaviour is rare among many species, and often of limited effectiveness (Estes 1991; Roberts 1996; Gerstenhaber and Knapp 2022). Some groups, like antelopes, are rarely or never witnessed using their sometimes enormous horns in defence against attacking carnivores, even when faced with certain death (Schaller 1972). Most deer seem to behave in a similar fashion, preferring to run or hide from predators despite their capacity to gore and kill conspecifics with their antlers. Indeed, there are indications that antlers may have a deleterious effect on prey species, with Geist (1966) reporting that antlerless moose are more capable opponents against wolves than their "armed" relatives. This may not be the case for all deer, however, with American elk proving more vulnerable to predators once their antlers are cast (Metz et al. 2018). But sometimes losing cranial weaponry makes no difference to predator vulnerability at all, as is the case for black rhinoceros. The necessary act of dehorning these animals to deter poachers shows that both adult and calf survivability are little affected by the removal of their horns (Chimes et al. 2022), suggesting that these structures have a non-essential role in thwarting predatory efforts.

When discussing anti-predation strategies involving horns, the African buffalo Syncerus caffer is one of the go-to species. And yet, these large, formidably armed animals are some of the preferred prey of lions, and are sometimes subdued by single individuals. Photo from Wikimedia, by Diamond Glacier Adventures, CC-BY 2.0.

Perhaps against expectation, not all animals with horn-like structures employ them in defence. Moose generally kick attackers, a behaviour they share with giraffes, who also lash out at predators with their powerful legs rather than bludgeoning them with their armoured, tri-horned heads (Gesit 1966, 1999). Kicking strategies are, of course, also available to species that we might mistake for being “defenceless” from their lack of horn-like anatomy. In some cases, these animals can be far more aggressive than their better-armed contemporaries. Horses, especially zebras, exhibit pronounced anti-predator aggression where they bite and kick attacking cats and hyenas (Kruuk 1972), and with such force that they may explain sightings of lions with shattered jaws (Schaller 1972). Zebras are also recorded as charging towards predators in a fashion that neutralises predatory effort. I like ecologist George B. Schaller’s account of this behaviour where he describes lions simply watching zebras running at them, as any effort to grab them would be “like jumping on a fast-moving train from a standstill” (Schaller 1972, p. 265).

We can augment our discussion further by switching our focus from prey species to their predators. If cranial weapons are effective predator deterrents, we might expect predators to avoid species with horn-like structures or, at least, those more likely to wield them aggressively. And yet, prey preferences seem largely determined by the energy investment demanded in animal capture (Schaller 1972) rather than the presence or absence of cranial armaments. Pouring cold water on romantic notions of life-and-death battles of horns and hooves vs. claws and teeth, field ecology suggests that large predators preferentially target species that are abundant, live in dense populations, and are of a size that provides a suitable reward against the effort of capture. Potential prey species are more likely to be ignored because they are too small, and thus do not provide enough nutrients for the predatory effort, or are too big, and will thus require an unreasonable degree of energetic investment to bring down. This is not to imply that the threat of injury isn’t factored into these behavioural calculations, but we just don’t routinely see predators avoiding horned, aggressive prey in modern ecosystems. On the contrary, both spotted hyenas and lions routinely attack African buffalo (lions especially), a species which is very well known for its horn-led predator defence (Kruuk 1972; Schaller 1972).

Clearly, the idea that horn-like structures serve as anti-predator devices is complicated by a lot of conflicting data. While no one doubts that these anatomies are sometimes used to deter predators, zoologists are engaged in a long-running scientific conversation about the extent and significance of their anti-predator role (e.g. Geist 1966; Estes 1991; Roberts 1996; Caro et al. 2003; Gerstenhaber and Knapp 2022). One especially important issue, which has implications for our discussion of dinosaurs, is whether the horn-like structures of female mammals exist primarily to deter predators (Bro-Jørgensen 2007; Stankowich and Caro 2009). While the formidable cranial weaponry of male mammals is often readily explainable through sexual selection (on which, see below), the function of the same anatomies in females is harder to fathom. Some argue that predator defence is their main purpose, which would imply a much wider role for this behaviour than has been documented in field studies. But others point to social selection as their principal adaptive driver (e.g. to dispute territories or mimic males) or simply regard them as non-functional, suggesting they only exist at all because of a genetic link to male horns. The latter must be the case for the females of some species, such as giraffes, which reportedly never seem to do much of anything aggressive with their heads.

I really like this image from Nikolay Spassov's (1979) paper on horned dinosaur evolution for its novel depiction of horned dinosaur combat. I especially like the interlocking of the frill spikes as we (or, at least, I) tend to forget about them possibly playing a role in physical competition. The horn-like structures of male mammals are shaped to match certain styles of intraspecific combat, and it's possible that ceratopsids were driven by similar evolutionary forces.

Thankfully, we can push most of these conflicting ideas and caveats aside to discuss the horn-like structures of male mammals. It is beyond doubt that intraspecific interactions have a far greater role in shaping these anatomies than interspecific ones, with the cranial ornaments of male giraffids, bovids, cervids and other taxa strongly influenced by sexual selection (Geist 1966; Bro-Jørgensen 2007; Knell et al. 2012). Their cranial structures are so strongly moulded by intraspecific adaptive pressures that they adopt sizes, shapes, textures and orientations that exclude them from effective predator defence (Estes 1991; Roberts 1996), instead becoming better suited to absorbing, catching and parrying the blows of rivals during physical intraspecific contests (Geist 1966; Packer 1983; Bro-Jørgensen 2007). They do not evolve these morphologies randomly, either, but change in response to specific fighting strategies and environmental circumstances. Horned female bovids may also engage in fights with other individuals (both male and female) of their species for resources, but their lessened behavioural emphasis on these bouts means their horns remain less developed than those of males — the significance of this is yet another area of discussion among zoologists.

So, having just thoroughly complicated this seemingly simple topic to a great extent, let’s bring this discussion back to dinosaurs and the assumption that horned dinosaurs wielded their horns like swords against dragon-esque theropods. While models of ceratopsids defending themselves with their horns are undoubtedly validated by the behaviour of some living species, a case can be made that we’ve overstated the importance of horns in predator defence among living animals and, by extension, dinosaurs. The message from the modern day is that horn-like structures can and might be used against predators, but that this behaviour is by no means ubiquitous. It may not even be that common, according to some researchers. It seems that intraspecific selection is more than sufficient to explain most horn-like structures among living species and that predatory influences, if present at all, are relatively minor for most species. We can’t know how much of this insight can be transferred over to dinosaurs, but if ceratopsid facial anatomy was being shaped by intraspecific factors (and we think it was; see above), then we have to entertain all that this brings. This means, in addition to the traditional view of horned dinosaurs being effective foils of predatory theropods, we have to consider some other possibilities suggested by their modern analogues. These could include, for instance, that only some horned dinosaurs actively fought predators; that their retaliations against attacks may have been ineffectual; and that some species may have rarely, and maybe never, turned their horns against other species. And this door swings another way: we have sufficient data from living animals to stop thinking that horned or spiked dinosaurs were the most formidable prey species and that “defenceless” dinosaurs like hadrosaurs and sauropods would be pushovers for their lack of obvious weaponry. Determining which fossil animals are “the most dangerous” from their raw anatomy overlooks the huge impact of non-fossilisable factors that contribute to anti-predator responses, such as temperament, prey awareness, physiology, intelligence, behavioural plasticity and so on. It’s a disappointing limitation of the fossil record that we can investigate what dinosaurs and other extinct animals were capable of, but we’ll never know what they were truly like. Questions about "the most dangerous dinosaur" and similar fall into that void.

Megasuperhypertheropod Tyrannosaurus encounters the unarmed sauropod Alamosaurus. "They said it was defenceless! Defenceless!"

So, in sum, the take-home here isn’t that anti-predator roles for ceratopsid horns are a non-starter, but that the behaviours of living animals complicate this seemingly simple hypothesis. If intraspecific evolutionary pressures on horns and related structures operate mostly independently of predatory pressures today, that has to be our model for Deep Time as well. This opinion comes loaded with caveats, of course, the biggest one being that we’re in a shifting landscape as goes determining the exact roles of horn-like structures in living species; as this changes, so might our ideas on extinct animals. And there’s a lot more we could discuss, too. There’s the aforementioned data about ceratopsid cranial functionality, there’s that healed, T. rex-bitten Triceratops horn described by Happ (2008) that is taken by some as evidence of defensive horn use (I’m not sure I agree; there’s no way of knowing the exact circumstances under which that horn was bitten), there’s the bigger picture of armed dinosaur co-evolution with different theropod clades… but we have to end here. I’ll conclude by borrowing a line from Farlow and Dodson (1975) who succinctly put the ceratopsid anti-predator hypothesis where it should be almost fifty years ago with the mere use of italics: “the evolution of ceratopsian cranial morphology probably reflects diversification through species-specific compromises among various selective pressures… and possibly predator resistance”.

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