Theropod dinosaurs were a bunch of buttheads: the evidence for and development of ideas around theropod cranial combat

When it comes to imagining aggressive behaviour between large Mesozoic theropod dinosaurs, the main game in town is head-biting: individuals grappling with one another by locking jaws around each other’s faces (Tanke and Currie 1998; Peterson et al. 2009). Such actions are well-evidenced by the sometimes significant trauma found on theropod skull fossils, including tooth marks, gouges, and even teeth embedded in facial bones. Clearly, head-biting was a dangerous, bloody activity that could lead to significant injury, and we have to wonder if theropods might have had means of physically settling their differences without tearing each other’s faces apart. After all, while some modern animals don’t have much behavioural grey space between ‘idle’ and ‘psycho’, many species employ means of fighting that prevent, or at least diminish, the need for more violent encounters.

One non-lethal means of theropod interaction explored in dinosaur literature is head-butting, where the various crests, horns and bosses adorning their skulls are transformed into bludgeons, clubs and shoving aids. We don’t see or read much about theropods employing these behaviours in dinosaur popular culture despite its obvious appeal - if you don't find the idea of heckin' big predatory dinosaurs shoving each other around with their heads at least a little bit awesome, you might want to see a doctor. But, actually, theropod head-butting has a reasonable foothold in dinosaur technical literature, with some studies even dedicated to determining its feasibility.

Among the earliest proponents of theropod head-butting were Robert Bakker and Gregory S. Paul, who've been illustrating this behaviour and the anatomy related to it for decades. From left to right: Bakker's (1986) diagram of theropod head ornaments related to head-butting; Paul's (1987, but published in 1988) fantastic flank-butting Ceratosaurus; and a more recent Bakker work showing head-butting Tyrannosaurus from the Beyond Bones blog.

So far as I’m aware, the concept of theropod head-butting was a product of the Dinosaur Renaissance, perhaps an unsurprising occurrence given that it involves regarding theropods as relatively sophisticated, behaviourally-complex animals rather than the pea-brained dullards of pre-Renaissance times. Two of the most influential figures of that revolution - Bob Bakker and Gregory S. Paul - were strong advocates for theropod head-butting. Their famous books The Dinosaur Heresies (Bakker 1986) and Predatory Dinosaurs of the World (Paul 1988) both proposed that theropods engaged in these behaviours (above). Bakker’s take interpreted the stout ridges and crests of theropods such as Allosaurus and Tyrannosaurus as weapons for butting bouts and stressed their significance as nonlethal fighting devices. Paul considered head-butting likely for most theropods with robust cranial ornaments, from the elaborately horned Ceratosaurus and Carnotaurus to the massively-skulled tyrannosaurs. Paul favourably compared the various horns and bosses of such animals to those of giraffes (which, of course, are renowned head clubbers) and has maintained such views in more recent works (e.g. Paul 2016). I’m curious as to whether Bakker and Paul's affinity for drawing theropods played into the formation of these ideas, as they both were among the strongest critics of pre-Renaissance artists for not capturing the diverse morphologies and ornaments of theropod skulls and produced many artworks showing more accurate theropod depictions. Might their more precise renderings of theropod skulls have made them ponder the function of the bosses, ridges and crests they were illustrating?

In subsequent decades theropod head-butting has been mentioned semi-often by theropod workers. A minority of workers have been negative and dismissive of the idea (e.g. Rowe 1989; Molnar and Farlow 1990), but it’s mostly brought up in a confirmatory sense. Most discussions have centred around a few clades - tyrannosaurids (e.g. Bakker 1986, Paul 1988, 2016), carcharodontosaurids (e.g. Sereno and Brusatte 2008; Cau et al. 2013) and especially abelisaurids (e.g. Novas 1989; Mazzetta et al. 1998, 2009; Hieronymus 2009; Snively et al. 2011; Delcourt 2018; Cerroni et al. 2021) - and we now have a suite of typical anatomical features identified as potential head-butting structures, as well as dedicated biomechanical investigations into the plausibility of theropod skull combat (Mazzetta et al. 1998, 2009; Snively et al. 2011; Xing et al. 2015). Evidently, the concept of theropod head-butting is not - as is sometimes the case - an over-eccentric hangover from a particularly adventurous period of dinosaur research, but a legitimate and current behavioural hypothesis supported by a growing amount of data.

The postorbital bone of the carcharodontosaurid Eocarcharia dinops and its prominent orbital boss: a structure made for lateral head-butting? From Sereno and Brusatte (2008).

Let's get into this a little more: what, exactly, are the structures theropod workers are linking to head-butting? As will be well-known to anyone reading this blog, theropods were prone to augmenting their skulls with ridges, bosses, crests and horns, most commonly along the top of the snout and above their eyes. It’s among these that we find potential head-butting structures, but it's not thought that any and all ornaments were suitable for head-butting. Some theropod crests were surely too delicate for physical aggression, probably even allowing for the strengthening properties of their overlying skin tissues (see below). It’s hard to believe that the thin, tall crests of Dilophosaurus were being smacked into other animals, for example, and they instead surely served a purely signalling role (Bakker 1986; Rowe 1989). Indeed, studies of the Dilophosaurus-like crests of Sinosaurus found that they were prone to structural failure under mechanical loading (Xing et al. 2015), and likely ill-suited to weaponisation.

The 'Trix' specimen of Tyrannosaurus, on display in Glasgow 2019, showing the massive postorbital bosses which dominate the posterior skull region of this species. Researchers have associated these with head-butting behaviour, which seems reasonable given their structure, location, and the general grumpy attitude evidenced for Tyrannosaurus.

Head-butting structures are better inferred from the sizeable, prominent and robust components adorning certain theropod skulls. In carcharodontosaurids, ornament indicative of head-butting consists of swollen, laterally-prominent bosses above their eyes (Sereno and Brusatte 2008) or, more unusually, dorsally-prominent domes in the same region (Cau et al. 2017). Unfortunately, taxa with these features - Eocharchia and Sauroniops - are not especially well-known so we can’t see if these features were somehow echoed across the rest of the skull: might they have sported other enhanced bosses and ridges as well? This is not a problem for tyrannosaurids, of course, which have skulls of extreme familiarity to theropod researchers. Both Bakker (1986) and Paul (1988, 2016) have linked the robust nasal bones (forming the top of the snout), lacrimal cornual processes (horns in front of the eye) and postorbital bosses (swollen, rounded structures behind the eye) of tyrannosaurs with head-butting behaviour. Predictions of armoured skin around their eyes and snout (Carr et al. 2017) are consistent with this idea, too. Tyrannosaurus needs special mention here for its uniqueness among theropods - even other tyrannosaurs - in having flattened its lacrimal cornual processes into swollen, highly-textured expansions of the skull roof and inflated its postorbital bosses to a great extent, sometimes even capping the latter with osteoderms (Carr et al. 2017). The enlarged postorbital bosses are among the tallest parts of the skull and also project outward from the eye socket by a considerable margin. Significant rugosities and other properties of these lacrimal and postorbital features indicate a covering of especially thick, toughened skin, but no true horns (despite what you see in lots of artwork and films, there’s nothing pointed or horn-shaped about the ornament of big T. rex specimens). If Bakker and Paul are correct in Tyrannosaurus having a head adapted for head-butting, it’s difficult not to see this reconfiguration as the skull of a theropodan bulldozer: a flattened, shovel-shaped cranium to heave and bash other animals around with.

Tyrannosaurus is unusual among tyrannosaurids for lacking a hornlet in front of the eye, but it compensated for this with armoured cranial skin and two prominent postorbital bosses. Bolted to something like six or more tonnes of angry tyrannosaur, this must have made for one heck of a battering ram. So let's see... if there's BRONTOSMASH!, ArSUMOitherium, then this must be... TYRANNOBUTT? Yes, make it so.

Neither tyrannosaurs nor carcharodontosaurid head-butting has received much in the way of dedicated research attention, however, leaving our best insights into this behaviour coming from abelisaurs, especially Carnotaurus. The head-butting potential of these short- and gnarly-faced, sometimes horned theropods has been remarked on for decades (e.g. Paul 1988; Novas 1989; Mazzetta et al. 1998) and, via Carnotaurus, encouraged several rounds of dedicated biomechanical investigation (Mazzetta et al. 1998, 2009; Snively et al. 2011; Méndez 2012). These initially regarded Carnotaurus cranial combat as a brutal affair, with spinal dampening allowing combatants to impact their heads at rhinoceros-grade speeds of 20 kph (Mazetta et al. 1998). But subsequent investigations have curbed these ideas somewhat, finding that Carnotaurus skulls were incapable of sustaining strong forces during simulated head-butting (Mazetta et al. 2009). Instead, their skulls were better suited to low-velocity impacts, blows that targeted softer parts of animal bodies (e.g. flank butting), or maybe even quasi-static bouts of head-shoving (Mazzetta et al. 2009). This isn’t too surprising: Carnotaurus is a relatively robustly-skulled theropod, but its cranium is still a relatively vacuous, air-filled structure adaptively balancing maximised strength and minimised weight. Compared to the massive and thickened skulls of more dedicated headbutters, theropod skulls look relatively delicate. Abelisaur necks are no longer considered shock absorbers for running impacts either, but their recognition as strong, powerful elements means they still have a role in head-butting hypotheses (Méndez 2012): the utility of a powerful neck in energetic, sustained use of a weaponised head is obvious (Delcourt 2018).

Epidermal correlates covering the heads of abelisaur species, from Delcourt (2018). Carnotaurus stands out with its indications of especially reinforced, cornified snout and horn tissues - a 'carnivorous bull' indeed.

Further information on abelisaur head-butting stems from their facial skin. The rugose surface textures of abelisaur skulls are indicative of particular skin types, such as cornified sheaths (think bird beaks and cow horns), cornified pads (muskox heads) or thickened, armoured dermis (rhino and hippo skin) (Sampson and Witmer 2007; Hieronymus 2009; Delcourt 2018; Cerroni et al. 2021; Hendrickx and Bell 2021). These are exactly the tough, reinforced skin types we’d expect to see in animals that used their heads as clubs or shoving implements, and it’s been noted that these dermal types, in extant animals, correlate with intraspecific fighting (Hieronymus 2009). We’ve touched on these data before, discussing that abelisaur faces may have looked quite different to their skulls thanks to thick skin. This is especially so for Carnotaurus, which had a snout covered by an extensive cornified pad. Because the evidence for thick, armoured skin on abelisaur faces is especially prominent compared to other theropods, it seems reasonable to infer a behavioural emphasis on head-butting within the group.

Carnotaurus sastrei, a species that - among theropods at least - is probably the most head-butting adapted of all. Exactly how its head, equipped with a cornified pad and sheathed horns, was used in aggressive behaviours has been interpreted differently over the years and boils down to assumptions of shock-absorption capability: was it a high-impact or low-impact headbutter, a shover, or a flank-butter? Whichever it was, it certainly looked awesome.

But these neat dermal insights bring complications along with insight, adding important caveats to the biomechanical studies outlined above. We know from living animals that head-butting capability is best modelled by factoring both bony and soft-tissue data (e.g. Drake et al. 2016), and that such simulations can turn bone-only models of skull strength on their heads. So far as I know, no studies of abelisaur cranial strength in head-butting simulations have modelled their armoured and cornified skin tissues yet, and so we await such analyses before real confidence can be drawn about abelisaur head-butting capability. The same is true, of course, of any other theropod we’d want to investigate head-butting behaviour for. I'm sure I'm not the only one thinking such studies, and other insights into this relatively unexplored but fascinating facet of theropod palaeobiology, would be very welcome additions to dinosaur research.

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References

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• 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.
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