October 21, 2016:

by Mark Witton

It is something of a trope that prehistoric animals must bare their teeth in palaeoart, even when their mouths are closed. Historically, the majority of palaeoartists covered the teeth of their subjects with lips, cheeks or other types of tissues and only select species – sabre-toothed carnivorans or mammoths – were depicted with exposed tusks or sabre-canines. This changed when artists working in the 1980s and 1990s - Paul, Hallett, Stout - and a certain 1993 movie started showing predatory dinosaurs with toothy overbites and perpetually exposed teeth. This convention has since expanded to all kinds of prehistoric animals, and some galleries of Deep Time now have more toothy grins than a holiday photo album. Theropod dinosaurs in particular are almost always shown with alligator-like overbites that perpetually expose their upper teeth, the large canines of stem-mammals protrude over their lower jaws, and even herbivorous animals with relatively unimpressive dentition (like sauropods) are shown without lips or other forms of dental covering.

Many words – mostly published at blogs, online mailing lists and social media - have been typed to discuss the credibility of lipless palaeoart, but the subject has traditionally received only cursory attention from academics. Happily for artists, this is starting to change. A small set of literature exists which debates the presence of extra-oral tissues in dinosaurs (e.g. Ford 1997; Knoll 2008; Morhardt 2009; Keilor 2013; Reisz and Larson 2016), and most this agrees that some sort of soft-tissue - at least 'lips' - covered their teeth. However, a running theme of these works is that reliably inferring soft-tissues of the face is not a simple task, and we really need more data to be sure of anything. Work on more recent fossil mammals shows more reliable inferences (e.g. Wall 1980; Antón et al. 1998), obviously benefiting from soft-tissue data from a range of extant, close relatives. New insights on the evolution of mammal cranial nerves are helping to understand the development of sensitive lips and cheeks in stem-mammals (Benoit et al. 2016). It's still early days for understanding fossil facial tissues, but at least it feels like we're off the line.

Collectively, there seems to be recognition among the academics interested in this topic that understanding the tooth coverage of fossil animals lies largely in understanding living animals. Attempts to understand tooth exposure from skulls alone - through making inferences about tooth size, jaw closure and speculations on how extensive soft-tissues can be before they become untenable - do not consider all necessary data. For example, Prehistoric Times palaeoart adviser Tracy Ford (1997) looked solely at the skulls of predatory dinosaurs to infer the absence of lips, suggesting their teeth were so long that they would pierce lip sheathing once the jaws were closed. This study assumed that predatory dinosaurs closed their mouths to the extent that the teeth of the lower jaw contacted the roof of the mouth, and that the preserved tooth configuration was the condition in life. These points are common issues raised against lipped dinosaurs, but there are several major problems. Dissections and CT scans of reptile heads show that jaw muscles and other soft-tissues have a major influence on mouth closure, to the extent that reptile jaw skeletons are typically loosely closed under their skin, even when the mouth is fully sealed. Taphonomic studies show that teeth slip readily from their sockets after death and often fossilise in far more vampiric states than they were in life. And undermining this further is that no extant taxa with lipped jaws were used to calibrate a limit for oral soft-tissues. Arguments about tooth coverage based on simply looking at skulls, without detailed consideration of modern animals and their anatomy, border on being arguments from incredulity : "I don't believe the anatomy could do that."

Modern animals and their tooth coverage
For an upcoming project, I've been trying to crystallise my approach to restoring ancient animal facial tissues, and deciding whether to cover their teeth or not is an important part of that discussion. I've been deliberately broad in this assessment to attempt to try to sort the wood from the trees: discussions of oral tissues can sometimes get lost in the minutiae of tissue types, uncertain osteological correlates and so on - and many of these discussions result in the same answer: they can't be resolved with current data. That's not to say they aren't important discussions, but it's helpful to step back to see if we can answer the simpler questions as well: what gauge of teeth can be covered by oral tissues? When are teeth actually exposed? And what questions should we, as palaeoartists, be looking to answer when restoring facial tissues?

Reviewing literature and galleries of modern animals, we can see that overwhelming majority of living tetrapods have covered teeth, including all amphibians, most mammals and most reptiles (excluding birds, naturally. Hey, if they wanted to be involved they shouldn't have lost their teeth). Exposed teeth are actually really rare, and a character completely absent in many major clades. The soft-tissues involved in covering the teeth are variable, but 'lips' – either slightly fleshy margins of skin, or skin overlying true muscle - are so universal among tetrapods, as well as living relatives like lungfish, that we might assume lip tissues of some kind were ancestral to the group, and breaching these with large teeth is a derived condition evolved independently in a minority of lineages. Crocodylians are the only living tetrapods with fully exposed teeth, but it's increasingly obvious that they're also pretty specialised/derived/downright weird (Grigg and Kirshner 2015). Far from being 'living fossils' frozen in evolution, they have so many anatomical nuances and specialisations that their use as model organisms for other extinct taxa is increasingly questionable. This applies to aspects of their facial anatomy too - we’ll discuss this in more detail below.

We owe many of these surprises to animal lips, which are generally much more extensive than we casually assume. Large teeth can slide into soft-tissue sheaths located between gums and lips, and these are quite visible in the open mouths of some species. Amphibians, lizards and many mammals have upper and lower lips of similar size which meet over the teeth and sheaths can form on either jaw, but some mammals – including most carnivorous forms - have very large, fleshy upper lips over thinner, tightly-bound soft-tissues of the lower jaw (Antón et al. 1998). In these species, the canine teeth overbite the lower lip but the upper ‘over-lip’ is large enough to obscure the fact that the tooth is outside the lower mouth tissues. I am unaware of a reversed situation with the lower lip covering a thin upper lip: this may reflect the fact that overbiting dentition is much more common than underbiting. Regardless of the specific configuration, it is clear that we should not underestimate the capacity for facial tissues to obscure even very large, sharp and ferocious-looking teeth. The assumption that all conspicuous teeth of fossil animals were on display in life is thus problematic and does not agree with what we can observe in modern animals (below).

What's up with crocodylians?
The elephants – or rather large semi-aquatic reptiles – in the room here are crocodylians: why do they have exposed teeth when all other tetrapods have largely covered mouths? Their teeth are not overly large, nor acutely angled. Some (Reisz and Larson 2016) have argued crocodylian dentition is only possible because of their semi-aquatic habits. The (unpublished, currently conference abstract only) Reisz and Larson hypothesis is that exposed teeth – specifically their enamel component – are at risk from desiccation and breakage without constant hydration from saliva or environmental water (Reisz et al. 2016). This is an interesting idea which potentially gives artists a useful, practical guide to restoring prehistoric animals: anything living outside water with enamel-covered teeth must have covered them with soft-tissue. Despite its unpublished status, this idea has already chimed with some quarters of the online palaeoart community who're restoring anything with enamel-covered teeth with full sheathing.

However, this proposal may not be as simple to implement as it first appears. For one thing, there is a real lack of consistency in tusk composition in living animals (see Hilson 2005). It is true that, as noted by Reisz et al. (2016), the tusks of elephants have caps of enamel and cementum that wear off rapidly, leaving their tusks composed of dentine alone. This would seem to support the desiccation hypothesis, it implying that enamel is a liability outside of the jaw soft-tissues. However, living elephants may not be typical in lacking enamel on their tusks, there being fossil and living mammals which do have substantial enamel components on their exposed teeth. For example, the tusks of several gomphothere species have broad bands of enamel along their lateral surfaces, even as adults (Padro and Alberdi 2008), while the canines of male musk deer are enamel covered on the external surface. The tusks of male wild boars and warthogs only bear dentine on the posterior surface and wear facet, the rest of these large, exposed teeth being covered in enamel. In all these species these are not just small bits of enamel that get worn off, but sustained coatings that persist on the tooth indefinitely and influence tooth wear (Koenigswald 2011). To confuse things further, walruses have dentine tusks like elephants, despite their aquatic habits seemingly precluding desiccation as a risk for their teeth, and the spiralling tusks of another marine mammal, the narwharls, are covered in enamel. If there is a relationship between enamel and tooth exposure, it is clearly a complicated one: the presence of absence of enamel in itself seems to have little bearing on tooth exposure in at least modern mammals. (Readers interested in tooth composition should check out the second edition of Samuel Hillson's Teeth (2005), for its extensive documentation and illustration of mammalian dentition).

Our second reason to be sceptical of the enamel desiccation hypothesis concerns crocodylian behaviour. It is not widely appreciated that several crocodylians species ‘hibernate’, or more accurately aestivate, for months at a time in dry underground burrows during the hottest summer months (Grigg and Kirshner 2015). During these intervals they do not access water at all. Other, South American species spend dry spells as fully terrestrial carnivores, abandoning aquatic habits and obtaining water largely from the prey they kill (Grigg and Kirshner 2015). These states have to be explained against the suggested need to frequently moisten crocodylian teeth, because they suggest dental desiccation is not as risky as we're all assuming it is. Alternatively, it suggests that the requirement for hydration is so relaxed – literally months can pass without getting the teeth wet – that it probably has little influence on tooth anatomy.

Furthermore, there are important caveats about crocodylian facial tissues that we have to factor into any discussion of their lipless configuration. Crocodylian faces are far more specialised and unusual than they first appear, and this may factor into their lipless mouths. Their highly keratinous facial skin undergoes a developmental pathway unlike that of any other amniote (their facial skin is essentially have one, highly 'cracked' scale) (Milinkovitch et al. 2013) and their heads are riddled with hyper-sensitive Integumentary Sense Organs (ISOs). ISOs are another unique crocodylian feature and are attuned, among other things, to sensing tiny vibrations in water (Soares 2002, see Grigg and Kirshner 2015 for a recent overview). In at least some parts of the crocodylian skull ISOs are situated over tiny foramina, presumably housing nervous tissues, and the overlying epidermis is thinned, with a reduced keratin component, to enhance their sensitivity (Soares 2002). We can thus see that ISOs do have a role to play in configuring crocodylian skin, and they present many questions that palaeoartists should be interested in. Are ISOs an important reason for crocodylian faces having such tight, contour hugging and lipless skin? Do the major functional and developmental distinctions of croc faces explain the lack of crocodylian lips? It might explain why virtually no other aquatic tetrapods have abandoned lips - aside from the the odd (and perhaps only) exception like the South Asian river dolphin*, there are no whales, snakes, seals or otters with crocodylian-like, fully exposed teeth. And given that no other lineages have osteological correlates for ISOs, should we put huge caveats around using crocodylians as models for facial tissues in anything other than their own ancestors? I don't know if anyone has answers to these questions yet, but they're food for thought when using crocodylians as ammunition for lipless reconstructions of fossil animals.

*Thanks to Ádám Lakatos for pointing out the toothiness of some river dolphins!

It's still very early days for the enamel/oral covering hypothesis, but modern animals suggest that interpretations of enamel precluding extraoral teeth are definitely more complicated than they first appear, and may even be flawed. If so, the simple presence of enamel on the teeth of fossil organisms may not be as useful to artists as some are currently suggesting. But this conclusion is preliminary, and we need to wait for this idea to mature before it's shot down entirely. We know, for instance, that there's more than one type of enamel among vertebrates. Reptilian enamel, for instance, is both thinner and of different microstructure to mammalian enamel, and these clades have rather different approaches to tooth longevity. This may mean something for enamel desiccation and long-term tooth exposure, and we may think differently on this matter once this research has been completed.

Predicting tooth exposure in fossil species

If this is so, only especially long teeth which project a considerable distance from the margins of the skull and lower jaw should be considered strong candidates for permanent exposure. Select examples might include the canines of certain mammalian carnivores (e.g. Smilodon and other machairodont felids), the tusks of fossil elephants and their relatives, and the larger tusks of dicynodonts. We should also note those fossil reptiles – such as certain crocodyliformes, pterosaurs and marine reptiles – where entire toothrows are composed of dentition so long that their tips extend well beyond the margins of the jaw skeleton. Such extensive dental apparatus would seem to preclude the development of any sheathing tissues, at least akin to those exhibited by from modern animals, and these animals probably had fully exposed toothrows in life. Of course, this conflicts with the observation that food-processing teeth are almost always covered in the modern day. However, we can defend this approach by arguing that their morphology gives a strong reason for ignoring this guideline: it answers the "why we shouldn't give them lips?" question.

We might also set aside this guideline when extant relatives of modern forms provide us with means to predict unusual lip anatomy. For instance, the aforementioned ‘over-lip’ of modern mammalian carnivores is common enough across this group to assume it was present in their fossil relatives. Because we understand how the lips of these animals work, we can make more specific predictions concerning tooth exposure in species with particularly impressive teeth. Thus, we can look at classic reconstructions of machairodontid cats like Smilodon with perpetually bared fangs as reasonable because, unless their lips were arranged differently to virtually all their living relatives, that’s simply how their lip tissues would respond to a massive set of canines. And yes, I'm aware of Dunae Nash's recent discussions about sheathing Smilodon: given that this rests heavily on enamel being a no-no in exposed teeth, I'm unconvinced for the reasons explored above.

The concluding caveat
Of course, it must be reinforced that these are just guidelines - and guidelines based on my own qualitative studies, nonetheless, your mileage with them may vary - and there are exceptions to the suggestions made above. As is well known, for all the suggestion that restoring sabre-toothed cats with exposed teeth is reasonable, one living cat species – the clouded leopard – does cover a set of long upper canines in a lower lip sheath. We would not predict this based on other cat species and, if known only from fossils, clouded leopards would probably be restored with a slightly exposed canine. Likewise, the exposed tusks of some deer are not especially massive, and if we followed the suggestions above we'd probably cover them up in a reconstruction. But palaeoart is ultimately a game of prediction and probability, attempting to restore what is most likely to fill gaps in our data, and any game of odds will have some failures. That’s not to say we shouldn’t ignore these exceptional examples - they show that guidelines can't be trusted all the time - but it makes sense for us to know where the guidelines are in the first place. As with all aspects of palaeorestoration, all of us stand a chance to be proved wrong about our artistic decisions: if and when that happens, the best we can hope for is to have been wrong for the right reasons.

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References
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