8Reconstruction Principles: Cave Art, Speculation and Tissue Depth
‘ When it comes to the reconstruction of fossil animals, there will always be a great many aspects of anatomy, behaviour and lifestyle that will remain unknown, and for which a modicum of reasonable speculation will be allowed, and even necessary’.
Our final chapter on the generalities of reconstructing fossil vertebrates covers three topics of relevance to all aspects of soft tissue reconstruction: ancient art as a record of extinct animal appearance, the role of speculation in palaeoart (Fig. 8.1), and mechanisms to determine the soft tissue volume of fossil animals. These fundamental aspects affect the entire reconstruction process and their discussion in our final chapter on restoring animal tissues does not imply that they should not be considered during the earliest stages of executing a reconstruction.
Fig. 8.1 Dimetrodon grandis, restored with a speculative mane of tubercles and protecting its offspring. Are such speculations useful in palaeoart, or should we stick to more conservative reconstructions? (M. Witton)
Ancient art as an aid to understanding extinct animal appearance
We noted in Chapter 6 that cave art – paintings and etchings made by ancient humans – play some role in understanding the colour of fossil animals, but this is far from the only details that it provides. Early artists who rendered now extinct animals on rocks were able to record specifics of their appearance that we could not predict from fossil bones. In addition to colouration and patterning, they captured specifics of hair length, feather distribution, posture, humps of soft tissue and even behaviour. Often, these features are consistent across several images, even when the art is stylized, implying the recording of genuine animal features observed by multiple artists.
Fig. 8.2 The role of ancient art in informing palaeoart. (A) Ancient Māori depictions of giant moa (Dinornis robustus) showing details of posture and soft tissue. (B) Depiction of giant moa informed by (A) (M. Witton). (C) Rock art of an Irish elk (Megaloceros giganteus) with details of body patterning.
Ancient art reveals that many well-known extinct species may have looked different to traditional palaeoartistic interpretations (Fig. 8.2). New Zealand’s famous flightless bird, the giant moa (Dinornis robustus), is depicted in rock art with upright, near vertical necks rather than – as is commonly assumed – necks held in front of their bodies (Worthy and Holdaway 2002), and with feathers largely covering their legs. The Irish elk, Megaloceros, had a shoulder hump that is not depicted in the classic, red deer stag-guise for this animal – jokingly referred to as the ‘‘Monarch of the Glen Megaloceros’ by palaeoart scholar Darren Naish. Naish has also noted that something like eighty ancient paintings show that the woolly rhinoceros, Coelodonta, had a large shoulder hump; a neck mane, a laterally-flattened and curving anterior horn, a shorter posterior horn with a rounded tip; longish hair that shortened the appearance of the legs, and a dark colour band across the mid-region of the torso (Naish 2013). Combined with what can be gleaned from mummified specimens (slender, rounded ears; a short tail; broad, shovel-like lips), ancient art allows us to very precisely restore Coelodonta, and it may be the most stellar example of palaeoartistry being augmented by observations by ancient artists.
Extrapolating anatomy and augmenting soft tissues
All palaeoartworks reach a point where the data runs dry, making speculation and extrapolation necessary in even the most conservative artwork. The degree of extrapolation and speculation could be minor – filling some missing skin details or having to imagine a suitable colour scheme – or significant, having to reconstruct major portions of missing anatomy. The degree of permissible speculation and extrapolation in a palaeoartwork is an important question for artists: too much might result in our artwork being more fantastical than factual, and too little can lead to anatomically peculiar, unconvincing animals (Conway et al. 2012). The trick is to use speculation and extrapolation logically and rationally. This frees us from having to make dull, conservative or uninspired reconstructions, but also demands that our inferences are carefully chosen and rationalized. We might break this topic down into two divisions: 1) using speculation and extrapolation to restore poorly known fossil species, and 2) augmenting restorations with details unrepresented in the fossil record.
Reconstructions from highly fragmentary fossil remains
A surprising number of the most famous fossil animals – and especially the biggest ones – are known from just a few bones, or even just bits of bones. These species represent problems for artists. Without a good amount of fossil material, we have nothing to base our soft tissue reconstructions on or the data required to extrapolate body size and proportions. Ironically, it’s often these poorly-known species which seem most exciting, and are often in highest demand by clients and audiences. Well known examples of such creatures include giant pterosaurs, Spinosaurus, the largest sauropods and many fossil hominids, including the giant ape Gigantopithecus.
When confronted with reconstructing such animals, two options present themselves. The first is to deem the species un-restorable until more information is available. This approach works well for those species which are known from especially anonymous sets of elements, or those which cannot yet be resolved to any specific taxonomic addresses. At most we might use these fossil as licence to include certain types of creatures as background elements, but we might be unwise to make them the main features of an artwork.
Fig. 8.3 Extrapolating characteristics of fragmentary fossils to broader anatomical frameworks. (A) The giant ape Gigantopithecus blacki is only known from jaw bones and teeth, but they are sufficient to suggest it ate a tough, fibrous diet. Modern analogues (B), such as the high-fibre specialist giant panda (Ailuropoda melanoleuca) show how this diet influences broader skull anatomy: shorter and stouter jaws, heightened space for jaw muscles, and reduction of carnivorous features. We might assume similar traits applied to Gigantopithecus, and can factor them into our speculative reconstructions of this species.
Fig. 8.4 Reconstruction of Gigantopithecus blacki and Homo erectus. The Gigantopithecus is largely speculative, but has large overall size as indicated by its fossils, as well as the powerful face, quadrupedal gait, and extensive gut of a high-fibre herbivore (M. Witton).
A second approach can be implemented when the fossil elements present characteristic anatomies despite their fragmentary nature (Fig. 8.3). Their size or structural properties may be particularly noteworthy, for instance, and allow us to imagine them as species of exceptional size and build compared to their relatives. We can apply general trends of animal scaling to the bauplan typical of their group to imagine how especially big or small variants of that clade may have looked, and implement necessary modifications to skeletal proportions and muscle bulk to facilitate carriage and stature at their body size. If we know some detail of their functional anatomy or behaviour, we can use that knowledge to infer likely differences from their relatives, such as changes in skull shape related to a different diet, or a change in limb construction reflecting a different means of locomotion or habitat preference. The results cannot be considered ‘reconstructions’ in the same sense as those based on more substantial material, but these ‘artistic extrapolations’ of fossil material have value in communicating basic interpretations and hypotheses about poorly known extinct animals (Fig. 8.4).
Speculations that augment life appearance
A central tenet of All Yesterdays (Conway et al. 2012) concerned the fact that many reconstructions of fossil species are very conservative – little more than restored musculoskeletal systems with skin over the top. It argued that modern animals show terrific variation in muscle bulk, development of fatty tissues, integument types and volume, as well as other forms of soft tissue elaboration which are largely absent from palaeoart. Conway et al. called for the use of ‘reasoned speculation’, whereby rigorous reconstruction approaches take us to the limits of fossil data, after which carefully considered but boldly applied speculative tissues are used to complete the restoration. This anti-conservative approach takes liberal inspiration from extant animals as well as fossil data to augment the life appearance of subject animals with structures such as combs or wattles, thick, body-contour obscuring coats of feathers or fur, integument types that suit certain climates or behaviours, and other speculative tissues that are not typically preserved in our subject fossils, but are fully consistent with the natural world. This may initially seem outlandish but – as the discussions in previous chapters have shown – multiple lines of evidence show or strongly infer completely unexpected anatomies in prehistoric animals, and this invigorates our licence to be adventurous with reconstructions. In embracing this idea, we might even be making our art more credible, as it negates inconsistencies between conservatively restored prehistoric animals and more elaborate living ones.
Fig. 8.5 Pachyrhinosaurus perotorum restored with a speculative fibrous coat to ward off cold Cretaceous Alaskan winters. A critical view of this image is that it conflicts with a good record of scaly horned dinosaur skin, while a supporting argument is that integument types can vary radically between closely related modern species. Such radical speculations are often the cause of discussion and debate among palaeoart aficionados. (M. Witton)
The tremendous critical success and influence of All Yesterdays proves that speculative anatomical augmentation has a place in palaeoart. However, the output of artists attempting to replicate, and in some cases out-do, the All Yesterdays approach highlights how easy it is to discredit our artwork with misapplied speculation. The original All Yesterdays project worked because it was nuanced, presenting scientifically credible interpretations of fossil animals that had soft tissues consistent with their phylogenetic bracket, predicted ecologies, and available fossil data. It was not an exercise in fantasy, an ‘anything goes’ approach to prehistoric animal art, or deliberate defiance of scientific convention. Reasoned, speculative variations in animal tissue reconstruction were used to bring additional vitality to prehistoric scenes, not create creatures from science fiction. In short, speculation was employed to fill gaps in data, not replace data itself. A lot of All Yesterdays-inspired work fails to appreciate this nuance. Artwork which directly contradicts fossil data or that creates wholly imagined anatomies without any modern or fossil basis cannot be considered successful. The line between adding reasoned speculation and removing credibility from an artwork is a fine one, and is best walked only after detailed research and consideration of animal anatomy has taken a reconstruction to its scientific apex (Chapter 10). The adage ‘learn the rules like a pro so you can break them like an artist’ rings especially true for palaeoartists looking to enhance their artwork with speculative elements: it’s critical to learn what can be deduced about the appearance of a subject species to determine what speculations will be valid, and which are immediately flawed. It is clear that some speculative palaeoartworks are created to break tropes and conventions instead of legitimately restoring an extinct animal. As discussed in Chapter 3, speculative palaeoartists must remember that not everything common in palaeoart is a stereotype or meme, or a convention waiting to be busted: well-verified components of animal biology are not stereotypes and tropes, and deviating from them opposes the goal of creating science-based artwork (Fig. 8.5). Tropes and memes should be circumvented through assessing reasonable, but unexplored, options about life appearance, not by deliberately disregarding science or special pleading against strong evidence.
There are some anatomies of extinct animals which lend themselves especially well to speculative variation and are fairly ‘safe’ to experiment with. The development of integumentary structures is highly variable among living animals and we can assume the same was true in the past: experimenting with the addition of naked patches, display filaments, variable scale density and fibre length and so on is fair game in any species where skin details are less than completely known (so long as, of course, any actual skin data is respected). Fibrous integuments are especially viable for this treatment as modern species show how hair and feather configurations can vary from sparse to extremely elaborate within clades, individuals and even seasons, and we should expect nothing less of prehistoric animals. Sparsely filamented skin, manes, trains, capes, and hoods. seem reasonable for fuzzy fossil animals. Fatty tissues are also fun speculative elements because they leave little anatomical record and can be applied imaginatively, although it’s important to keep an eye on relevant phylogenetic and functional data. Outgrowths of soft, fleshy skin – which may take the form of dewlaps, combs, caruncles, air sacs, or wattles – leave little or no evidence on animal skeletons and can be added to many tetrapod faces without violating data concerning facial anatomy, so long as correlates for other tissue types are not present in relevant regions.
Such speculations give us potential to restore prehistoric animals in elaborate ways, but, before rendering every prehistoric animal like they’re on their way to Mardi Gras, it’s worth reminding ourselves that truly ostentatious species are not superabundant in the modern day. It’s unlikely that every species in prehistoric landscapes were covered in fleshy outgrowths, ornate fibre capes and so on. Many soft tissue augmentations have adaptive or functional implications that should be considered before their implementation. For example, animals that live in cold climates are unlikely to have lots of loose, dangling caruncles as these are prone to losing heat, and ambush predators are less likely to have highly visible display structures than species unconcerned about being conspicuous. ‘Reasoned speculation’ does not only cover where and what type of augmentation is possible for extinct species, but also demands that speculative anatomies meet adaptive predictions of lifestyle, climate and habitat preference of the subject species.
‘Playing dress up’: Using living species as precise models of colour and anatomy
If palaeoart benefits strongly from looking at living animals, is it sensible to adorn fossil species in colours and integuments taken directly from living species? ‘Dressing up’ extinct animals in the guises of living ones happens with some frequency, with common examples including Mesozoic theropods dressed as cassowaries or living raptors; early tetrapods depicted with the colours of frogs or salamanders; sauropods dressed as giraffes or in elephant skin; early hominins rendered as specifically chimpanzee-like; Mesozoic mammals rendered with opossum or Tasmanian devil colours; entelodonts restored as giant boars, and Mesozoic marine reptiles rendered with cetacean colour schemes.
Few will argue that this technique is inappropriate when restoring the extinct close relative of a living species, and especially so if their habitat and lifestyle was were similar. Some fossil cats or bovids, for instance, may well have shared colours and patterns with close modern relatives living in similar habitats. ‘Dress up’ methods are harder to rationalize for distantly or wholly unrelated animals, but a favourable view is that it presents colours and anatomies known to exist in nature, and that the resulting appearance can be more convincing than a purely fabricated one. After all, although there are few firm rules about animal colouration and patterning, it is possible to colour animals in unconvincing ways, and using a living colour scheme can sidestep this pitfall. Furthermore, ‘dressing up’ approaches can imply attributes of character and habits as well as augment appearance: what better way to convey that an extinct animal might be an ecological or behavioural analogue of a living species than to give it the same outward appearance? This can confirm audience suspicions as well as undermine them, such as by restoring a typically fearsome animal in the clothes of a bumbling living species. ‘Dressing up’ fossil animals can help audiences re-imagine fossil species through the context of a living one.
Arguments against ‘dressing up’ fossil animals are that it is distracting and scientifically questionable. While we cannot firmly falsify applications of modern colour schemes or specifics of integument to fossil taxa, ‘dressing up’ fossil species in this way implies near identical phylogenetic and adaptive factors shaped the anatomy of both species – which is demonstrably untrue in most cases. Indeed, we might be guilty of presenting misleading hypotheses with some ‘dressing up’ conventions. Clothing entelodonts with specifics of pig anatomy is one example, as entelodonts are more closely related to hippos and whales than pigs, and whale-like marine reptiles ignore the fact that most sea-going reptiles alive today (including snakes, some crocodylians, various lizards and turtles) have not converged on cetacean colouration. In these examples, our intention to add borrowed real-world authenticity to an artwork undermines another scientific message. We might also be wary of leaning too heavily on one species as a model and ignoring other possible analogues. In the aforementioned pig-entelodont analogy, for example, we see a strong bias towards using boars and warthogs as entelodont models (perhaps because they are the most fearsome looking suids) and little consideration of other pig species, even though they might be just as apt for this purpose.
Fig. 8.6 Deinotherium giganteum restored without elephant-like epidermal features and colouration. Though related to modern elephants, deinotherids diverged from the elephant-line long before crown-group elephants evolved, and it might be presumptuous to assume identical external anatomy. (M. Witton)
Further scientific concern for ‘dressing up’ concerns the appropriation of anatomies from living animals that had yet to appear in Deep Time. (Fig. 8.6) As we have seen in previous chapters, and will explore again in Chapter 9, evolution took time to equip animal lineages with the anatomy we see today and it is not a given that fully ‘modern’ anatomies are viable restoration options for a given subject species. Restorations of fossil mammals are particularly prone to this issue, where it is assumed that features of modern species were present in their distant ancestors, even those at the base of synapsid evolution. This is a case of ‘dressing-up’ overriding good palaeoartistic sense: we can reliably determine that many features of living mammals are different to those of early synapsids, and maybe even some of our comparably recent relatives from the Mesozoic (see Chapter 9). Subject species should be restored with soft tissue anatomies congruent with their fossil data, not simply draped in the clothes of their closest living relatives.
Criticisms that this approach is distracting are also valid. ‘Dressing-up’ fossil species risks our reconstructions looking more like distorted versions of living animals than unique entities, and might draw unnecessary attention to the mechanism of anatomical inference. The blatant transference of animal appearance from living to dead species is a visual reminder to our ignorance of fossil animal appearance and, like seeing the strings on a movie spacecraft or the flowers up a magician’s sleeve, the illusion and atmosphere of an artwork can be lessened if the viewer can spot the mechanism behind its creation. This issue can be especially noticeable in palaeoart photocompositions, where parts of photographed animal bodies are assembled to create the appearance of prehistoric animals. Unless the components represent anonymous, generic anatomies, or are significantly modified, the body parts of living animals can be obvious and detract from the impact of the artwork. Borrowing too liberally or blatantly from the modern day can turn perusal of a palaeoart gallery into a game of ‘spot the reference material.’
On balance, while there is nothing ‘right’ or ‘wrong’ about speculatively transferring superficial anatomies from living to dead species, it is neither a subtle technique or one that is welcomed by all. Modelling extinct animals very closely on living ones can be effective in the right circumstances, and might even be appropriate for some species. However, it can also compromise the credibility and impact of our artwork when misapplied or overused. These are not issues we encounter when generating original colour schemes and superficial anatomical details so, if in doubt, use living species for inspiration and ideas, but inject some originality into the appearance of an extinct subject.
The question of tissue depth and the convention of ‘shrink-wrapping’
Palaeoartists typically restored ancient animals with relatively deep soft tissues until the end of the twentieth century. Ideas that the deep tissue anatomy of extinct animals might have been conspicuous on living creatures had been explored (restorations of ichthyosaurs from the mid 1800s, for instance, show visible sclerotic rings as it was once thought that these plates lay only superficially within the facial tissues) – but were exceptions to the general trend. The conservative reconstruction approaches popularized in the 1970s started a contrary convention where artists restored prehistoric animals with minimal soft tissue volumes and skin tightly wrapped over their musculoskeletal systems. This approach showed clear details of the muscle layout and major skeletal contours which, at its most extreme, included clearly visible ribs and vertebrae, sunken tissues in skull openings, and prominent hip bones (Fig. 8.7). Thicker or more elaborate soft tissues were only added when their presence was indicated by fossil data or strong phylogenetic inference, and fur and feathers were cropped short to hug body contours. The somewhat skeletal appearance of these animals can be augmented in some renderings by skin textures and colours which match the topography of the underlying bones, such as using scale size or colour to mark out skull openings. In 2010, dinosaur palaeontologist Mathew Wedel coined the phrase ‘shrink-wrapping’ for this convention, comparing the contour-hugging soft tissues of these restorations to items wrapped in tight plastic for transport.
Fig. 8.7 The convention of ‘shrink-wrapping’ on the face of the sauropod dinosaur Europasaurus holgeri. Shrink-wrapping conventions include minimal tissue depth, sunken facial features, exposed teeth and visible skeletal contours, often creating a malnourished, ‘zombie-chic’ appearance (top). The reconstruction below shows a more generous – and anatomically plausible – allocation of soft tissues. (M. Witton)
Shrink-wrapping has proven very popular among palaeoartists. Many, perhaps most, of the restorations produced in the late twentieth century used this approach and it is still conspicuous in artwork produced today. The extent of this shrink-wrapping is variable. Some artists, like Gregory S. Paul and Mark Hallett, tend to show only slight contours of the skull features alongside very lean, though well-muscled, bodies and limbs. Others, like Ely Kish and William Stout, take shrink-wrapping to a level where soft tissue volumes are almost certainly unrealistically low. Gaping vacuities exist between neck vertebrae, rib cages and limb girdles bulge from the torsos, arms and legs – though adorned with a suitable muscle layout – are extremely thin, and faces are lipless and gaunt. It’s difficult not to look at some extremely shrink-wrapped palaeoartworks and not think of starving animals or even decaying remains; they do not look like healthy, virile beings.
We might assign three reasons for the popularity of shrink-wrapping. The first is that development of this style coincided with a reinvention of dinosaurs as bird like, active and powerful animals rather than oversized, under-muscled cold-blooded creatures. The athletic appearance of shrink-wrapped dinosaurs chimed with this renaissance in thinking and helped contrast newer art from the plodding, perhaps over-voluminous animals of previous artistic generations. Secondly, images of prehistoric animals as heroically-built, lean beings are preferred by many merchandisers and palaeoart fans because they suit the popular portrayal of prehistory as a savage struggle for survival, where only the most powerful animals survived. Thirdly, shrink-wrapping allows palaeoartists to ‘show our work’, demonstrating that the anatomy underlying the skin of a restored animal matches osteological information provided by fossils.
In recent years, shrink-wrapping has become increasingly unfashionable because artists and scientists have noted that the results of this convention compare poorly to living animals. Champions of the rigorous reconstruction movement were right to draw attention to the true shapes of fossil animal form, and to emphasize that these shapes should be apparent in our art, but we must also appreciate that skin, muscle, fats and other tissues can obscure details of skeletons instead of hugging them. The fact is that most animals do not have sunken tissues over skull fenestrae, distinctions in skin colour and texture correlating with skull anatomy, nor are the detailed contours of their skeletons visible through skin topography. Even reptiles, which are meant to have appearances closely resembling their skeletons, can have elaborate soft tissues including voluminous fat deposits; large amounts of wrinkly, saggy skin; eyes which bulge prominently from their sockets; deep lip tissues which fully sheath their teeth; jaw muscles which completely fill, and sometimes swell from, their fenestrae; thick or pointed scales and prominent cartilaginous nasal tissues.
Fig. 8.8 How extremes of integument hide musculoskeletal contours, demonstrated by an Eurasian eagle owl (Bubo bubo) and a yak (Bos grunniens).
Skeletal elements provide an important foundation for soft tissue distribution, but overlying skin, muscle, fat, and integumentary structures determine the actual body shape. In some cases, soft tissues bury skeletal elements to the extent of being ‘misleading’ about underlying deep anatomy (Fig. 8.8). It is a common mistake, for example, to interpret avian ankle joints as ‘backwards knees’ because the actual knee joint is well hidden beneath their feathers. Similarly, the necks of many birds and small mammals are often flexed at much higher angles than we would assume based on their external appearance, but their feathers and fur are so thick that the neck skeleton posture is entirely hidden (Taylor et al. 2009). The muscles and bones of major anatomical elements – such as necks and proximal limb segments – can also be completely obscured under skin, fat and integumentary structures. Structures such as horns, spikes, spines, combs, humps, armour, fins, and webbing are often entirely composed of soft tissue, and large, savage-looking teeth of mammals and lizards can be completely obscured by facial tissues. Palaeoartists are encouraged to look at x-rays of living animal species to see the extent that bony anatomy can be hidden by external soft tissues.
Fig. 8.9 Were the bodies of fossil animals shrink-wrapped? Preserved soft tissue outlines suggest not. (A) Polycotylid plesiosaur Mauriciosaurus fernandez. (B) Ichthyosaur Stenopterygius quadriscissus. (C) Paravian dinosaur Sinornithosaurus millenii. (A) After Frey et al. (2017); (B) after McGowan and Motani (2003).
Fig. 8.10 Fossils show that the life appearance of many bird-like dinosaurs, including Serikornis sungei, was dominated by large volumes of fibres and feathers, completely obscuring the shape of the skeleton beneath them. Reconstructions of these animals with naked faces, pockets of fluff, or thin veneers of body contour-hugging fuzz are not credible. (E. Willoughby)
Fossils preserving entire body outlines of extinct animals suggest our observations of modern soft tissue depths also applied in Deep Time (Fig. 8.9). These rare specimens show that the skeletons of extinct animals were also buried deeply in skin, muscle and fat and suggest some extinct animals bore little resemblance to their bony anatomy. We have direct evidence that the bodies of ichthyosaurus (Stenopterygius) and mosasaurs (Prognathodon) bore tall fins and paddle extensions that vastly exceed the limits of their skeletal margins (McGowan and Motani 2003; Lindgren et al. 2013). Preserved body outlines of ichthyosaurs and plesiosaurs show deep tissues which created smooth, streamlined torsos that are much bulkier than the underlying skeleton (Frey et al. 2017). Fossils of early horned dinosaurs (Psittacosaurus) and ‘mummified’ hadrosaurs (multiple taxa) show extensive muscle volume that bury their skeletons deeply within body tissues as well as elaborate structures – soft tissue filaments, combs and skin membranes – that defy ‘shrink-wrapping’ conventions (Osborn 1912; Vinther et al. 2016). The feather outlines on innumerable theropod fossils (including extinct birds) show that they were just as densely feathered as modern avians (Fig. 8.10), and the fuzzy ‘halos’ of fossil mammals and pterosaurs suggest they were also adorned with deep layers of filaments. Several pterosaur fossils (Pterodactylus, Pterorhynchus) also preserve unexpectedly broad neck tissue outlines which contrast against their thin, tubular neck vertebrae, as well as elaborations of crests and skin fibres that create body outlines which more voluminous than those predicted from musculoskeletal restorations (Frey and Martill 1998; Frey et al. 2003). These, and many other fossils, suggest that soft tissue volumes of extinct animals were comparable to those of extant species and not ‘shrink-wrapped.’
Fig. 8.11 Predicting tissue depth on animal skulls. Skull identifications, from top to bottom: Burchell’s zebra (Equus quagga burchellii); water monitor (Varanus salvator); American alligator (Alligator mississippiensis) and Arrau turtle (Podocnemis expansa).
But while shrink-wrapping is not a sensible base assumption for our reconstructions, this does not mean that animals do not have areas of shallow tissue depth or that their skeletal contours are not sometimes visible. Some body parts might be described as ‘regionally shrink-wrapped’: they include the ends of limbs and tails, the midline of the chest, and some facial regions: the frontal and nasal areas, ‘cheekbones’ (the jugal region in birds and reptiles, and the zygomatic arch in mammals), and the lower margins of the bottom jaw. The relationship between osteoderms and scales described in Chapter 6 is essentially equivalent to shrink-wrapping, and some living animals have almost entirely shrink-wrapped faces – including certain lizards, fish, turtles and crocodylians – where their life appearance is not dissimilar to their skull morphology. Palaeoartists thus need to know when to predict minimized tissue volumes from fossil bones, and especially in areas where tissue depth is prone to variation, such as animal faces. This is challenging, not the least because phylogenetic bracketing is of limited use: the cranial architecture of some extant animals is just too different from their extinct relatives to reliably infer soft tissues with this technique. We must thus look for bony features which correlate with facial tissue depth in living species and use these as a guide for our restoring fossil subjects. Contrasting the anatomy of ‘softer-faced’ animals like mammals, monitor lizards and certain birds, to species with shrink-wrapped faces, like turtles, crocodylians, chameleons and well-ossified fish, suggests several characteristics linked with facial tissue depth (Fig. 8.11).
Openness of skull architecture
The skull openings of softer-faced animals – including the temporal muscle openings, orbits and nares – tend to be large. At their most extreme – the orbits of mammals, the lower temporal fenestrae of lizards – these openings are not fully surrounded by bone. Larger skull openings suggest that a greater fraction of the face structure is composed of soft tissue, such as muscle, organs, and cartilage, and they build upon the contours of the bony skeleton. The nasal cartilages of monitors and mammals, as well as the bulging jaw muscles of mammals, are examples of this. Conversely, shrink-wrapped species have smaller cranial openings, limiting the potential for emerging soft tissues to form the shape of the face.
Rugosity
Soft-faced animals tend to have smooth bone textures with limited or no areas of rugosity, whereas the skulls of shrink-wrapped animals have large areas of rugose textures, often corresponding to scales or keratinous sheaths. This factor largely seems to reflect the proximity of epidermal tissue, which can leave characteristic textures in species with tightly-bound skin (Witzmann et al. 2010). Soft-faced species generally lack this rugosity because muscles, fat and fibrous integuments don’t leave large areas of osteological scarring, or simply because their skin is displaced too far from the bone to alter its surface. We might also note that the skull contours of soft-faced species are generally more rounded than those of shrink-wrapped species, which are often crisp and sharp. Rugosity is a particularly useful criterion because it can show the presence of tight skin tissues with some precision: if one part of a skull is rugose, and another isn’t, there’s a good chance that the smoother region had a different, and possibly more vouminous tissue configuration.
Pits, grooves and foramina
Shrink-wrapped species tend to have large numbers of perforations in their skulls, while soft-faced species show the opposite. (Morhardt 2009) This is particularly evident around the jaws. This presumably reflects the greater capacity for soft-faced animals to carry nutrients and sensory information through their soft tissues, whereas shrink-wrapped animals are forced to run nervous and vascular networks through their facial bones.
Correlates for epidermal projections
Elaborate skin projections – such as soft-tissue horns or crests – can alter animal faces quite substantially from the underlying skull shape. The characteristic osteological signatures of epidermal projections (Chapter 6) are thus a good indication of voluminous tissues, whereas absence of such features is predicted for shrink-wrapped species.
Fig. 8.12 Predicted facial tissue depth of fossil animals. Facial tissue volume is not a binary condition, but a continuum between extremes. The facial tissue depths of fossil animals are predicted to occupy all parts of this continuum, at least according to the criteria outlined in this chapter.
As is so often the case with biological subjects, there are enough exceptions to these observations that no single criterion seems to predict tissue depth (for example, smooth bone textures can underlie thin naked skin, so is not always a hallmark of deep tissues) but, assessed collectively, they might give a general insight into facial tissue volume. (Fig. 8.12) Applying this logic to fossil taxa implies that the roomy, smooth-boned skulls of cynodont-grade synapsids and fossil mammals match predictions for a ‘softer-faced’ species, and this might be true of some fossil reptiles – like sauropod dinosaurs – too. The highly textured bones and solidly built skulls of ankylosaurs and temnospondyls have cranial features indicating shrink-wrapped faces.
Careful examination of fossil skulls might also allow us to predict partial or regionalized shrink-wrapping in species where some aspects of their facial anatomy conformed to the underlying bone, and others did not. An example of this configuration is demonstrated in some living lizards, like gila monsters, which have skull textures strongly indicating minimal tissue depths over much of their skulls, but a sharp contrast to smooth, foramina-lite bone around their jaws. In life, these animals have vast, fleshy lips beneath their bony snouts, which we could predict using the criteria outlined here. Similarly regionalized shrink-wrapping seems apt for many fossil species. Gorgonopsians, for instance, might not have soft faces like living mammals as their snouts and foreheads have a low rugosity and their nasal openings are small, indicating the presence of tighter skin. However, they have few labial foramina and relatively open regions for jaw musculature, so they were likely fleshier around their jaw margins (perhaps lipped) and at the back of the face. Tyrant dinosaurs have skulls with relatively small openings compared to some of their theropod relatives, rugose snout textures, several hornlets, as well as a slightly elevated foramen count. This cranial anatomy is consistent with somewhat reduced tissue depth in several areas, but is not consistent with a fully lipless, crocodylian-like degree of shrink-wrapping. Many pterosaurs show pitting and vascular canals imbedded into their jaw margins, and some species have indications of tight keratinous sheathing on their crests and jaws, but the presence of striated bony crests – correlates for epidermal projections – as well as large skull openings and smooth bone textures in other parts of the skull, indicate that their facial appearance might not have been entirely skeletal.
Tissue depth is difficult to accurately assess in extinct animals, but it is not entirely unknowable or mysterious: we can measure it directly from exceptionally preserved specimens and, in lieu of such remains, careful analysis of bony anatomy holds clues to tissue volume. This, and what we know of living species, shows that there is no ‘universal truth’ about animal tissue depth, and artists should not approach their restorations with a preconceived idea of likely soft tissue volume. It should be standard palaeoart practice to use fossil data and modern anatomical analogues to rationalize soft tissue thickness for each subject, and to apply what is most defensible instead of what is preferred or fashionable.
