4General Reconstruction Principles, Skeletons and Trackways
‘ …without knowledge of what constitutes the life, attitudes, and psychology of a modern animal, it is impossible to infuse similar attributes into the drawing or modelling of a fossil creature.
Over the next five chapters we will examine the process of restoring a fossil animal (Fig. 4.1). Our discussion will be framed around reconstructing the life appearance of a vertebrate as these are the most popular subjects for palaeoartists, but the principles outlined here are essentially the same for reconstructing any fossil organism. All organisms are reconstructed in a stepwise fashion, beginning with understanding the skeleton, then reconstructing the deepest soft tissues and working to the most superficial. For vertebrates, soft tissues include layers of muscles, visible organs and fatty tissues, and finally the skin and integument. This process may seem arduous and long winded, but art of any living subject (including humans) must understand skeletons, muscles, and body contours to accurately capture elements of three-dimensional form. But whereas artists of living subjects can witness these forms directly, palaeoartists must restore them from fossil data before creating a convincing visage of an extinct form.
Fig. 4.1 Azhdarchid pterosaurs wander over a Korean Cretaceous sand bar, leaving Haenamichnus tracks as they go. This reconstruction factors knowledge of skeletal anatomy and trackways to predict the posture, anatomy and gait of these animals – a process that will be discussed in depth over subsequent chapters (M. Witton).
Most palaeoartworks have a significant amount of work taking place ‘behind the scenes.’ Many of the calculations, sketches and diagrams produced in pursuit of compelling anatomical reconstructions will not contribute directly to a final artwork, but it is often clear which palaeoartworks have received careful and considered reconstruction approaches and which ones have not. Superior quality and credibility is the reward for putting time into the reconstruction process. We should not think that palaeoartworks designed for non-expert audiences are excuses for lax attitudes, either. Even when looking at an unfamiliar animal, many viewers will question badly reconstructed anatomy, either in terms of peculiar or asymmetric proportions and physical anomalies, or biologically untenable poses and tissue depths.
The constraints of a linear narrative mean we must discuss the tissue reconstruction process in defined stages, though in reality these aspects are not divorced from one another and palaeoartists may not execute works in the order of consideration used here. More commonly, data sources and ideas are considered holistically during the reconstruction process, with artworks arising from simultaneous consideration of multiple lines of evidence and interpretations. Our discussion will cover: 1) fundamental principles, skeletons and trackways; 2) muscles and fatty tissues; 3) skin and colouration; 4) specifics of facial anatomy; and finally, 5) aspects of tissue depth and speculative anatomy.
Predicting missing anatomies
The patchy quality of the fossil record dictates that an ability to reliably predict missing anatomy is a vital tool for the palaeoartist. This could be something as simple as the length of a bone or the number of toes, or something more complex, like the configuration of musculature or a probable skin type. The least speculative mechanism to make such predictions is phylogenetic bracketing, where we look to infer the state of unknown anatomy in extinct taxa by their position in an evolutionary tree (Fig. 4.2). This technique owes much of its origin and early application to palaeontologists Harold N. Bryant and Anthony P. Russell (1992) and its best-known champion, Larry Witmer (Witmer 1995). Application of this process has given insight into ancient animal anatomy, like integument evolution and muscle layouts, as well as behaviour, colours and patterns, and many other palaeobiological concerns relevant to palaeoartists.
Fig. 4.2 An overview of phylogenetic bracketing, one of the most powerful tools in the palaeoartist’s arsenal for predicting unknown characteristics of fossil organisms.
A simple evolutionary scenario demonstrates how phylogenetic bracketing works (Fig. 4.2). In a simple phylogeny consisting of three species, A, B and C, species B and C form a clade, with species A being their sister taxon (see Fig. 3.6). Species B is our subject and is poorly known, whereas species A and C are well understood; they are either modern animals or species represented by excellent fossils. It is simplest to assume that any shared anatomical and behavioural characteristics of species A and C were present in their last common ancestor, and the evolutionary branches containing these species form a ‘bracket’ for that trait in their region of the evolutionary tree. Any species evolving within this bracket can be inferred to have the same shared anatomies or behaviour common to that bracket. Thus, we would predict that species B would have the same trait we identified in A and C, because it evolved within their phylogenetic bracket. This has allowed us to predict the missing aspects of species B in a relatively objective fashion, rather than simply guessing, extrapolating from one taxon, or biasing our reconstruction to a preferred interpretation. The only times when we would not restore species B with those traits is if we had good evidence to the contrary, such as direct fossil evidence that it differed from A and C in that characteristic, or strong reasoning to think that trait might not apply to species B (perhaps if it was of such radically different size or lifestyle that the characteristic may not apply).
Although phylogenetic bracketing is very useful, it has some limitations. Its chief issue is a struggle with evolutionary novelty (Knoll 2008) because it assumes the subject taxon will either have, or not have, the same anatomy as our subject species, and cannot predict a third state (such as a novel anatomy). For example, phylogenetic bracketing fails to predict the fleshy, muscular cheeks of mammals from our bracketing taxa, amphibians and reptiles (Upchurch et al. 2007). This issue is compounded when brackets are drawn from contrastingly-adapted or distantly-related species. Adaptive pressures can change organisms radically, especially over long timeframes, and living animals can be so modified from their fossil relatives that their anatomical relevance is questionable. Facial tissues are again, a good case study here. Crocodylians and birds are each other’s closest modern relatives but their facial tissues – tight skin in crocodylians, beaks and loose, feathered skin in birds – sends conflicting signals about the facial tissues of the animals they bracket (which includes non-avian dinosaurs and pterosaurs). Details of fossil dinosaur and pterosaur skulls seem to confirm that their facial tissues were not identical to their extant relatives, and we have to conclude that phylogenetic bracketing struggles to predict their facial appearance (see Chapter 7 for more on reconstructing animal faces). A final major issue concerns the lack of consensus phylogenies for many fossil groups. When new research moves taxa around evolutionary trees, their brackets shift with them, sometimes dramatically altering the predicted characteristics of our art subjects. All this said, phylogenetic bracketing is no more flawed than any other scientific technique. Every method has limitations and requires testing through alternative analyses, and our palaeoartworks should – like good science – be informed by multiple lines of investigation.
Creating a skeletal reconstruction
The foundation of a vertebrate palaeoartwork is the skeletal reconstruction, a diagram that figures carefully drawn skeletal elements of a fossil species in a pose suited to that species (Fig. 4.3). They factor predictions of skeletal form, proportion and articulation from multiple lines of evidence and carefully predict the proportions of missing elements to fill gaps in fossil data. A well-produced skeletal reconstruction is a hypothesis of basic fossil animal appearance and, as with any model of fossil animal structure, they are prone to revision as new data is unearthed. These illustrations are typically clearly-rendered monochrome artworks that emphasize bone shape over shading and finer detail, and though they are conceptually simple, they can be challenging to execute. Perhaps contrary to expectation, there is rarely one universally agreed way to reconstruct a fossil skeleton, and not all artists produce skeletal diagrams of equal quality. Readers particularly interested in this aspect of palaeoart should read the Skeletal Drawing blog, written by one of the finest skeletal restorers of modern times, Scott Hartman, for a thorough understanding of this process. A guide to the names of major bones of the skull and skeleton are provided in Figs. 4.4 and 4.5, and anatomical terms of the orientation of bones (or any other anatomy) are overviewed in Fig. 4.6.
Fig. 4.3 Examples of skeletal restoration formats commonly used in modern palaeontological papers. Top, the strange Triassic reptile Tanystropheus; bottom, a selection of variably known azhdarchid pterosaurs. Pterosaur skeletons from Naish and Witton (2017).
Fig. 4.4 Major bones of the tetrapod body, exemplified by (A) troodontid and (B) opossum skeletons.
Fig. 4.5 Structure of the tetrapod skull. (A) The cynodont Kayentatherium. (B) and (C) The avialan Archaeopteryx showing names of cranial bones (B) and openings (C). Skull illustrations after Sues and Jenkins, 2006 (A) and Rauhut, 2014 (B and C).
Executing a skeletal reconstruction is made easier by basing it on the most completely known specimen of a subject species. Such specimens allow the artist to transfer, not calculate, aspects of bone shape and proportion directly into their diagram. Sadly, entire skeletons of fossil vertebrates are not especially common and artists often have to composite skeletons from several individuals to reconstruct a single animal. This requires cross-scaling: the transference of proportions between specimens, appropriately scaled to match the ‘donor’ specimen to its recipient. Cross-scaling can be a complicated task and it is encouraged to keep diagrams of each specimen separate from the composite illustration in case something goes awry and you need to start afresh.
Fig. 4.7 Animals adopt different proportions as they grow, a phenomenon known as allometry. Despite this hatchling Protoceratops – just 12cm long in life – being well-developed compared to the offspring of many birds and mammals, it still has a lot of proportional changes to make before it resembles the adult in Fig. 2.1 (M. Witton).
Cross-scaling is achieved by scaling bone elements common to two specimens to the same size and assuming that non-common elements can be scaled to the same degree. Not all elements are equally reliable indicators of size, however. Small bones are to be avoided as they are easily distorted through fossilization as well as easily mismeasured, leading to errors in a restoration. Larger bones are preferable, but not all are useful because animals often grow with allometry: different body parts growing at different rates. Allometry is extremely common because of the effects of increasing body mass on bones: larger, relatively heavy individuals need proportionally larger bones to support their bodies. Conversely, some organs – such as our eyes – do not benefit as much from increasing in size, so they remain similarly proportioned throughout growth even if our surrounding tissues change. The phenomenon of allometry is the main frustration of cross-scaling, and a major source of error: we should not assume juvenile specimens will be ideal models for adults, or vice versa (Fig. 4.7).
The most reliable bones to use for cross-scaling are those of the upper limb, as these elements are less prone to runaway allometry than other body parts. The femur is the bone of choice as it’s big (and thus easy to measure), and has proportions that correlate well to body size in many species. A second tip is to cross-scale similarly sized specimens to avoid allometric influences. If enough specimens are available, measurements can be compiled to assess relationships between size and proportion, and plotting these values in graphs and tables can predict unknown bone proportions, even at poorly represented body sizes. Spreadsheet software can be very useful for plotting bone dimensions against one another and, via simple scatter graphs, trend lines will predict missing proportions. When possible, drafted proportions should be checked against actual data – there may be fossils that could not be used directly in your scaling calculations, but are sufficiently complete to provide a reality check of your projected measurements.
It is important to draw the bones of the specimen in the same view. This is challenging for specimens which have been badly crushed and even more so when bones are flattened in unhelpful attitudes. Additional specimens, sometimes of close relatives, may be called upon to reconstruct these bones. Drawing bone shapes is best done through first hand observations of relevant fossils or, at least, access to very good photos, drawings and 3D scans. Bones often have to be restored because of distortion or breakage, and this can be achieved using paired skeletal elements – including skull bones, ribs, limb girdles and limb bones – by forming a reconstruction using the most complete bones from each side of the body. Strongly distorted skulls can be challenging to rebuild as they are made of complex three-dimensional elements. Restoring symmetry, carefully ascertaining the nature of the distortion and breakage and understanding the likely configuration of the skull (perhaps from a close relative) are the best methods here. Axial bones – the vertebrae – can be checked against neighbouring elements and their size and proportions estimated from these. Restoring bones is sometimes little more than an educated guess, especially when working with poorly known or aberrant species. In these instances, the best we can do is make informed and defensible attempts, and then wait for confirmation from better specimens.
Fig. 4.8 Teeth slipping from sockets is a common decay phenomenon that is easy to detect in modern skulls, but can be misleading in fossils. Top images show a water buffalo (Bubalus bubalis) skull with slipped and restored teeth, lower images show Sinornithosaurus with the same conditions. Sinornithosaurus skull after Gong et al., 2010.
Special mention should be made of deformation that occurs to the socketed teeth of some reptiles and mammals (Fig. 4.8). ‘Tooth slippage’ is a common distortion of many vertebrate fossils where, after gums and connective tissues rot away, their teeth became loose in their sockets and partially slip out. This is not a unique phenomenon of fossilization but a common occurrence even among recently deceased animals: skull specimens from living species often have teeth that rattle and slip around their sockets. The effect of tooth slippage is to create an impression of teeth being far longer than they were in life, so artists should make sure their reconstructions only show the tooth crown (the part of the tooth which protrudes from the gum), and not the root (the region buried in the jaw).
Fig. 4.9 Artists sometimes restore fossil animals without knowledge of their girth, seemingly applying a generically ‘average’ width to their torsos. In fact, animal girth is extremely variable and reflects many adaptive factors. Left, the lithe form of a cheetah (Acinonyx jubatus); right, the rotund water buffalo (Bubalus bubalis).
Fig. 4.10 Torso cross-sections through living and extinct reptiles: note their considerable difference in shape – there is no ‘standard’ shape of reptilian body. Images after Brown (1908); Holland (1910); Borsuk-Bialynicka (1977); and O’Keefe et al. (2011).
Fig. 4.11 The fantastically wide Asian ankylosaur Pinacosaurus grangeri. Ankylosaur girth has been evident from fossils for over a century, but it is only in recent decades that artists have acknowledged this in art. (J. Conway)
Skeletal reconstructions most commonly depict lateral views, but if fossils permit it, artists are encouraged to reconstruct animals in multiple views. This brings far more confidence when restoring an animal at oblique angles or in three dimensions. As with modern species (Fig. 4.9), fossil animals are enormously variable in girth, particularly with regard to torso shapes and limb girdle widths, and these attributes are not easy to predict from lateral aspects alone. Unusual proportions in non-lateral views characterize some species (Figs. 4.10, 4.11) and these taxa allow us to identify those artists who skip understanding their animals in three dimensions. Some artists are particularly prone to imbuing their reconstructions with a ‘generic rotundity’ – a generally oval or round torso cross-section that has little resemblance to the real animal shape.
Articulating bones and choosing a pose
The pose chosen for a skeletal reconstruction should consider biological plausibility, comparability to other restorations, and visual clarity. Generally, a fairly ‘neutral’ pose is preferable to a dynamic one as they run less risk of being shown as implausible and, in not forcing the anatomy into a contorted state, viewers can see skeletal details more clearly (4.12). Concessions might be made for animals with long appendages, where neutrally-posed long elements (such as wings, necks or long, whip-like tails) force restorations down to tiny sizes to fit them on sheets of paper. Here, deviations from ‘neutral’ poses serve a practical purpose: dipping the end of a long tail or elevating a neck can make much better use of available space. Restoring pterosaur skeletons in take-off, though dynamic, compacts their lanky forms into a smaller space than a more neutral flying pose, while also leaving their anatomy visible (Fig. 4.3B).
Fig. 4.12 Skeletal reconstruction of the avemetatarsalian archosaur Teleocrater rhadinus. Note how the chosen pose maximizes clarity of the bones and is functionally ‘safe’: a simple walking pose is unlikely to be proven untenable in future studies, giving this skeletal potential for scientific longevity. (S. Hartman)
Fig. 4.13 Skeletal reconstruction of the plesiosaur Thalassomedon haningtoni. Plesiosaurs are challenging to articulate because of the number of bones and some highly cartilaginous regions. (S. Hartman)
Whatever pose is chosen, the bones should be drawn in correct articulation (Fig. 4.13). This is where first-hand observation of fossils becomes critical as bone joint surfaces are invariably complex and have impacts on articulation that are difficult to appreciate from photos or illustrations. Limb bones, for instance, look simple to articulate but must nestle properly into a limb girdle socket (which might require the bone to project at an odd angle), articulate appropriately with multiple neighbouring bones (which might rotate and twist as the joint moves), and then be depicted in a correct orientation by a skeletal artist. A common error in palaeoartworks stems from artists assuming that extinct animals have limbs that articulate and move like those of familiar, living animals (mostly mammals) whereas, to the contrary, palms and soles can face inwards, limb skeletons can be mechanically linked (for instance, when one joint is worked, those further along the limb must also move) and limb bones can swing medially and laterally when their joints are open and closed. It is important to make sure limb functionality is understood before beginning this part of the restoration. One aspect of limb articulation that skeletal restorations generally ignore, however, is the slight lateral bowing of elbows and knees in erect-limbed species. These bones tend to be depicted as fully vertical in lateral view, which is incorrect, but the bowing of these joints is subtle enough that we needn’t worry about this too much. Non-lateral views must account for these oblique orientations however, as the bowing is more pronounced in these views and the skeleton will not articulate properly without it.
Articulated specimens provide useful insights into the arc of the spine in life. If a fossil skeleton has not been unduly disturbed they can record where, and to what extent, vertebral joints could move and, in species that had relatively inflexible torso vertebrae (many early tetrapods, most reptiles), they might give some indication of the arc of the back. When reconstructing the vertebral arc from scratch, pay close attention to how ‘wedge shaped’ each vertebra is in lateral view. Specifically, look at the length across the centrum – the typically blocky, cylindrical structure at the base of the bone – and that across the zygapophyses – the enlarged processes that project fore and aft from the vertebra to maintain contact with their neighbours as the spine moves. If the centrum is consistently longer than the length across the zygapophyseal region (for example, the bases of the vertebrae are longer than the upper regions), the vertebral column will likely bow downwards. If the opposite is true (the bases are shorter than the upper regions), the column will arc up.
Fig. 4.14 One of the most famous sauropod dinosaurs, Brachiosaurus. The neck posture of sauropods remains the subject of debate because details of bone articulation are difficult to predict from fossils (J. Csotonyi).
Articulating neck vertebrae can be very challenging because the degree of articulation can vary tremendously among living species (Taylor and Wedel 2013), and it is not always clear what strategy was employed by fossil species (Fig. 4.14). Some living animals have tightly bound neck bones with lots of zygapophyseal overlap, but some mobile-necked species – such as birds and camels – almost disarticulate their neck skeletons in commonly adopted poses. At present, we struggle to fathom the degree of neck motion and habitual neck postures of prehistoric animals. Nevertheless, one commonality of all terrestrial tetrapods is that their necks are held upright, with an upward arc at the neck base and a downturned arc at the top (Taylor et al. 2009). This trait is exaggerated in species with erect limbs (mammals and birds), and, given their ubiquitousness in living species, we might assume these principles were true for fossil animals as well.
Fig. 4.15 Skeletal reconstruction of the Cretaceous teleost Ichthyodectes ctenodon. Restoring the skeletons of animals such as fish, early tetrapods and armoured species is complicated by the by the sheer number of bones – sometimes even the vertebrae are composed of multiple elements. (S. Hartman)
Among the more challenging aspects of reconstructing a skeleton is understanding how the torso bones fit together. Torso skeletons are jigsaws of vertebrae (which can be composed of several bones in some species. Fig. 4.15), ribs of several kinds (including gastralia, or belly ribs), multiple cartilage elements, as well as limb girdles. It is especially important to position the pectoral (shoulder) girdle correctly because it ties much of the anterior trunk together and influences how we place and move the forelimb. The pectoral girdle is a series of interlinked bones: the scapulae – the shoulder blades – wrap over the ribs and articulate with the chest via the coracoids and clavicles (if present); the coracoids link to the sternum, and the sternum contacts the ribs or their cartilages. The complex articulations of pectoral girdles are not only artistic headaches but an area of active research for palaeontologists. It is generally realized that understanding the length, shape and curvature of pectoral girdle elements, data on their position in articulated fossil skeletons, and comparisons to living relatives are needed to make meaningful predictions of shoulder girdle position. It is difficult to give any definitive rules on orientating the pectoral girdle, but Schwarz et al. (2007) noted three commonalities across most living tetrapods: 1) the long axis of the scapula is typically close to vertical; 2) the coracoid must contact the sternum and 3) the orientation of the sternum tends to parallel the shape of the anteriormost rib terminations (for example, if the first few ribs are of similar length, the sternum lies horizontal, if they increase in length posteriorly, the sternum is inclined). These observations are not universal (birds, for instance, have horizontal scapulae) but they are useful starting points for rebuilding forelimb girdles in many extinct groups.
Cartilage is another major consideration for the skeletal restorer. Sometimes, cartilage is just a thin veneer over a bone joint which aids bone motion; other times, it’s a thick wedge of tissue that forms the articular surface itself. The former tends to occur in well-sculpted and defined bones, the latter on rounded, simple articular surfaces. The amount of cartilage that can occur in a skeleton should not be underestimated. Holliday et al. (2010) found that cartilage accounts for 4–8% of alligator limb bone lengths, while Taylor and Wedel (2013) found inter-vertebral cartilage thickness between 2.6 and 24% of vertebral length in birds and mammals. The latter team also noted that mammals seem to have thicker vertebral cartilage than birds, and that juveniles have proportionally more cartilage than adults. In short, cartilage is a significant component of the skeleton, and its decay before fossilization is a clear hindrance to understanding skeletal dimensions and articular surfaces. Unfortunately, our worst fears about the implications of cartilage loss seem to apply to many fossil animals. Certain dinosaurs and marine reptiles, for example, have rounded, simple limb articulations that must have been augmented in life with thick, complex cartilage caps (Holliday et al. 2010). We can roughly estimate their size by measuring the distance between bones in articulated remains, and these details can then be factored into both skeletal reconstructions as well as considerations of bone mobility. Note that even modest amounts of cartilage can radically alter ranges of bone motion and ‘neutral’ bone poses (Taylor 2014).
Fig. 4.16 Skeletal reconstruction of the pseudosuchian Stagonolepis robertsoni. Both skeletals show the same animal, albeit with the armour removed in the lower image to more clearly show the rest of the skeleton. This is a common convention for armoured fossil animals. (S. Hartman)
Producing a publishable skeletal restoration
A clear, reliable skeletal reconstruction represents a lot of work and can be considered a work of art in its own right. They are worthy of publishing on their own and will almost certainly be used by other artists and scientists. Useful components to add to a skeletal destined for publication include a scale bar (a horizontal or vertical line indicating a measurement proportionate to your drawing); details of which specimens were used; indications of where bones were extrapolated or inferred; and a Knightian body silhouette to show how the skeleton might relate to the overlying soft tissues. Some particularly rigorous skeletal artists, such as Scott Hartman and Gregory S. Paul, often present two skeletal restorations side-by-side: a ‘rigorous’ one, showing what is actually known of an animal (or a specimen), and an ‘inferred’ one, presenting a complete osteology. This is a very useful approach for their fellow artists, allowing us to know aspects have been hypothesized to fill missing data, and what is essentially incontrovertible data about the subject species. Two skeletals are sometimes also published for species covered in bony armour (Fig. 4.16) because, with armour in place, little can be seen of the underlying osteology. In such cases, a ‘naked’ skeleton often accompanies one that shows the distribution of armour in the skin. Hartman (2011) further suggests emphasizing when a skeletal reconstruction is ‘schematic’ – mostly inferred from other species or when bone outlines and proportions are rough – versus ‘realistic’ – skeletal diagrams produced with good data pertinent to that species, and rigorously drawn bones and proportions. There is no shame in producing either – schematic or mostly inferred skeletals are the best we can produce for some animals – but it’s useful to fellow artists to be explicit about how reliable a skeletal restoration is.
Fig. 4.17 How footprints and trackways aid the reconstruction of gait, stance and foot anatomy. The depicted tracks are those of the pseudosuchian Cheirotherium, and the restored animal is Mandasuchus.
The role of footprints, trackways and other trace fossils in extinct animal restoration
Footprints are very useful sources of palaeoart data, providing information on animal posture, behaviour, step patterns and the anatomy of feet and hands (Fig. 4.17). They are thus important throughout the palaeoart process, aiding reconstructions at a very basic level (helping to pose and position animals) as well as at in later stages (adding tissues to their feet and hands). These attributes stem from ichnology: the study of ‘trace fossils’ such as footprints, burrows, root impressions and other evidence of ancient organismal activity.
Fig. 4.18 Some common gaits of quadrupedal animals, and their relationship to trackways. These gaits can be employed by sprawling or erect limbed species. “H” in rectangles shows placement of hindlimb in hypothetical track, “F” in circles shows forelimb.
Sequences of footprints are known as trackways. Although it is rare for trackmaker species to be known with certainty, most tracks and prints are referable to specific fossil groups and provide good artistic guidelines about typical stances and gaits in those clades (Fig. 4.18). Artists looking at tracks might note:
•Which parts of the animal left impressions? Footprints are parts of virtually all trackways, but what about hand-prints, tail drags or belly scrapes? Did the feet clear the ground between steps, or are long drag marks left in the sediment, indicating fingers or toes not entirely clearing the surface when the animal took a step?
•How far are the tracks from the midline of the trackway? Did the animal move by placing its feet directly under its hip and shoulder joints, with the feet under the midline of the body, or displaced far apart, as if the animal was sprawling? If the animal was a quadruped, take note of whether there are spatial differences between the hand and footprints; particularly in sprawling animals, longer limbs can lead to wider tracks for one limb set than another. Be aware that narrow track spacing does not always equate to upright limbs: sprawling animals that swing their bodies laterally as they run can also produce narrow tracks, and placement of hands and feet at the midline does not necessarily mean the elbows or knees were not bowed outward as the animal moved.
•Which way do the hands and feet face? There is a lot of variation in print angulation: some prints project fully sideways, with toes or finger impressions at 90 degrees to the direction of travel; others, like our own footprints, more or less point in the direction that we’re moving in; and many species are somewhere between. Incorrectly orientated appendages are one of the commonest mistakes in palaeoartworks despite, in many instances, trackways giving us a clear indication which way their feet and hands faced when walking or running.
•Do the fingers and toes splay from the base of the print, or are they parallel with one another? How much of the hand or foot contacted the ground? Was the animal locomoting with ankles and wrists touching the ground, or using only its toes and fingers?
•What is the spatial relationship between the hand and footprints? This tells us a lot about gait – the order in which the limbs moved as the animal walked or ran (Fig. 4.18). It’s common to see fossil animals restored with step cycles conflicting with their tracks, so artists should pay close attention to this aspect of track data. Different gaits leave characteristic track patterns that we can use to predict the appearance of limbs in the step cycle. Tracks that are symmetrical across the midline, for instance, indicate an animal moving with left and right limbs in tandem – perhaps through hopping, bounding or galloping (depending on speed, degree of symmetry and number of limbs being used). If the prints are asymmetrical across the midline, the limbs were moving alternately. Of particular interest is whether footprints ‘overstep’ handprints – has the footprint landed on or in front of the corresponding handprint? Unless the footprints are medially offset from the handprints (representing the legs swinging under the body and between the arms), this indicates that the hand was moved before the foot touched the ground, requiring both limbs on one side of the body to move in tandem. This – known as the lateral sequence walk – is a common gait for longlimbed animals like greyhounds, horses, giraffes, camels and – judging from trackways – pterosaurs. Diagonal sequence walks – which are common in many reptiles and amphibians – see the footprint placed behind the handprint, and reflect animals moving diagonally opposing limbs at once (for instance, front left and back right, then front right and back left). Variations to these gaits reflect differences in anatomical form, movement speed and so on, and can be detected with close examination of track data.
•Finally, some context about the track itself should be considered. Animals adapt their gait to the substrate they are moving over, so tracks left in soft, sticky mud will likely capture a different gait to those left in a harder, less restrictive substrate. The tracks of a running animal may be rather different to those of a trotting or walking one. Understanding what behaviour and circumstances are represented by a trackway is crucial to establishing its relevance to an artwork.
Fig. 4.19 High-quality footprints can record unexpected anatomical details, including the nature of the tissues around the toes. These images show exceptionally preserved casts of a Cretaceous theropod foot from Cretaceous strata of Spain – see Huerta et al. (2012) for details.
Anatomical information from foot or handprints is often available from tracks, but frequently ignored in artwork (Fig. 4.19). Good-quality tracks often show cushioning pads we are unable to predict from bone anatomy and, more rarely, casts of entire hands or feet can be left in rock, allowing us to see 3D representations of ancient foot and hand anatomy. Surprises abound once we start looking closely at print morphology. In theropod dinosaurs, for instance, some foot casts show extensive tissues around the bases of claws and pads on the underside of toes. Sauropod dinosaur hands, in contrast, left roughly horseshoe-shaped prints that lacked large cushioning pads: contrary to many palaeoartworks, their hands were not like those of elephants or even other heavyset dinosaurs. Tracks and prints also show whether digits were ‘free’ on the hands and feet or bundled into soft tissue, and give direct insight into whether claws and nails were present. In especially well-preserved prints, details of integument – such as scales, skin ridges and other epidermal structures – can be discerned.
Not all traces are footprints and trackways, however. Other behaviours leave different trace types which are also artistically informative. Impressions of crouching or reclining animals provide details of how fossil animals sat or rested, which is useful data for posing reclined individuals. Feeding traces on shells, bones, leaves and substrata reveal feeding motions and mechanics; and nests and eggs provide detailed insights into ancient reproductive behaviour. Ichnology has a lot to offer palaeoartists, allowing us to fit our pictures around data created by the behaviour of subject organisms. Incorporating this into our artwork is an essential component of executing our restorations credibly.
