Miocene pelagornithid Pelagornis chilensis parents and chicks, what fantastic animals they must have been. Note the lack of pseudoteeth on the chicks, recent work suggests they didn't develop until the cusp of adulthood. Not that I drew them in and removed them at the last minute when researching this post, of course. Given how often I've written about giant pterosaurs, it's peculiar that I've never thought to cover the only flying animals which have ever come close to challenging their size, the pelagornithids: long-winged, often gigantic birds which attained wingspans exceeding 6 m. And yes, a 6 m wingspan is a metric that many pterosaurs - not even just the big azhdarchids - would find endearingly cute, but it's the largest wingspread of any bird and well above the size of any living flying animal. In public engagement pelagornithids are mostly wheeled out to gawk at their size and weird pseudoteeth before being put away again, but there's lots of fascinating anatomy under those big wings, and they deserve a longer period in the spotlight. Pelagornithids - which are sometimes called (the now defunct name) "pseudodontorns" - were a long-lived and globally distributed group, their fossils ranging from Palaeocene - Pliocene rocks of Eurasia, both Americas, Africa, Antarctica and New Zealand (Mayr and Rubilar-Rogers 2010; Bourdon and Cappetta 2012). They were a group of large-bodied pelagic soarers, seemingly adapted for extended periods of flight over seas and oceans. Most of their fossils are - as is typical for birds - pretty fragmentary, but a number of species are are relatively well represented, especially members of the genus Pelagornis. Their soft-tissue anatomy is virtually unknown, save for primary wing feather impressions associated with the holotype of P. orri (Howard 1957). Many pelagornithids are known from single bones or a few pieces of skeletal shrapnel. In having good cranial and limb material, the Oligocene taxon Pelagornis sandersi is among the better known species. Note the difference in size of the humerus (e-f) vs. the hindlimb bones (j-q; femur is j-k, tibiotarsus is l-m, and carpometatatsus is o-p). From Ksepka (2014). Much uncertainty and confusion surrounds the composition of pelagornithid taxa with numerous genera being considered invalid or synonymous with others. This problem is rooted in over-enthusiastic naming of undiagnostic, often crushed scraps of bone as well as a lack of comparable anatomies between holotypes. Evaluation of Neogene pelagornithid material has suggested that most genera are weakly supported, leading Mayr and Rubilar-Rogers (2010) to suggest that all Miocene and Pliocene taxa (which includes Osteodontornis, Pseudodontornis, Neodontornis and possibly Cyphornis) should be sunk into Pelagornis. It's a little difficult to say how speciose Pelagornithidae is given the fluidity of their taxonomy but, as a rough figure, it seems to be composed of a little over a half-dozen Neogene Pelagornis species and a handful of Palaeogene representatives. All pelagornithids share the same basic "long-winged and pseudotoothed" bauplan, but the characteristic anatomies and proportions of the group are most expressed in later taxa, such as Pelagornis. The confusion over pelagornithid systematics is not confined to generic relationships. Their placement among other birds has been the source of much discussion and controversy, and it's perhaps best to regard their affinities as currently uncertain. Initially regarded as possible relatives of Pelecaniformes (classically thought to contain pelicans, cormorants, gannets and so on - the situation has changed since then), Procellariiformes (tube-nosed birds, including albatrosses) or Ciconiiformes (storks and allies), Bourdon (2005) found stronger evidence linking pelagornithids with Anseriformes - the same group that includes ducks, geese and screamers. Numerous features of the skull and forelimb support this affinity, as do some features of skull development (Louchart et al. 2013). An affinity with waterfowl might seem bizarre for these ocean-going giants but Anseriformes have a long and varied evolutionary history: this is the same branch of avian evolution that (probably) begat the giant, flightless gastornithids and mihirungs, as well as the wader-like Presbyornis. Viewed from a geological perspective instead of a modern one, Anseriformes are not just birds that honk and quack. But while an anseriform affinity for pelagornithids has not being dismissed out of hand, the idea is not without critics. Some pelagornithid anatomies - such as their sterna - are not anseriform like (Mayr et al. 2008), and other features imply a position outside the anseriform-galliform clade (Galloanserae: crudely, the duck-chicken group) without qualifying for entry into Neoaves (all living birds but ratites and galloanserans) (Mayr and Rubilar-Rogers 2010; Mayr 2011). So while these studies broadly agree that pelagornithids emerged from fairly rootward stock among Aves, and that they are not closely related to any modern soaring birds, further work, and maybe more fossils, are needed to clarify their actual phylogenetic position. Pelagornithid primary wing feather impressions associated with the holotype of Pelagornis orri. It's not known is these represent the longest feathers of the wing, but they still have a useful role to play in reconstructing pelagornithid wingspans. From Howard (1957). Size-off: Pelagornithids vs. Argentatvis All pelagornithids are characterised by large size with even the earliest, smallest taxa being comparable to big albatrosses in wingspan (Bourdon 2005). But how big did they get? I know several readers are already sharpening their comment knives about my introduction suggesting that pelagornithids are avian wingspan record holders, thinking I've forgotten about the giant, 7 m wingspan Miocene teratorn Argentavis magnificens. But that's not a mistake: pelagornithids really should be considered the record holders for avian wingspans, and Argentavis isn't as big as most people imagine. Classic image of teratorn researcher Kenneth E. Campbell posing with a 25ft wingspan (7.62 m) silhouette model of Argentavis magnificens at the National History Museum of Los Angeles. Alas, Argentavis wasn't quite as big as depicted here. From Campbell (1980). Some giant pelagornithids, such as Pelagornis chilensis, are unusual among giant fossil fliers in being represented by relatively good skeletal material and their feathered wingspan estimates of 6 m or more can be considered trustworthy, reliable figures. Giant Argentavis magnificens, on the other hand, are known from fragmentary remains and some degree of uncertainty surrounds their wingspans: estimates have ranged from 5.7 to 8.3 m (e.g. Campbell and Tonni 1983; Chatterjee et al. 2007). Recent workers have suggested that the lower range of these estimates is more likely. When describing P. chilensis, Mayr and Rubilar-Rogers (2010) noted that the 82 cm long humerus of their pelagornithid was vastly bigger than the 57 cm long Argentavis humerus, and that scaling the latter to proportions seen in smaller teratorns yields a wing skeleton length of 183 cm. If so, the bony wing spread of the largest Argentavis might have struggled to reach 4 m, and this is before assuming any flex in the wing bone joints. And no, the addition of feathers does not bring Argentavis into record-breaking territory. Ksepka (2014) predicted that the primary feathers of Argentavis would need to be about 1.5 m long to reach a 7-8 m wingspan, a length that would exceed the primary feather: wingspan ratio of all living birds as well as contradict the observation that primaries tend to scale with negative allometry against wingspan. Accordingly, Ksepka (2014) suggested Argentavis was more reliably sized at a 5.09 - 6.07 m wingspan, with estimates at the lower end of that range being predicted in most models. In contrast, the wing skeletons alone of P. chilensis and P. sandersi easily exceed wingspans of 4-5 m, and the addition of conservatively estimated feather lengths easily raise these wingspans into the 6-7 m range. Skeletal reconstructions of giant pelagornithids: the holotype of Pelagornis chilensis (ventral view) and P. sandersi (dorsal view). That bird to the right of the image is a little thing called the wandering albatross, which has the largest wingspan of any extant flying bird. Pelagornithids must have been amazing to see in life. Images from Mayr and Rubilar-Rogers (2010) and Ksepka (2014). Despite their size, pelagornithids were not heavy animals. Mass estimates for giant pelagornithids are in the region of 16–29 kg for P. chilensis and 21.9–40.1 kg for P. sandersi (Mayr and Rubilar-Rogers 2010; Ksepka 2014). Given that these birds are twice the size of albatrosses, the fact that their predicted masses match, or only double, the maximum masses of extant flying birds are surprising. Remember that mass increases by a factor of eight for every doubling of a linear dimension so, if we scaled wandering albatross (using masses given at Wikipedia) to Pelagornis proportions we’d expect a mass of 50-96 kg - well above those predicted figures. The pelagornithid weight-watching secret seems to lie in their unique wing proportions: even more than albatross, pelagornithids have extremely long wings compared to the rest of their bodies, and can thus attain giant wingspans while keeping their masses low. Similar tactics were also exploited by giant pterosaurs: maximising wing area while keeping the body small is a great way to maintain volancy at large size. Extremely thin walled bones also helped pelagornithids maintain low masses (and also explains why so many pelagornithid fossils look like they've been hit with a bulldozer). Biological sailplanes The largest pelagornithids were of a size which exceeds some theoretical flight limits for albatross-like birds (e.g. Sato et al. 2009), though the plainly obvious flight adaptations of their skeletons suggest this problem lies with our calculations and not the concept of pelagornithid flight itself. Indeed, glide analyses of P. sandersi indicate a supreme soaring capability with a very low sink rate (the rate at which altitude is lost during gliding) and high glide speeds, a combination that would facilitate extremely wide-ranging, energy efficient flight (Ksepka 2014). Their flight performance seems generally more akin to that of albatross than other pelagic birds, so reconstructions of pelagornithids riding air currents between waves, buzzing along the water surface and cruising on ocean winds seems sound. Reduced hindlimb proportions indicate that pelagornithids were probably not capable walkers or runners however, and we might envisage them only landing infrequently, perhaps most commonly when nesting. Curiously flattened and wide toe bones recall those of birds which use their feet as air brakes when landing (Mayr and Rubilar-Rogers 2010; Mayr et al. 2013), and may also have aided stabilisation on land (Mayr et al. 2013). Predictions of glide ability and lift:drag ratios in P. sandersi from Ksepka (2014). Note how both models compare very well to albatross flight (black), but less well with frigate bird (red) or raptor flight (green). Maintaining flight is a relatively easy part of aerial locomotion: how pelagornithids became airborne is trickier to fathom. This is mostly because of several indications of a limited flapping ability in the largest Neogene species, which are also the ones that would struggle the most with launch. Scaling of muscle energy availability predisposes all large flying animals to a relatively limited flapping capability and, like all fliers operating at the upper limit of their respective bauplan, pelagornithids likely relied on short-lived bouts of powerful anaerobic muscle activity to perform flapping (Ksepka 2014). But there is some question over whether they could flap their wings at all: several osteological features suggest pelagornithids had reduced shoulder/humeral motion (including a lack of rotatory capability) and lessened downstroke musculature (Mayr et al. 2008). A lack of dynamism is also seen elsewhere on the wing in that the articulation for the alulua was weakly developed, prohibiting spread of this structure during takeoff and landing (Mayr and Rubilar-Rogers 2010). The alula, when extended, allows the wing to function at higher angles of attack (the angle of the wing relative to the direction of airflow) and is thus very useful in initiating flight, controlling landing and general aerial manoeuvrability. Its immobility in pelagornithids would have impacted their range of flight dynamics quite considerably. The large size of pelagornithids means that a very limited, maybe absent flapping ability may not be as detrimental as we intuitively predict. Flapping motions - both frequency and amplitude - reduce against increasing wing area and flight speed (the latter being predicted as high for any giant flier) so, as the largest flying birds of all time, pelagornithids may not have missed flapping as much as you'd think. But nonetheless, a significantly reduced flapping capacity and limited alula motion may have demanded fairly specialised launch and landing behaviour. Pelagornithids may have been limited to launching by simply extending their wings and using running, gravity or headwinds to find sufficient glide velocity. Landing, by contrast, may have involved low-angle approaches, slowing as much as possible (a dangerous game, as slower gliding also brings higher sink rates) and ditching to the ground. I can entirely believe that undignified semi-crash landings were common in this group. If our understanding of pelagornithid flight is accurate, typical seabird behaviours like cliff-nesting - demonstrated here by northern gannets (Morus bassanus) - can be ruled out. Long winged seabirds are not the most agile fliers, but many still have enough control over their initiation and cessation of flight to land on small ledges. Pelagornithids trying this may have ended up splattered, Wile E. Coyote-style, on the side of a cliff. Photo by Georgia Witton-Maclean. Being so light relative to wingspan would assist in both takeoff and landing, but nevertheless question marks hang over their ability to achieve flight in some conditions, such as escaping water (Ksepka 2014). Perhaps, like frigate birds, pelagornithids avoided entering water (though the former struggle with water escape because of waterlogged feathers rather than restricted flapping kinematics). I wonder if this is the case however, it being historically proposed that (unrealistically lightweight) giant pterosaurs could achieve flight from water by simply spreading their wings and catching wind (e.g. Bramwell and Whitfield 1974). The predicted pterosaur masses, wingspans and wing area models used in these old pterosaur studies are not far off those modelled today for giant Pelagornis: if so, could pelagornithids have escaped water with the same wing-spreading trick? It would be interesting to see this modelled biomechanically for a pelagornithid-specific model. On land, we may assume that pelagornithids favoured open space that permitted full 6-7 m wing spreads for launching and landing, and it would not be surprising if they favoured windy, elevated coastal regions that provided environmental launch assistance. I'm not sure what their prospects for flight in continental habits are, but it probably wasn't good: they almost certainly stuck to oceanic soaring, as suggested by the skew of their fossils to marine sediments. They're only pseudoteeth, but I like them We’ve made it all the way through this post without discussing the other characteristic anatomy of pelagornithids: their ‘pseudoteeth’. These structures are bony outgrowths of the jaw bones which strongly resemble actual dentition, though histological studies have verified that they lack all tissues associated with true teeth (Howard 1957; Louchart et al. 2013). It’s thought that pseudoteeth compensated for a well-developed hinge in the lower jaw that permitted wide horizontal bowing during feeding. This allowed for large prey to be swallowed, but compromised overall jaw integrity and potentially risked the loss of slippery seafood prey. A set of variably sized spikes along the margins of the beak is a great way to ensure snagged foodstuffs - thought to be mainly surface-seized fish or squid - did not slip from their beaks. Though many of the spikes are hollow, jaw bone surficial textures indicate that the entire jaw - pseudoteeth and all - was covered with a cornified sheath typical of other bird beaks (Louchart et al. 2013). As we discussed when looking at mammal horns, that’s a pretty potent combination for maximising lightness with strength, though the bone forming the pseudoteeth was mechanically weak and, despite their ferocious appearance, they were not adapted for tackling large, formidable prey (Louchart et al. 2013). Holotype skull of P. chilensis in lateral view: check out those pseudoteeth. From Mayr and Rubilar-Rogers (2010). That pelagornithid teeth functioned well as fish-grabs is suggested in their similarity in size and distribution to the dentition of other fish eaters, including certain crocodylians, large predatory fish, pterosaurs and temnospondyls. Quite how pelagornithids caught their prey is not well understood: if they could enter the water, they may have foraged from the water surface or dived; if not, they may have snatched prey from the water surface or stole it from other birds. Further research into pelagornithid flight capabilities and launch kinematics would narrow down this range of possibilities. Recent studies have shown that pseudoteeth erupted from the jaw relatively late in pelagornithid growth (Louchart et al. 2013), meaning juvenile Pelagornis would have looked like regular, cute baby birds before developing their toothy smiles as adults. This has several interesting implications for pelagornithid growth and ecology. The first is that the cornified beak tissue covering their jaws must not have hardened until after the teeth had fully developed (recall from a previous post that cornified sheaths, on account of being inert, dead tissue, can’t be easily modified once deposited). This characteristic is not common among birds, but occurs in a number of Anseriformes. This observation is not a deal clincher for the pelagornithid-anseriform phylogenetic hypothesis, but it's an interesting connection nonetheless. Secondly, studies show that the emerging pseudoteeth were relatively delicate and potentially unable to withstand stresses imparted by thrashing fish or squid until late in development. This being the case, Louchart et al. (2013) proposed that pelagornithids might have been altricial, feeding regurgitated food to their offspring until they were fully grown and able to forage for themselves; or else that the juveniles were foraging on different foodstuffs. Altriciality would be unusual behaviour for a stem-neoavian as most bird species of this grade have precocial offspring that feed themselves straight after hatching. Insight into these hypotheses would be provided by fossils of juvenile pelagornithids but these remain extremely rare. I wonder if these animals were like living pelagic birds and nested atop cliffs in isolated offshore settings? If so, I wouldn’t hold your breath waiting for fossils of their hatchlings. Enjoy monthly insights into palaeoart, fossil animal biology and occasional reviews of palaeo media? Support this blog for $1 a month and get free stuff! This blog is sponsored through Patreon, the site where you can help online content creators make a living. If you enjoy my content, please consider donating $1 a month to help fund my work. $1 might seem a meaningless amount, but if every reader pitched that amount I could work on these articles and their artwork full time. In return, you'll get access to my exclusive Patreon content: regular updates on research papers, books and paintings, including numerous advance previews of two palaeoart-heavy books (one of which is the first ever comprehensive guide to palaeoart processes). Plus, you get free stuff - prints, high quality images for printing, books, competitions - as my way of thanking you for your support. As always, huge thanks to everyone who already sponsors my work! References Bourdon, E. (2005). Osteological evidence for sister group relationship between pseudo-toothed birds (Aves: Odontopterygiformes) and waterfowls (Anseriformes). Naturwissenschaften, 92(12), 586-591. Bourdon, E., & Cappetta, H. (2012). Pseudo-toothed birds (Aves, Odontopterygiformes) from the Eocene phosphate deposits of Togo, Africa. Journal of Vertebrate Paleontology, 32(4), 965-970. Bramwell, C. D., & Whitfield, G. R. (1974). Biomechanics of Pteranodon. Phil. Trans. R. Soc. Lond. B, 267(890), 503-581. Campbell Jr, K. C. (1980). The world's largest flying bird. Terra, 19(2), 20-23. Campbell Jr, K. E., & Tonni, E. P. (1983). Size and locomotion in teratorns (Aves: Teratornithidae). The Auk, 390-403. Chatterjee, S., Templin, R. J., & Campbell, K. E. (2007). The aerodynamics of Argentavis, the world's largest flying bird from the Miocene of Argentina. Proceedings of the National Academy of Sciences, 104(30), 12398-12403. Howard, H. (1957). A gigantic" toothed" marine bird from the Miocene of California. Santa Barbara Museum of Natural History, Department of Geology Bulletin, (1), 1-23. Ksepka, D. T. (2014). Flight performance of the largest volant bird. Proceedings of the National Academy of Sciences, 111(29), 10624-10629. Louchart, A., Sire, J. Y., Mourer-Chauviré, C., Geraads, D., Viriot, L., & de Buffrénil, V. (2013). Structure and growth pattern of pseudoteeth in Pelagornis mauretanicus (Aves, Odontopterygiformes, Pelagornithidae). PloS one, 8(11), e80372. Mayr, G. (2011). Cenozoic mystery birds–on the phylogenetic affinities of bony‐toothed birds (Pelagornithidae). Zoologica Scripta, 40(5), 448-467. Mayr, G., Hazevoet, C. J., Dantas, P., & Cachão, M. (2008). A sternum of a very large bony-toothed bird (Pelagornithidae) from the Miocene of Portugal. Journal of vertebrate Paleontology, 28(3), 762-769. Mayr, G., & Rubilar-Rogers, D. (2010). Osteology of a new giant bony-toothed bird from the Miocene of Chile, with a revision of the taxonomy of Neogene Pelagornithidae. Journal of Vertebrate Paleontology, 30(5), 1313-1330. Mayr, G., Goedert, J. L., & McLeod, S. A. (2013). Partial skeleton of a bony-toothed bird from the late Oligocene/early Miocene of Oregon (USA) and the systematics of neogene Pelagornithidae. Journal of Paleontology, 87(5), 922-929. Sato, K., Sakamoto, K. Q., Watanuki, Y., Takahashi, A., Katsumata, N., Bost, C. A., & Weimerskirch, H. (2009). Scaling of soaring seabirds and implications for flight abilities of giant pterosaurs. PLoS One, 4(4), e5400.

Today we in the Open Culture Program are releasing a new publication: Don’t be a Dinosaur; or, The Benefits of Open Culture.

Europasaurus holgeri - twice. These portraits are of the same animal using the same specimen and the same view, but one is restored with extreme shrink-wrapping (above) and the other has a more generous amount of facial tissue (below). But which one is more plausible, and can we even tell from fossil bones alone? You can't move around palaeoart circles on the internet nowadays without someone being criticised for 'shrink-wrapping' their reconstruction. This refers to the convention of restoring extinct animals with minimised soft-tissues, allowing details of muscle layouts and major skeletal contours to be seen in allegedly healthy living animals. At its most extreme, this includes clearly visible ribs and vertebrae, tissues sunk into skull openings, ultra-prominent limb girdles and skinny, sinewy legs. We owe the term 'shrink-wrapping to sauropod expert and SV:POW author Mathew Wedel who, in a 2010 article, compared the contour-hugging soft-tissues of these restorations to items wrapped in tight plastic for transport. Shrink-wrapping is a well known convention among those interested in palaeoart but is a relatively modern invention. Palaeoartists restored ancient animals with relatively bulky soft-tissues until the end of the 20th century to an extent where visible deep-tissue anatomy is genuinely exceptional in pre-modern palaeoart (a well known exception are ichthyosaur sclerotic rings, reflecting erroneous interpretation of these structures among early palaeontologists - see Buckland 1836). Shrink-wrapping became popular as conservative reconstruction approaches became dominant in the 1970s and went on to become a standard palaeoart convention soon after. Many, perhaps most, of the restorations produced by late 20th century artists employed shrink-wrapping and it remains conspicuous in artwork produced today. It has even spawned related traditions, such as tightly cropping fur and feathers to ensure animal shapes remain obvious, and has influenced approaches to restoring colour and skin texture, these elements being used to outline the topography of underlying bones. Famous shrink-wrappers include artists like Gregory S. Paul and Mark Hallett, who tend to be on the less dramatic side of the tradition, showing slight contours of the skull features alongside lean, though well-muscled, bodies and limbs. More extreme shrink-wrappers, like Ely Kish and William Stout, have works where shrink-wrapping is taken to a wholly unrealistic level. Gaping vacuities exist between neck vertebrae; rib cages and limb girdles bulge from the torsos; limbs are extremely thin and faces are lipless and gaunt. It’s difficult not to look at some of these works and not think of starving animals or even decaying remains: they do not look like healthy, virile beings. William Stout's Quetzalcoatlus, posted at Love in the Time of Chasmosaurs, has to be the most shrink-wrapped being ever rendered in paleoart. If it had any less tissue we'd be looking at moulds of the internal organs. We might assign three reasons for the popularity of shrink-wrapping. The first is that its development 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 and contrasted newer art from the plodding, perhaps over-voluminous animals of previous generations. Shrink-wrapping is not a dinosaur-exclusive tradition of course, but the popularity of these reptiles means that palaeoart conventions applied to dinosaurs are inevitably followed in artworks of other species. Secondly, images of prehistoric animals as heroically-built, powerful beings are preferred by many merchandisers and palaeoart fans, these interpretations most closely matching the erroneous but 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 the osteological information provided by fossils. How shrink-wrapping became unfashionable Nowadays, shrink-wrapping is losing popularity among some parties as scientists and artists note a simple, but obvious problem: modern animals are generally not shrink-wrapped in the way we draw their extinct relatives. The most famous counter-shrink-wrapping arguments are in All Yesterdays (Conway et al. 2012) but something of an anti-shrink-wrapping movement was underway from the mid-2000s onward. Some now argue that, while champions of the rigorous reconstruction movement were right to draw attention to the true shapes of fossil animals and to emphasise their form in art, they might have gone too far in thinning out skin, muscle, fats and other tissues. Few animals have deeply sunken tissues over skull fenestra or distinctions in skin colour and texture correlating with skeletal anatomy, and no animals witnessed outside of veterinary clinics have detailed limb bone outlines projecting through their skin. Even reptiles - meant to be the living poster boys of shrink-wrapping - have a suite of elaborate, contour-altering soft-tissues. They include 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 swell from their skull housing; thick or pointed scales and, in some species, even expansive, mostly cartilaginous noses. Matt Wedel's touching plea to end shrink-wrapping, from 2011. The struggle is still real: if you have spare paint, pixels clay or graphite, please donate generously. Nowadays, many view skeletal elements as providing an important palaeoartistic foundation for soft-tissue shape, but concede that overlying tissues must have smoothed-over skeletal contours to produce 'softer' body forms. Indeed, there's something of an collective interest in knowing how deep extra-skeletal tissues can get. The answer, it seems, is 'very'. The necks of many birds and mammals are often flexed at much higher angles than we would assume based on their external appearance because their overlying tissues are so thick that the entire neck skeleton posture is hidden (Taylor et al. 2009). The muscles and bones of major anatomical elements – such as necks and proximal limb segments – can also be obscured under skin, fat and integument. Contour-altering structures like horns, spikes, spines, combs, humps, armour, fins, and webbing are often composed of soft-tissue, and the large, savage-looking teeth of mammals and lizards can be completely obscured by facial tissues. We need only look at x-rays of living animal species to see their often-startling lack of correlation between external appearance and internal anatomy. Even seals get in on this action, as evidenced from this Irish Seal Sancutary x-ray. Their site appears to be down at time of writing, but SV:POW! has this image hosted there for the time being. It's from this general train of thought that a push for more bulk, fuzz and fat in palaeoart has been born, and this general philosophy is lining up well with fossil data. We have direct evidence that the bodies of ichthyosaurus (Stenopterygius) and mosasaurs (Prognathodon) bore tall fins and paddle extensions that vastly exceeded 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), Tanystropheus and ‘mummified’ hadrosaurs (multiple taxa) show extensive muscle volume that bury their skeletons as well as elaborate structures – soft-tissue filaments, combs and skin membranes – that defy ‘shrink-wrapping’ conventions (e.g. Mayr et al. 2002; Renesto 2005; Bell 2014). The feather outlines on innumerable fossil theropods show that they were just as densely feathered as modern avians, and the fuzzy ‘halos’ of fossil mammals and pterosaurs suggest they too 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 crest tissues that create body outlines more voluminous than those predicted from musculoskeletal restorations (e.g. Frey and Martill 1998; Czerkas and Ji 2002). The 'shrink-wrapping hypothesis' is being falsified with regularity. Select fossilised body outlines of exinct taxa: no shrink-wrapping here. A, plesiosaur Mauriciosaurus fernandezi, B, ichythyosaur Stenopterygius quadriscissus; C, dromaeosaur Sinornithosaurus millenii. A, after Frey et al. 2017; B after McGowan and Motani 2003. Anti-anti-shrink-wrapping But while cries of 'bulkier, deeper, fuzzier!' are generally well-placed in palaeoart discussions, we should be careful not to overshoot the mark. Amid the cry for deeper tissues, we might be overlooking the fact that some living creatures are somewhat shrink-wrapped - at least in some regions. In fact, virtually animals have areas where their extra-skeletal tissues are shallow and skeletal contours are visible. Common areas of thin tissue include the ends of limbs and tails; the midline of the sternal region; and some areas of the face, such as the frontal and nasal regions; the ‘cheek region’ (over the jugal in birds and reptiles, and the zygomatic arch in mammals), and the lower margins of the bottom jaw. Our own anatomy is no exception to these trends, as is borne out by the extremely well-studied tissue depths of human faces (e.g. Stephan and Simpson 2008) or the simple act of looking in a mirror. The osteoderms of sauropsids are another example of close interaction between skin and bone: as with modern armoured reptiles, extinct scaly sauropsids with extensive osteoderm arrangements probably looked pretty darn like their fossil remains - in other words, kinda shrink-wrapped. There is no tissue, only Zuul. In reality, there is a spectrum of tissue depth in living species and some are more 'shrink-wrapped' than others. While no healthy living animal attains the most extreme levels of shrink-wrappery portrayed in palaeoartworks, certain lizards, fish, and crocodylians have anatomies which are more shrink-wrapped than average, possessing large areas of relatively thin, skull-hugging tissues which recall shrink-wrapped art. These thin tissues are highly characteristic of these species and are something something palaeoartists would want to capture if restoring these animals from fossils. We would miss this, however, if we assume that all animals have their tissue volume settings cranked up to maximum. These observations mean we have to be careful with applying a general philosophy to shrink-wrapping rather than scientific investigation. Tissue depth is evidently not a matter of palaeoartistic style or fashion, but a biological variable we should be aiming to predict and infer. If we're aiming to approach this topic like scientists, we should look to see what fossils and comparative anatomy can tell us about tissue depth to make informed, specific predictions about extinct animal appearance and avoiding a one-size-fits-all 'anti-shrink-wrap' philosophy. So, is there anything in the fossil record that elucidates how deeply buried animal skeletons were under muscle, skin and so on? Looking for clues of 'shrink-wrapped' tissues Frustratingly, one of the first lines of evidence we have to jettison are those body outline fossils. As great as they are, they can be of limited use for determining subtle variation in tissue thickness as their shapes are readily altered by taphonomy, preservation styles and even our own preparation work. Regions of thin tissue depth will be were especially sensitive to destructive processes and are easily obliterated by imperfect preservation or human error, so their chances of preservation are minimal. Phylogenetic bracketing is also of limited utility because the vastly different cranial architecture of extant and extinct animals makes such investigations almost meaningless. Non-avian dinosaurs, for instance, have skulls which are neither truly croc-like or bird-like, and it's probably not sensible to assume their extant relatives provide reliable insights into their facial tissues. Predicting regions of thin tissue is thus largely left to comparative anatomy - predicting minimised tissue volumes using fossil bones and the living structural analogues. Among extant species, we see shrink-wrapping largely applying to animal faces, so if we investigate the skulls of ‘soft-faced’ animals like mammals, monitor lizards, snakes, and certain birds, and compare them to species with shrink-wrapped faces, like turtles, crocodylians, chameleons and well-ossified fish, we might find characteristics that correlate with facial tissue depth. These will then give us some criteria to assess tissue depth in fossil species. I've had a go at this, and suggest that osteological attributes related to facial tissue depth include: How might we predict shrink-wrapping in fossil animals without good soft-tissue remains? It's challenging, but these attributes might give a general idea. From top to bottom: Burchell's zebra (Equus quagga burchellii); water monitor (Varanus salvator); Alligator mississipiensis and Arrau turtle (Podocnemis expansa). 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 these openings are not fully bordered by bone (e.g. many mammal orbits and nares, the lower temporal fenestrae of lizards). Larger skull openings necessitate a larger fraction of face structure be composed of soft-tissue, such as muscle, organs, and cartilage, and this overwhelms the contours of the bony skeleton to make a 'soft-faced' species. The nasal cartilages of monitors and mammals, as well as bulging mammalian jaw muscles, are examples of this. Conversely, shrink-wrapped species have smaller cranial openings, which impose physical limitations on how much soft-tissue can form the shape of the face. Muscles and organs might protrude from these somewhat, but their impact on facial structure is less than that of species with large skull openings, and more of the face shape reflects bony contours Rugosity. Soft-faced animals tend to have smooth bone textures with limited or no areas of rugosity, whereas the skulls of shrink-wrapped species have large areas of rugose textures, often corresponding to specific epidermal features (e.g. scales or keratinous sheaths - see below and Hieronymus et al. 2009). This factor largely seems to reflect the proximity of epidermal tissue, which can leave characteristic textures in species with tightly-bound skin. Soft-faced species generally lack this rugosity because muscles, fat and voluminous integuments (fur and feathers) don’t leave broad osteological features (Hieronymus et al. 2009), or simply because their skin is displaced far enough from the bone that it doesn't 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 can be 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 tissue configuration which could - among other things - reflect a deeper or 'softer' facial covering. Fossil skulls - like those of the centrosaurine Centrosaurus apertus - are covered with features that allow us to predict aspects of their facial skin. Often - as is the case here - they suggest fairly low-volume structures, like scales and horn sheaths, which generally don't deviate too much from the underlying bone (yes, I know there are exceptions, but we're looking for major trends here). Centrosaurus skull redrawn from this Wikipedia photo, data on facial tissue correlates from Hieronymus et al. (2009). 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 their jaws and 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 face skeletons. Correlates for epidermal projections. Elaborate skin projections – such as soft-tissue horns or crests - leave characteristic osteological signatures (Hieronymus et al. 2009). Given that these projections can alter animal faces quite substantially from the underlying skull shape, the presence of these is a clear indication that the species was not shrink-wrapped. We would expect a lack of correlates for epidermal projections in shrink-wrapped species. As is often the case with zoological topics there are exceptions to these observations that preclude using any one of these criteria in isolation to determine tissue depth (e.g. smooth bone textures can underlie thin naked skin, so are not always a hallmark of deep tissues). However, applied collectively, they might give a general insight into how shrink-wrapped or 'soft-faced' an extinct animal was. I'm encouraged to see that these proposed osteological features of soft- and shrink-wrapped faces covaried in the past as much as they do for modern species. This doesn't mean these criteria are 'correct' as goes their relationship to tissue depth, but at least shows there's variation in their skull architecture that we can recognise as equivalent to that of modern species, and it isn't unreasonable to think the variance might reflect the same anatomical factors. If we apply these criteria to some fossil taxa, what predictions might we make? The roomy, smooth-boned and foramina-lite skulls of cynodont-grade synapsids and fossil mammals match predictions for ‘softer-faced’ species, and this might be true of some fossil reptiles – like sauropod dinosaurs - too (this is not a new conclusion: both Matt Wedel and Darren Naish have been saying similar things about sauropods for years). If right, the 'soft-faced' sauropod that greeted you at the start of this post might be more likely that the shink-wrapped toilet-headed version we're so familiar with. At the other end of the spectrum, the highly textured, pitted bones and solidly-built skulls of ankylosaurs and anamniotes meet our criteria for shrink-wrapping very well, and they likely had facial anatomy tightly conforming to their skull shapes. Applying the criteria outlined above might help us roughly sort predict 'shrinkwrapped', 'soft-faced' or intermediary conditions in extinct taxa. The placements of the animals here are only rough, but give an indication of their relation to the tissue-depth criteria outlined above. Fingers crossed that some of these will be corroborated or refuted with soft-tissue discoveries in future. Careful examination of fossil skulls allows us to also predict partial or regionalised 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 depth over much of their skull but smooth, foramina-lite jaw margins. In life, these animals have shrink-wrapped dorsal skull regions and snouts, but vast, fleshy lips, which is what we might predict based on their skull anatomy. Partial facial shrink-wrapping seems apt for many fossil species. Gorgonopsians, for instance, might not have soft faces like living mammals as their snouts and foreheads are quite rugose and their nasal openings are small (e.g. Kammerer 2016). These features might indicate the presence of tighter skin over the snout. However, they have few jaw foramina and relatively open regions for jaw musculature, so they might have been fleshier around their jaw margins and at the back of head (below). Tyrant dinosaurs have skulls with relatively small openings compared to some of their theropod relatives, rugose snout textures, several hornlets (Carr et al. 2017), as well as a slightly elevated foramina count (Morhardt 2009). This cranial anatomy is consistent with tighter tissue depth in several areas, if someway short of a fully lipless, crocodylian-like degree of shrink-wrapping. Many pterosaurs show pitting and vascular canals embedded into their jaw margins, and some species have indications of tight 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 faces might not have been entirely skeletal. Was gorgonopsian Inostrancevia shrink-wrapped or soft-faced? According to the criteria of this post, maybe a little from column A, a little from column B. Time and testing will tell whether these criteria are a genuinely useful means to predict facial anatomy. I hope - as with other aspects of extinct animal appearance - that genuine research into this issue will be carried out one day. Criteria to predict tissue-depth are a desirable tool for any palaeoartist as it's simply more honest and scientific: if we're serious about this reconstructing extinct animals gig, predictive methods and sound hypotheses are infinitely better than sticking to our personal hunches, guesses or erring on what looks coolest. Regardless of whether we can predict tissue depth or not, the take home here is that we should not approach our artwork having already decided how thin or fat the tissue volumes of our subjects will be. There is probably not a single ‘universal truth’ that can be said about restoring tissue depth for all animals, whether we err toward thicker or thinner: the right tissue depth is the most defensible and best rationalised on for each subject and its constituent body parts. Enjoy monthly insights into palaeoart and fossil animal biology? Consider supporting this blog with a monthly micropayment, see bonus content, and get free stuff! This blog is sponsored through Patreon, the site where you can help online content creators make a living. If you enjoy my content, please consider donating $1 a month to help fund my work. $1 might seem a meaningless amount, but if every reader pitched that amount I could work on these articles and their artwork full time. In return, you'll get access to my exclusive Patreon content: regular updates on research papers, books and paintings, including numerous advance previews of two palaeoart-heavy books (one of which is the first ever comprehensive guide to palaeoart processes). Plus, you get free stuff - prints, high quality images for printing, books, competitions - as my way of thanking you for your support. As always, huge thanks to everyone who already sponsors my work! References Bell, P.R. (2014). A review of hadrosaurid skin impressions. In D.A. Eberth and D.C. Evans (eds.) The Hadrosaurs: Proceedings of the International Hadrosaur Symposium. Indiana University Press, Bloomington and Indianapolis, pp. 572–590. Buckland, W. (1836). Geology and mineralogy considered with reference to natural theology (Vol. 1). Carey, Lea and Blanchard. Carr, T. D., Varricchio, D. J., Sedlmayr, J. C., Roberts, E. M., & Moore, J. R. (2017). A new tyrannosaur with evidence for anagenesis and crocodile-like facial sensory system. Scientific Reports, 7. Conway, J., Kosemen, C. M., Naish, D., & Hartman, S. (2013). All Yesterdays: Unique and Speculative Views of Dinosaurs and Other Prehistoric Animals. Irregular books. Czerkas, S. A., & Ji, Q. 2002). A new rhamphorhynchoid with a headcrest and complex integumentary structures. Feathered Dinosaurs and the origin of flight, 1, 15-41. Frey, E., & Martill, D. M. (1998). Soft tissue preservation in a specimen of Pterodactylus kochi (WAGNER) from the Upper Jurassic of Germany. Neues Jahrbuch fur Geologie und Palaontologie-Abhandlungen, 210(3), 421. Frey, E., Mulder, E. W., Stinnesbeck, W., Rivera-Sylva, H. E., Padilla-Gutiérrez, J. M., & González-González, A. H. (2017). A new polycotylid plesiosaur with extensive soft tissue preservation from the early Late Cretaceous of northeast Mexico. Boletín de la Sociedad Geológica Mexicana, 69(1), 87-134. Hieronymus, T. L., Witmer, L. M., Tanke, D. H., & Currie, P. J. (2009). The facial integument of centrosaurine ceratopsids: morphological and histological correlates of novel skin structures. The Anatomical Record, 292(9), 1370-1396. Kammerer, C. F. (2016). Systematics of the Rubidgeinae (Therapsida: Gorgonopsia). PeerJ, 4, e1608. Lindgren, J., Kaddumi, H. F., & Polcyn, M. J. (2013). Soft tissue preservation in a fossil marine lizard with a bilobed tail fin. Nature Communications, 4, 2423. Mayr, G., Peters, S. D., Plodowski, G., & Vogel, O. (2002). Bristle-like integumentary structures at the tail of the horned dinosaur Psittacosaurus. Naturwissenschaften, 89(8), 361-365. McGowan, C. & Motani, R. (2003). Part 8 Ichthyopterygia. Sues H–D (ed.) Handbook of Paleoherpetology. Munchen: Verlag Dr. Friedrich Pfeil. 175 p. Morhardt, A. C. (2009). Dinosaur smiles: Do the texture and morphology of the premaxilla, maxilla, and dentary bones of sauropsids provide osteological correlates for inferring extra-oral structures reliably in dinosaurs?. Western Illinois University. Renesto, S. (2005). A new specimen of Tanystropheus (Reptilia Protorosauria) from the Middle Triassic of Switzerland and the ecology of the genus. Rivista Italiana di Paleontologia e Stratigrafia (Research in Paleontology and Stratigraphy), 111(3). Stephan, C. N., & Simpson, E. K. (2008). Facial soft tissue depths in craniofacial identification (part I): an analytical review of the published adult data. Journal of Forensic Sciences, 53(6), 1257-1272. Taylor, M. P., Wedel, M. J., & Naish, D. (2009). Head and neck posture in sauropod dinosaurs inferred from extant animals. Acta Palaeontologica Polonica, 54(2), 213-220.

Dromaeosaurus albertensis is a Deluxe Raptor figure from the Raptor Series. The figure measures approximately 12″ long by 5″ tall, with 26 points of articulation, includes a display base, interchangeable closed running toes, plus a unique background display insert. The material for the wings and tail are softer plastic and the tail is posable. In the second run of the Raptor Series, color adjustments were made to the figure. See gallery.