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Thursday, 16 August 2018

Those terrific pelagornithids

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.

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  • 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.

Tuesday, 31 July 2018

Introducing The Palaeoartist's Handbook: Recreating Prehistoric Animals in Art: out next month!

In just under a month I have a new book out: The Palaeoartist's Handbook: Recreating Prehistoric Animals in Art, published by Crowood Press. This is a big (280 x 220 mm, 224 pages), full-colour, densely illustrated soft back entirely dedicated to the subject of palaeoartistry: its history, methods, execution and philosophy. It's going to be available internationally from the 27th of August in both physical and digital formats, and online retailers are already taking pre-orders for the cover cost of £22 or less (Amazon sale links: UK/US). I plan on having stock to sell signed copies from my website very soon, and a signing event is planned for TetZooCon 2018 - get your tickets for that here.

With the release impending, I figure it's time to start talking about the book to generate some buzz. The handbook is essentially a palaeoart textbook, containing a history of the genre, an overview of the process of reconstructing an extinct animal, notes on the life appearances of popular extinct taxa, discussions about the artistic and scientific requirements of the discipline and giving practical advice to aspiring palaeoartists. The goal of the book is to be accessible to newcomers while also interesting to veterans and enthusiasts. Sections demystifying geological and palaeontological jargon or introducing important concepts (finer divisions of geological time, phylogenetic bracketing etc.) should be useful to those just entering the discipline, while the detailed discussions, diagrams and citations should interest enthusiasts and professionals.

Emily Willboughby's Microraptor welcomes you to the first chapter of the handbook. I'm very happy with the overall look of the book: it has a good text/figure ratio, is suitably 'dense' without being cluttered, and has lots of nice details like the colour graded panels beneath the chapter openers. The designers have done a really good job.
The idea for the handbook came in January 2016 when I was reading Jackie Garner's excellent Wildlife Artist's Handbook (2013, Crowood Press). It occurred to me that, like conventional natural history art, palaeoart has a long history, its own theory and methods, good and bad practise, as well as a large body of practitioners, and yet we lack texts which discuss palaeoart as a learnable skill or discipline. Virtually all palaeoart books are collections of artwork, historic overviews or 'how to draw dinosaur' volumes, the latter often being of dubious scientific merit. The most detailed discussions of palaeoart theory are found in book chapters or articles, but they're limited in detail because of their lack of space. In writing this blog I've found that there's scope for long, detailed discourses on everything palaeoartistic: if even arcane topics such as extra-oral tissues or predicting horn shapes can justify a few thousand words a piece, then writing about the entire discipline would easily fill a book. Being impressed with the quality of The Wildlife Artist's Handbook, I contacted Crowood about creating a palaeoart equivalent and, 2.5 years later, we're almost at that August release date.

As you may expect from a book about artistry, the handbook is heavily illustrated. It has about 200 figures, photographs and paintings, as well as a large number of annotated diagrams. Not all the artworks are my own, however. Though happy to handle the diagrams and many of the paintings myself, I felt it would be inappropriate to illustrate the book exclusively with my own work - I fear giving the impression of putting my own work on a 'here's how to make palaeoart' pedestal. To that end, I reached out to eight of the most talented and interesting palaeoartists working today: Raven Amos, Julius Csotonyi, John Conway, Johan Egerkrans, Scott Hartman, Rebecca Groom, Bob Nicholls and Emily Willoughby, each of whom graciously donated several pieces of artwork. Their contribution not only makes the book a heck of a lot prettier but also demonstrates a broad stylistic range. The list of contributing artists could easily have been twice as long but, as I'm sure you can appreciate, finding content for this book was never a problem: fitting it all into a reasonably sized package was. Indeed, I had to request more words from the publishers midway through writing and the project ended up being 20,000 words longer than originally intended. This is not to say that the book is cluttered or over-stuffed - to the contrary, I actually find the layout quite comfortable to look at - but simply that we really pushed this one as far as we could go.

Contents page for The Palaeoartist's Handbook. Much of the book is devoted to the reconstruction process, but many other topics - history, composition, professional practise etc. - also feature.
Questions about the handbook's content are best answered with a tour through its chapters. The book opens with a chapter introducing the genre: its scope and depth, its bias towards charismatic fossil vertebrates and how we might distinguish palaeoart from other visual media pertaining to extinct animals. Much focus is given to the line between palaeoart and palaeontologically-inspired art. This subtle distinction is an important one, being the cause of much frustration and confusion among those of us who care about realistic depictions of the past and public education. Ultimately, we have to concede that the creative forces behind the prehistoric animals of movies and toys are rarely on the same page as us: they aren't making 'palaeoart', but 'palaeontologically-inspired art'. These are works that use preferred and marketable aspects of palaeontology to achieve a goal, but ignore components that conflict with their objective. A take home from this is that anyone seriously wanting to be considered a 'palaeoartist' needs to create art of extinct subjects based on evidence and data, not gut feelings, what the latest Jurassic movie is doing, or what we think looks cool.

Chapter 2 is one of my favourite parts of the book: a history of palaeoart from the pre-scientific period right up to the modern day. So many histories of palaeoart are short and selective, often jumping from Duria Antiquior to Hawkins' Crystal Palace models, saying hello to Knight and Burian and then calling it a day. Such treatments omit many important details in the development of palaeoartistry - and I'm not just thinking about the reinvention of palaeoart inspired by the Dinosaur Renaissance. It should be more widely appreciated, for instance, that De la Beche's Duria Antiquior is not the oldest piece of palaeoart. It is widely labelled with this title but a number of works undeniably qualifying as palaeoart pre-date it by 30 years. De la Beche's painting broke new ground in some respects, but the terrain had already been cracked by several other scholars and artists. Another example: historic overviews often focus so much on Knight that they overlook other significant developments taking place in the early 20th century, such as the invention of hybrid 'scientist-palaeoartists' and their strong influence in the genre. While Knight was painting murals Harry Seeley was publishing Dragons of the Air (1901) and Gerhard Heilmann was producing The Origin of Birds (1926), books which contained very progressive takes on pterosaurs and dinosaurs and are clear precursors to the way we illustrate these animals today. I've tried to cram the handbook's overview of palaeoart history with as much information as possible and I feel it's a more comprehensive treatment than you'll find in many venues. It also features a brief section on palaeoart prior to science - my recent blog posts on griffins and cyclopes stemmed from research for this section.

Hendry De la Beche's 1830 artwork Duria Antiquior: A more Ancient Dorset: definitely a landmark illustration for palaeoartistry, but not the first piece of palaeoart. The pre-1830 history of palaeoart gets a lot of discussion in the handbook. Image in public domain.
The third chapter is a crash course in how to research palaeoart. This part of the book will hopefully benefit folks who're new to the discipline and struggling to make sense of the often technical information that informs a palaeoartwork, an especially daunting task for those lacking a background in geology or palaeontology. There's a lot of explanatory text in this chapter, explains (for example) what terms like 'functional morphology' and 'stratigraphy' are, giving advice on how to read a cladogram, and outlining why researching geology and fossil provenance are just as important as understanding anatomy. There are also discussions of where to find information relevant to palaeoart and how to verify it reliability. There's a lot of junk and erroneous information out there, especially online, and these tips should help you to sift some useful information from the detritus.

We talk a lot about epidermal correlates at this blog (see here and here for recent examples) but they aren't as widely used as they should be. They're best known in centrosaurine horned dinosaurs thanks to Hieronymus et al. (2009), but occur widely across tetrapods. We're probably getting a lot of reconstructions wrong by ignoring them. Image from Witton (2018).
Chapters 4-8 outline the process of reconstructing extinct vertebrates. Collectively, these chapters represent a major chunk of the book. They start with the prediction of missing anatomies, building skeletal reconstructions and determining plausible postures. Muscles and fatty tissues are then considered, followed by skin: how we can predict skin types when they aren't present in fossils as well as what we can determine from fossil skin itself. A whole chapter is devoted to facial tissues: extra-oral tissues (lips, cheeks etc.), eyes, ears and noses. Our precision for reconstructing animal faces is something of a mixed bag as some features are much easier to predict than others. We have robust means to predict how much eyeball tissue should be visible, the likely positions of reptile nostrils, and when trunks or proboscides were present, but ask about the shape of extinct mammal ears or what sauropod noses really looked like and we're less certain. Chapter eight deals with hot topics like shrink-wrapping and the role of speculation in soft-tissue reconstruction. Both have roles to play in palaeoart, but both can be 'overdone': the handbook has some food for thought about when, and when not, to make use of these conventions.

Chapter nine drills down into the specifics of restoring tetrapod taxa. I originally envisaged this section as being bigger and encompassing more animal types, but non-tetrapods had to be cut to save space. The alternative would have been to include very brief notes on more taxa, but I fear the sin of error through omission: more detail about popular palaeoart subjects seemed the best compromise. Most major tetrapod groups are included, with specific sections on dinosaurs, pterosaurs, marine reptiles, different 'grades' of synapsids, temnospondyls and others.

'Rictal plates' - the structures that cover the corner of tetrapod mouths - are among the topics discussed in the handbook. Though often mentioned in discussion of dinosaur 'cheeks', they also have relevance to suction feeders, such as the placodont Henodus chelyops. Understanding a subject's functional morphology can guide speculative reconstruction of unknown anatomies. Another image from Witton (2018)
The tenth chapter moves away from restoring animals to considering their environment. As with chapter three this section is aimed partly at newcomers, bringing them up to speed on how ancient environments are understood through sedimentology, stratigraphy and palaeoclimatology. This is not to say Chapter 10 is a geology lecture however: it's more a bluffers guide which explains useful terms and phrases to allow non-geologists to glean information from research papers on the palaeoenvironment of their subject species. Plants are also briefly covered in this chapter. I'm afraid the handbook is not the text that overturns palaeoartisty's general short shrift to palaeobotany, but there is guidance for how to research ancient floras as well as some need-to-know information about plant evolution.

Raven Amos' Nemegt Sunrise shows an entirely typical palaeoart topic - a foraging dinosaur (specifically, Conchoraptor) - but in awesome style. Palaeoart which is scientifically credible but strongly stylised is relatively new to the discipline. Will it become more widespread in future? Raven's excellent image features prominently in the book.
Chapter 11 addresses the 'art' in 'palaeoart', talking about the interplay between science, composition and style. Discussions of palaeoart rarely stray into these areas, but they're important: there's no point getting your scientific details spot on if your artwork is an uncompelling mess. This chapter covers how our ideas about animal behaviour, their arrangement in a scene and relationship to the viewer are critical to making effective artwork, it being argued that some common palaeoart practises - extremes of perspective, and unrealistic shoutyroaryfighty behaviours - can make artwork less credible. Through liberal use of art by the contributing artists, choices of style and the advantages of different approaches are discussed. I'm a big fan of artists who push the stylistic boundaries of palaeoart and, after two centuries of relatively conservative approaches, consider bold stylisation to the next frontier of the medium. The utility of such styles is discussed, including whether they may sometimes be more 'honest' than our default approach of photo-realism (or, at least, in the orbit thereof): when animals are poorly known, is it more representative of our knowledge to use simpler, or looser styles than to hone every scale or hair to precision? This is becoming more of an issue as some species become incredibly well known to scientists and artists. Do we risk 'diluting' the impact of discoveries where we can plot every scale and pigment cell with certainty if we restore every animal as if this were the case?

The final, concluding chapter takes a look at the professional world of palaeoartistry. This section is aimed at those who commission artworks as well as those who create it, tackling subjects like what information artists need to plan and price a commission, the importance of feedback, and that all important topic: how to make a living from palaeoart. I'm afraid this chapter doesn't have an easy answer for the latter: hard work, talent, luck and shameless promotion remain hurdles between us all and palaeoart success. What a jip.

A page from Chapter 9's mosasaur section. Diagrams or illustrations such as these appear on almost every page of the book. When I started the book I figured I'd mostly use 'off the shelf' art, but I ended up creating a lot of new images to illustrate points made in the text. This is why is took two years to write, folks.
And that probably tells you everything you need to know if you're wondering whether this is a book for you. My ultimate aim was to make a book comprehensive enough to cover most questions anyone could have about how palaeoart is made, or at least give some idea where the answer could be found in other literature. It is, of course, impossible to cover everything about a topic as broad as palaeoart in a single book, but by placing an emphasis on methods as well as raw information I figure readers should gain sufficient knowledge of the field to answer questions on their own. And that's probably the most important lesson in the handbook: palaeoart is reliant on an evolving, changing set of data, so what's considered 'accurate' in 2018 may not be in ten years time. Training yourself to think scientifically, and to check information no matter where it's from, is just as important as learning how to paint or sculpt in palaeoartistry. If that's the message you take home from this project, I'll consider my job done.

The Palaeoartist's Handbook: Recreating Prehistoric Animals in Art, will be available internationally on August 27th, published by Crowood Press. Pre-orders can now be made at Amazon (UK/US) and at other retailers.

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My work - including the writing of educational books like the handbook - is supported through Patreon, the site where you can help online content creators make a living. If you enjoy my content, please consider checking out my Patreon site - subscriptions start at $1 a month. That might seem pretty trivial, but if every reader pitched that amount I could work on books, artwork and other educational content full time. In return, you'll get access to my exclusive Patreon content: regular updates on research papers, books and paintings, including previews of another upcoming book. 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 - without your help, the Palaeoartist's Handbook may not exist.


  • Garner, J. (2013). Wildlife Artist's Handbook. Crowood Press.
  • Heilmann, Gerhard (1926). The Origin of Birds. London: Witherby.
  • 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.
  • Seeley, H. G. (1901). Dragons of the air: an account of extinct flying reptiles. Methuen & Company.
  • Witton, M. P. (2018). The palaeoartist's handbook: recreating prehistoric animals in art. Crowood Press.

Sunday, 24 June 2018

Ricardo Delgado's Age of Reptiles at 25: a palaeontological retrospective

With the 25th anniversary of Jurassic Park cascading through dinosaur social media you could be forgiven for overlooking another influential dinosaur franchise celebrating the same vintage this year. Unveiled in 1993, this long-running series stood out for showing dinosaurs as fast, agile and intelligent animals, and immersed us in an then-unparalleled expanded prehistoric narrative, rich in detail and huge in scale. I'm talking, of course, about Ricardo Delgado's Dark Horse comic series Age of Reptiles.

The award-winning Age of Reptiles series comprises multiple, unconnected stories published throughout the last 25 years, with a bit of a hiatus between its earliest and latest incarnations. The series comprises four serials (Tribal Warfare, 1993; The Hunt, 1994; The Journey, 2009; and Ancient Egyptians, 2015) and two shorter pieces (The Body, 2011; Baby Turtles, 2014 - regrettably, I haven't seen these in entirety). It has a number of fans among those of us who research and illustrate fossil reptiles, and judging from the calibre of movie makers who've contributed endorsements to the comics, it is well regarded in the movie industry, especially for its entirely 'silent', visual means of storytelling. Despite some relatively complex narratives, large casts and use of multiple locations, not a word of dialogue or descriptive text is used to explain plots or character motivations. The 2011 documentary Dinosaur Revolution and its 2012 spin-off Dinotasia were inspired by Age of Reptiles, with Delgado having director credits on two episodes of the former. It's fairly well known that, until late in production, Dinosaur Revolution was effectively meant to be Age of Reptiles: the TV show, but studio cold feet revised the programme into a more conventional documentary.

Cover art for various editions of the Ricardo Delgado's Age of Reptiles comic series, borrowed from Dark Horse Comics. Their website which has a full back catalogue of the series for your browsing and purchasing needs. Sorry for the low res images - I'm deliberately using officially released artwork in this post to avoid unintentional piracy of Age of Reptiles content. Entire sequences of the Age of Reptiles comics have been uploaded without authorisation to the web, and that's not cool, folks: it's stealing.
Age of Reptiles was an important influence on my childhood drawing and, with the series hitting the big two-five this year, I thought I'd share some of what I think makes the series special. It helps, I think, to set the stage in which I first met Age of Reptiles as an eight or nine-year old* dinosaur obsessive in the early 1990s. Though not so long ago, this was a different age for palaeontological media because rendering life-like prehistoric animals for TV or film was much harder than it is today. Making a realistic film or documentary chronicling the lives of prehistoric animals would not only have been very difficult, but also very expensive. Jurassic Park may have broken new ground for dinosaur animation in 1993, but it needed sophisticated animatronics, then-radical computer generated imagery and a Hollywood-grade budget to achieve its visuals. It took several years for this technology to become more widely affordable, with the the bridge between Jurassic Park and our living room viewing - Walking with Dinosaurs - not appearing until 1999. Thus, if I wanted to see 'living' dinosaurs, rather than just disconnected pictures in dinosaur books, I had to made do with the short vignettes with wobbly puppets on the A&E Dinosaur! series, or else hope to find a showing of a Harryhausen dinosaur film on TV.

*My birthday would have been around the time I first saw the comic, and I can't remember if I'd graduated to nine years old when I first saw it.

It was into this landscape that Age of Reptiles placed a weighty clawed foot. I first encountered the first story, Tribal Warfare, through the UK's take on the Jurassic Park comics. Coinciding with the movie release, the international branch of Dark Horse comics published a weekly Jurassic Park magazine that contained a comic story of the movie as well as two other series: Xenozoic Tales (a post-apocalyptic sci-fi story with minimal dinosaurs - I never really got on with it) and something called Age of Reptiles. Juxtaposed against the talky, high-tech worlds of Jurassic Park and Xenozoic Tales, Tribal Warfare immediately stood apart with its silent, patient and entirely immersive opening. Without a word of introduction, we see a sleepy Pteranodon wake up, spread its wings, and then launch over a huge, double-page vista of unspoilt trees and bluffs. The pterosaur sails past a foraging sauropod, which we soon learn is being stalked by a group of dromaeosaurs. We watch the sauropod flee and ultimately fall to its attackers, before a huge tyrannosaurid arrives to claim the carcass from the smaller predators. The animals were colourful, dynamic, imposing and vicious, and played out their drama without interruption from a narrator, talking head or visiting caveman. It was unlike anything I'd ever seen.

Sorry, Walter Cronkite: your A&E dinosaur puppets had nothing on this. Page from the opening comic of Age of Reptiles: Tribal Warfare, where a pack of Deinonychus bring down a titanosaur, while Pteranodon soars past. Anachronistic fauna be damned: for an early 1990s dinosaur fan, this was ambrosia from the loftiest peaks of Olympus. Borrowed from Dark Horse Comics.
Reading the comic in entirety, it was apparent that this was not a story about prehistoric animals living with humans, or anthropomorphic cuddly dinosaurs learning lessons about friendship. Age of Reptiles was the extended, unadulterated prehistoric drama every '80s kid wanted but film or TV had yet to produce. OK, it was in comic format rather than animated on screen, but Delgado's experience with storyboarding and film illustration gave our brains little work to do as we filled in the action between panels.

Age of Reptiles continued to resonate for years after I first encountered it. Much of the dinosaur art I drew for the next 5, 10... 25 years was influenced to a greater or lesser extent by Delgado's creation, to the extent that I rank him as one of my top artistic influences. It may not be as obvious in my modern work as it was 20 years ago, when I was a teenager liberally borrowing from his style (below), but it's still there. Every now and then a Delgadoesque waterfall or critter still sneaks into one of my paintings and I still have a lesson in composition and visual storytelling whenever I re-read his work. Those of you with eagle eyes may have noticed praise for Age of Reptiles in my 2017 book, Recreating an Age of Reptiles, the title of which was chosen as much for its relevance to the comic as it's palaeontological and paleoartistic connotations. Though I don't think Age of Reptiles can qualify as pure palaeoart on grounds of taking a few too many artistic liberties with palaeontological data, it contains many lessons about effective depiction of fossil animals and, 25 years on, I still regard it as some of the best 'palaeontologically-inspired art' (as opposed to entirely science-led palaeoart) out there.

Revisiting some of my 20 year old drawings (I would have been about 13 when I drew these) shows many Delgadoisms. These would all have been influenced by Tribal Warfare, I didn't have The Hunt. The tree outlines, cliffs, waterfalls with sharply defined mist, the hatched scalation, eye shapes and so on were my best attempts to execute an Age of Reptiles style.

Telling palaeostories

As noted above, some of the heaviest praise for Age of Reptiles stems from its ability to tell complex stories without any text. They are not conventional narratives about dinosaurs either, the Age of Reptiles stories recalling cinematic westerns, Mafia dramas and Mad Max-style journeys through post-apocalyptic wastelands. Essays penned by Delgado for some of the comics discuss these influences, often citing classic films as inspiration. It's quite a feat to make western where cowboy hats are traded for scales and, yes, the characters are somewhat anthropomorphised to achieve this, but it rarely feels overdone. Anthropomorphism also lessens as the series continues, just one of many aspects that seems to change - we might say 'mature' - as the series has continued. The animalistic behaviours of Age of Reptiles' characters are aided by Delgado not being afraid of making them real scumbags, as well as a dark sense of humour and regard for his their wellbeing that even Game of Thrones might consider a bit harsh. These attributes make Age of Reptiles a closer approximation of the natural world than other franchises where we see dinosaurs engaging in day-to-day behaviour, and brings a moral ambiguity to his characters. This makes it difficult to root for any one character entirely, but I think that's the point: these aren't comics with moral lessons about human values, but stories about animals that have to be strong and sometimes violent to survive. Executives wondering what to do with dinosaur narratives for documentaries or films could learn a lot from Delgado's work: dinosaurs can do more than just search for those far-flung lush valleys, folks.

Dark Horse's 2015 motion comic is slightly different from the original opening of Tribal Warfare, but it captures some of the arresting cinematic style and dinosaur behaviour of the very first Age of Reptiles comic. From Dark Horse Comics' official YouTube account.

The Age of Reptiles series has paid increasing attention to science since its 1993 debut. Tribal Warfare has anachronistic casting with a mix of dinosaurs and other reptiles from across time and space: Tyrannosaurus rubs shoulders with Deinonychus, Deinosuchus, shastosaurid ichthyosaurs, Saltosaurus, Pternanodon, Parasaurolophus, Carnotaurus and others - it's a grab bag of fan-favourite Mesozoic animals. Their behaviour is also among the most simplistic and anthropomorphised of the series too, with the tyrannosaurids and dromaeosaurids acting like rival gangs from some gritty, gory 70s exploitation film. But in later serials more attention has been paid to real species compositions and animal behaviour is more nuanced. This has been implemented most successfully in Ancient Egyptians, where efforts have been made to feature the correct fauna and palaeoenvironment of mid-Cretaceous Africa, and the depicted behaviours are relatively animalistic. The characterisation of some species also runs against stereotyped portrayals of dinosaurs in popular media, subverting tropes of 'harmless herbivores' and so on. The giant titanosaur Paralititan, for example, is the primary antagonist in the story, being an aggressive, violent species bristling with antagonism in every frame. Annoy these sauropods and you're in trouble, even at risk of being crushed to death under the massive tonnage of their forelimbs. This is a very different role for a sauropod dinosaur in popular media, even contrasting with prior Age of Reptiles stories where they are little more than background animals or prey species. The idea of large herbivores being badass mothertruckers isn't silly either, this being the case for many living herbivores like hippos, certain bovids, and some elephants.

Ancient EgyptiansParalititan in full angry mode. Note the blocky neck profile, distinctive facial tissue, correctly positioned nostrils and distinctive scarring - great stuff. From Dark Horse Comics.
Elsewhere, a male Spinosaurus - the anit-hero for the story - kills the offspring from another male before siring his own (in stark contrast to the nurturing parent-juvenile relationships of earlier Age of Reptiles) and sometimes communicates using rumbling vocalisations emitted from its throat rather than always using open mouth roaring. This is progressive stuff, and - particularly as someone who's experienced pushback against new ideas when working on dinosaur media projects - very refreshing to see in a popular dinosaur product. We can't pretend that Age of Reptiles is a documentary - if it were entirely true to life, 95% of the series would be dinosaurs chewing leaves and pooping - but Delgado deserves full kudos for pushing his creation towards more credible faunal compositions and not holding back when depicting new ideas about dinosaur behaviour. Hollywood, take note: thus far, we've seen no evidence that having half an eye on science has impacted his ability to tell great stories.

Evolving anatomy and Age of Reptiles

Delgado's animal designs have also crept towards realism and scientific credibility since 1993. His reptilian cast is 100% post-Dinosaur Renaissance, and thus has always been appropriately posed, agile, and dynamic, but his creative approach seems to have changed between 1990s and 21st century entries into the Age of Reptiles canon. The taxonomic identities of his animals have always been apparent and his animals look 'realistic', in the sense that they don't look anatomically implausible, but the creatures of Tribal Warfare and The Hunt have a certain 'augmented' quality that is not apparent in later serials. The theropods, for example, are always long-legged beasts with boxy, robust skulls and large, prominent teeth, as well as heavy scalation and exaggerated ornaments. They're recognisable as their real-life counterparts, but look like superpowered versions of the real species. Though not all the animals in the first Age of Reptiles serials received this treatment (most of the herbivorous species are pretty darned good approximations for our 1990s views of these animals, with minimal embellishment) the overwhelming impression is still one of prehistory on steroids. I'm reminded somewhat of William Stout's 1990s palaeoart: Stout's work is probably on the more credible side of the scientific fence, but shares an emphasis on gnarly, enhanced features with Delgado's creations. I wonder if Stout's work was referenced in those early comics.

The Journey and subsequent stories feature more scientifically credible restorations which seem more carefully modelled on their real-life counterparts. The tyrannosaurs in The Journey, for instance, have longer bodies and skulls, and stouter legs, than the 1993 versions and thus look much more like the real deal. The abelisaurs in Ancient Egyptians show the peculiar short arms and blunt heads particular to this group, unlike the fairly 'generic' Carnotaurus we met in 1993. I especially like the titanosaurs of both The Journey and Ancient Egyptians, their designs having robust, wide necks, rotund bodies and stout limbs, as they should. Smaller details are well captured too, with eyes, ears and nostrils being in the right places - not something to be sniffed at in any public-facing dinosaur art.

Cover of the first issue of Age of Reptiles: The Journey. Note the improved tyrannosaurid anatomy compared to that of Tribal Warfare, which you can see in the video above. Also, so many footprints! - another hallmark of later Age of Reptiles art. From Dark Horse Comics.
Additional positive trends include less shrink-wrapping on many species (the pterosaurs, in particular, have a lot more meat on their bones in later stories), closer attention to the anatomy of non-dinosaurian species, and more natural-looking colour schemes. I am curious to know if this reflects influence from broader palaeoart trends, or if Delgado has independently moved away from some of the retrospectively questionable reconstruction choices of early 90s palaeoart. Whatever the influence, though some liberties are taken to create recognisable individual characters or convey thoughts and actions, the tighter, more believable take on these animals is welcome. Within the constraints of creating a comic about prehistoric animals, I think Delgado is doing an increasingly good job of balancing the demands of narrative with science.

If I have one complaint about the accuracy of the animals, it's that several species have remained scaly even when their fossils now unequivocally show feathers or filaments. I hope this changes in future. To the series credit, feathers have crept in here and there (indeed, they've been in the series since 1993) but voluminous, bird-like feather shells have yet (to my knowledge) to feature in animals we know had them, such as maniraptorans and ornithomimosaurs. Still, I admit that I find this less irritating than I do the lack of feathers in that other major dinosaur franchise launched in 1993, mainly because Age of Reptiles doesn't employ consultants to give the prestige of scientific credibility, nor does it make lame excuses about why it's animals look like they do. It is what it is, and never made any claim for being 100% scientifically credible. Moreover, Age of Reptiles has spent the last 25 years trending in the right anatomical direction, whereas the modern Billy and the Cloneosaurus movies are stuck in the past, sometimes taking deliberate steps away from palaeontological science.

Page from Age of Reptiles: The Journey, featuring the unluckiest sauropod hatchling ever committed to print. Age of Reptiles often has a dark sense of humour and the plight of this little guy is both funny and tragic - you'll have to buy the comic to find out what happens. From Dark Horse Comics.

Worlds of space and detail

Moving away from science and into the art itself, there are also lots of subtle details in Delgado's illustrations which enhance the believability of his prehistoric landscapes and bring character to his actors. It's here where Age of Reptiles can teach conventional palaeoartists a few tricks, as reasoned speculation and imaginative concepts are used to bring Delgado's Mesozoic to life. I could list many examples, but one of my favourites is the association of a small preening pterosaur with a specific female tyrannosaurid in Tribal Warfare - a charming addition to a sometimes violent character. Elsewhere, small creatures - bugs, fish, birds, pterosaurs and so on - frequent most frames, sometimes playing out their own minor dramas against the backdrop of the main narrative. Variation in colour, injuries and integument between his animals give each different personalities, as well as unique visual identities. From The Journey onward we see sauropods sleeping in rings with their necks draped over one another, and in one of Age of Reptiles' rare visits to the marine realm, Delgado's giant mosasaurs are covered with parasitic fish. Plus - because why the heck not - the Araripesuchus in Ancient Egyptians are almost always relieving themselves. These small, sometimes inconsequential details really sell the richness of the Age of Reptiles universe and the individuality of each character.

Another page from Ancient Egyptians. The low angle and shading gives the Paralititan a terrific presence in this panel, leaving us in no doubt that a) it's absolutely huge, and b), that Spinosaurus is in trouble. Note the improved pterosaur anatomy vs those in Tribal Warfare (see images, above). Borrowed from Dark Horse Comics.
The composition and framing of Age of Reptiles is also excellent, creating a sense of atmosphere, scale and motion that rivals the greatest palaeoartworks. Delgado's experience in the world of movies and television brings a truly cinematic quality to some parts of Age of Reptiles, and I strongly recommend these comics just to see how varying viewpoints, animal poses and colouration influence the portrayal of ancient species. If Age of Reptiles was a movie, we could imagine it as one with particularly liberated camera motions that swoop, cut and jump between viewpoints and distance. Delgado is not afraid of placing subjects in the middle or even far distance, often at the expense of fine detail but working terrifically for conveying size, motion and character. My favourite images of the series are those with the viewpoint pulled right back to show enormous landscapes, his animals reduced to fractions of the frame (think Douglas Henderson palaeoart, in comic form). His liberal application of footprints - and their role in communicating information about the nature of a scene - becomes apparent in such views. Close-ups are variably used in more intimate, tense of energetic moments, and we see a lot of variation in light and setting to alter atmosphere and and tone. In all, Age of Reptiles is an excellent demonstration of how a strong eye for composition can enhance artwork of prehistoric animals, and how we can tell entire stories in single images.

Age of Reptiles is not, and is not meant to be, a scientifically rigorous take on Mesozoic life, but it skirts the edge of palaeoartistry and palaeontological science close enough that those interested in these topics should check it out. It's among the most creative and consistently interesting palaeontological products I'm aware of and, if you like dinosaur science, or dinosaur art, you're going to find something to like here. An omnibus of the first three serials is available, as is the collected issues of Ancient Egyptians - all are still in print and very affordable. Fans might also want to check out Ricardo Delgado's blog, which has a lot of 'behind the scenes' content from the series. CGI might have made it easier to create dinosaurs for film and TV since 1993, but the still-picture storytelling of Age of Reptiles competes with, and often outdoes, the best prehistoric drama that Hollywood can throw at us.

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Thursday, 31 May 2018

Why we think giant pterosaurs could fly

Giant azhdarchid pterosaur in flight. Images like this virtually always trigger discussions about the validity of giant pterosaur flight hypotheses.
Every so often the idea of flightless giant pterosaurs circulates in the press or on social media. It doesn't take much to ignite these discussions: a new giant pterosaur fossil, a PR event from a museum, or simply artwork emphasising the size of giant flying reptiles will see someone, somewhere, questioning their flightworthiness. These suggestions are often made with strong conviction, to the extent of dismissing or even arguing with scientists who study pterosaur anatomy and biomechanics. After all, how can any sensible individual think that animals with 10 m wingspans and body masses hovering around 250 kg were capable of flight? At most they were gliders, or flighted as juveniles and flightless as adults, right?

Confession time: as someone actively involved in research and outreach on these animals, I often find these discussions frustrating, for two reasons. The first is that, among those who actually study pterosaur functional morphology – that is, those who make detailed observations and measurements of pterosaur fossils, compile biomechanical data and use computer modelling to objectively test their flight capacity – there is no controversy about the volant nature of these animals, even at their maximum size. Peer-reviewed claims that pterosaurs were flightless are genuinely rare (perhaps limited to Sato et al. 2009; Henderson 2010 and Prentice et al. 2011 in the last decade) and have a consistent record of being flawed on some critical anatomical or functional detail (Witton and Habib 2010). There is no debate about giant pterosaur flight among those of us who study their fossils: the press and social media fuss about the topic is a genuine palaeontological nontroversy.

The second source of frustration is that, away from technical literature, discussions of giant pterosaur flight frequently suffer from major cases of Dunning-Kruger effect, especially when parties have knowledge of planes. I've experienced this a lot in my career, and not just in the wilds of social media: many of my TV and film consultancy jobs have required defending basic tenets of pterosaur anatomy - even their basic, there-for-all-to-see proportions preserved in articulated fossils - to folks who just can't or won't believe what the fossils show. Having a casual understanding of engine-driven man-made flying machines does not equate to knowing all there is about everything that has ever flown, but you would not know this from some conversations.

Our controversial giant azhdarchid friends Arambourgiania philadelphiae (middle) and Hatzegopteryx thambema (right), compared to a record-breaking giraffe and the Disacknowledgement.
Whether through naivety of palaeontological theory or unwillingness to accept good data, the lack of accessible overviews of current thinking on giant pterosaur flight probably fuels this ongoing nontroversy. What's needed, it seems, is a synthesis of modern ideas on giant pterosaur flight and justification for why pterosaur experts don't challenge this idea. I've attempted this below, including sufficient methodological detail and references so that anyone wanting to understand these ideas will have a useful jumping-off point, and to establish what needs to be overturned to challenge the null hypothesis of giant pterosaur locomotion. Our focus will be on the largest of all pterosaurs, giant members of the clade Azhdarchidae, as they are the main focus of most flightless claims (above). They are also among the most familiar giant pterosaurs, their number including species such as Quetzalcoatlus and Hatzegopteryx. The general points made below pertain to all large pterosaurs, however.

Evidence from comparative anatomy

Giant azhdarchids are invariably known from scant remains, sometimes a handful of fragments representing bones from across the skeleton or, in the case of Quetzalcoatlus northropi, an incomplete left wing (e.g. Lawson 1975; Frey and Martill 1996; Buffetaut et al. 2002; Vremir 2010; Martill and Moser 2018). Ordinarily, fragmentary remains are a barrier to interpreting the locomotory strategies of extinct organisms but flighted lifestyles adapt animal bodies to such an extreme degree that just a few bones can betray volant habits. It’s evident that even the largest pterosaurs bore wing anatomy comparable to their smaller, incontrovertibly flightworthy relatives. Although no complete giant wings are known, our fragments indicate similar linear forelimb bone proportions to smaller azhdarchids. Their wing joints – including details of their elbows, wrists and wing finger knuckle – are well understood, and indicate typical properties of pterosaur wing motion and function. We can make a number of predictions concerning muscle extent for giant taxa, the most important being related to the presence of a huge deltopectoral crest on their humeri. This broad flange of bone, situated at the proximal end of the humerus, anchored many muscles running from the shoulder to the wing and powered flapping motions in flight (Bennett 2003), so is a clear correlate for powered flight in giant species. The seemingly-small deltopectoral crest on the Hatzegopteryx humerus is sometimes raised as evidence of reduced flight ability: it’s actually just badly preserved with lots of bone missing on all margins (Buffetaut et al. 2002).

A collection of giant azhdarchid bones: A, cervical vertebra of Arambourgiania philadelphiae; B, humerus of Quetzalcoatlus northropi, C-D, the rather broken proximal humerus of Hatzegopteryx thambema. Other than their huge size (scale bars represent 100 mm) and some details of robustness, these bones are identical to those of smaller, incontrovertibly flying pterosaurs.
The only significant difference between the wings of giant and smaller azhdarchids concerns bone robustness, especially that of their joints. We would predict expanded wing bone diameters in large fliers as they enhance resistance to bending, and as giant animals they are going to experience proportionally greater bending stresses. If they flew, giant pterosaurs should have very large wing bones indeed, and - as is evident from the adjacent images - this is exactly what we find. Witton and Habib (2010) noted that the humeral shafts of giant azhdarchids are comparable in diameter to those of giant mammals, like hippos, despite the pterosaurs being a fraction of their weight (Witton 2008; Henderson 2010). Giant azhdarchid wing bones perform exceptionally well in bending strength tests, being able to resist multiple bodyweights before failing (Witton and Habib 2010), and their expanded diameters maintaining relative failure levels comparable to those of small or mid-sized pterosaurs, despite their size (Witton et al., in prep).

Pterosaur humeral scaling: as pterosaurs got bigger, their wing bones and joints expanded disproportionately to accommodate greater stresses incurred in flight and launch. Image from Witton (2013).
But for all their expansion, giant azhdarchid wings retain exceptionally thin bone walls. Even in Hatzegopteryx, the most robust of the group, they’re only 4-7 mm thick. This is a high value for a pterosaur, but still (in relation to bone diameter) at the low end of the cortical thickness spectrum. Other giant species have cortices of just 2 mm or so, values only just slightly larger than those of mid-sized pterosaurs. Expanded but bone-lite wings are another feature of flying creatures, being common to most flying birds and all large pterosaurs, and offering their owners a lightweight but bending-resistant flight skeleton. However, what optimises these skeletons for flight compromises resistance to buckling forces, which is why most non-volant animals tend to have much thicker cortices. The correlation between thin bone walls and flight is not watertight (Hutchinson 2001) but it's a feature we would predict for any seriously large flying animal, and is thus consistent with volant habits in giant pterosaur species.

Flight models

It’s often asked how animals as large as the biggest azhdarchids could attain and sustain flight. It’s important to stress that no-one imagines giant azhdarchids as breezy fliers flitting around Cretaceous plains like busy songbirds. As animals operating close to the size limits of flight for the azhdarchoid bauplan (Marden 1994; Habib and Cunningham pers. comm. in Witton 2010; Habib 2013) we should assume a flight frequency comparable to our largest modern fliers – creatures like bustards, geese, swans, albatrosses and so forth. Though different in flight mechanics these birds are united in their relatively low launch frequencies, taking to the air when they must (such as to evade danger) or when they have long distances to travel. Launch is very energy-demanding because of their great body masses, and in some cases specific environmental conditions are needed (such as space for taxiing in albatross), limiting their options for frequent takeoff. We should assume the same was true for large azhdarchids: their functional morphology and trackways show strong terrestrial abilities (Hwang et al. 2002; Witton and Naish 2008, 2013) and they probably spent a lot of time grounded, only flying when harassed, or wanting to move far and fast.

When imagining giant pterosaurs flying, we need to have birds like the kori bustard in mind: large, powerful animals which are strong fliers, but unable to flit about the sky like small songbirds or bats. When these guys take off, they mean it. Photo by Arnstein Rønning, from Wikimedia, CC BY 3.0.
Indeed, in all likelihood giant pterosaurs couldn’t launch every few moments. Flying animals tend to allocate about 20-25% of their body mass to flight musculature, which gives our large azhdarchids 50 kg or so of flight muscle to use in launch and flight (Paul 2002; Marden 1994). Even so, models of muscle energy availability show that giant pterosaurs could not launch aerobically (that is, using muscle contractions supplied with oxygen) and they had to rely on stronger, but less endurable, anaerobic muscle contractions. Anaerobic muscle power is essential to launch in the largest birds and almost certainly played a role in extinct giant insect flight, too (Marden 1994, see graph below), so its inferred use in giant pterosaurs is quite plausible. This reliance on anaerobic muscle power would necessitate resting periods between launches (hence the inability to launch continuously like a small flyer) as well as after vigorous bouts of flapping. Witton and Habib (2010) predicted that the hard flapping window for a giant azhdarchid was about 90 seconds, after which a rest was needed. So, does that limit our giants to turkey-like burst flights?

Launch for giant azhdarchids - like Quetzalcoatlus northropi - would be no more challenging than it is for large birds. The dotted line on this graph represents the minimum muscle energy output needed for flight. Using the same mechanism of anaerobic muscle power as large living fliers, giant azhdarchids are on the right side of that line. From Marden (1994).
Probably not. One of the world's leading experts on animal flight, Mike Habib, found that Colin Pennycuick’s freeware Flight programme – software designed to model bird flight - can be easily modified to predict pterosaur gliding and soaring capabilities, even accounting for the differences between feathered and membranous wings (see Witton and Habib 2010 for this methodology). Using this software, Mike and I predicted that giant azhdarchids were supreme soarers, easily able to sustain long-distance gliding even at body masses of 180-250 kg (Witton and Habib 2010). Predicted giant flight velocities exceeded 90 kph and, in that 90 second flapping burst, giant azhdarchids would cover several kilometres - plenty of distance to seek areas of uplift such as deflected winds or thermals. Having located these, azhdarchids could easily adopt energy-saving soaring to recover their flight muscles, their glide ratios being consistent with those of large soaring birds such as storks, Procellariiformes and raptors. Mike has presented calculations that these giants would have sufficient on-board energy resources to travel the planet, their speed and flight range being sufficient to ignore most geographical barriers. Note that these models assume modern day parameters of atmospheric density and gravity: we do not need to modify these to keep giant azhdarchids airborne. Sure, if you did change these parameters you might make the job easier, but the giants already have very strong flight performance without it. If you don't buy this, remember that you can play around with Flight yourself: download the program, get the method and pterosaur parameters from our open access paper and go at it. None of the science behind these animals is mystical - the methods are entirely conventional and repeatable.

Too heavy to fly?

For all this talk of modelling pterosaur flight at quarter-tonne masses, two sets of authors have proposed that giant pterosaurs were simply too heavy to attain flight. Sato et al. (2009) based this on their understanding of procellariiform takeoff, modelling a maximum possible volant mass of 40 kg for these birds and assuming the same limit must apply to pterosaurs. The next year, Don Henderson (2010) compiled a series of volumetric estimates of pterosaur mass including a 450 kg Quetzalcoatlus. Don – probably correctly – assumed that such an animal would be too heavy to fly.

Mike and I addressed both these proposals in a 2010 publication about giant pterosaur flight. On Sato et al. (2009), we found numerous problems with the overt biomechanical links drawn between bird and pterosaur flight. Avian and pterosaur anatomy is comparable enough to assume some broad analogies in wing shape and flight styles (e.g. Hazlehurst and Rayner 1992), but the detailed kinematics of flight – including launch – are too distinct to assume that the size limits of one group apply to the other. There are reasons to think pterosaurs launched in a very different way to birds (see below) and were subject to a different set of scaling regimes and size limits (Habib 2008, 2013; Witton and Habib 2010). Sato et al. (2009) may have predicted a flight mass limit for long-winged, dynamically soaring birds, but the application of this limit to flying reptiles is not supported by our understanding of pterosaur and avian biomechanics.

Don Henderson's (2010) Quetzalcoatlus model compared to the articulated skeleton of the small, completely known azhdarchid Zhejiangopterus linhaiensis. Note the clear distinction in torso size, and the actual torso length of the fossil pterosaur compared to the humerus. Images from Henderson (2010) and Cai and Wei (1994).
We found a much simpler issue with Don Henderson’s half-tonne Quetzalcoatlus model: its body was simply too large. Don based his work on a silhouette in Wellnhofer’s (1991) pterosaur encyclopaedia, a reasonable decision given the paucity of reconstructions of this animal at the time, but ultimately a problematic one for making accurate mass estimations. Azhdarchoids were relatively poorly known in the early 1990s and Wellnhofer’s silhouette reflects this, being a mostly imaginary pterosaur only accurate in wingspan. Crucially, its body is monstrously oversized at 1.5 m long. Complete azhdarchoids discovered since this time, including that of the azhdarchid Zhejiangopterus, have shoulder-hip lengths only 30-50 % longer than their humeri (above) and - in lieu of giant pterosaur torso fossils - we have to assume this was true for the giants, too. The 544 mm long humerus of Q. northropi translates to a predicted torso length of just c. 750 mm – a fraction of the size used in Don’s estimate. Mike and I adjusted Don's calculations to a more reasonable body proportion and, presto, the predicted mass was in the more familiar quarter-tonne range, a value flight models are happy to see launching and soaring without difficulty (Witton and Habib 2010).

The key to everything: quad launch

A critical hypothesis for giant pterosaur flight concerns recent interpretations of their launch strategy. This idea is that pterosaurs – probably all of them – took off from a quadrupedal start, not a bird-like bipedal one. The quad-launch hypothesis has origins in technical literature dating back to 2008 (Habib 2008, 2013, Witton and Habib 2010) but has a longer history through Mike H’s and Jim Cunningham’s contributions to the Dinosaur Mailing List. In retrospect, quad launch can be seen as a unifying hypothesis in studies of giant pterosaur flight, the piece of the jigsaw that allowed us to see how data from comparative anatomy, body masses and relative bone strength fit together. Before quad-launch, pterosaur flight models struggled to transfer giant azhdarchids from the ground to the air and were forced to cap their body masses at unrealistically low values (e.g. 75 kg in Chatterjee and Templin 2004) in order to launch them like big birds. Other than the fact that masses of 75 kg are untenable for creatures the size of giraffes (they’d need to be something like 70-80% air; Witton 2008), bipedal launch models suffer from several biomechanical issues involving bone strength, limb bone scaling and muscle size, as well as inconsistencies concerning pterosaur gaits. It's these issues which Mike and Jim investigated in their studies, making them the first researchers to approach pterosaur launch with objectivity, rather than a priori assuming an avian launch model, and bending pterosaur palaeobiology until it fit.

Ratios of limb bone strength in birds and pterosaurs. Positive values trend towards strength in humeri vs. femora, while negative values skew towards stronger femora vs. humeri. I've left the caption on for greater explanation. From Habib (2008).
Let's unpack these points in a little more detail. Firstly, the main launch limbs of flying animals are - above body masses of 500 g - stronger than their non-launching counterparts, and scale with more pronounced positive allometry (Habib 2008). This reflects launch being the most demanding part of flight. Look closely at a launching animal (high speed video helps) and you'll see that flight does not begin with a flap, but a leap: something like 80-90 % of launch effort stems from a powerful jump initiated by the main launch limbs. This explains why birds have proportionally robust and strong hindlimb skeletons but relatively slender wing bones: as they increase in size, their legs must become proportionally stronger to initiate flight at greater masses (Habib 2008). Pterosaurs, in contrast, show the opposite condition: their forelimbs are larger and stronger than their legs, with this relationship increasingly pronounced in larger species. Mike's 2008 study quantified this distinction, showing that the section modulus - a value proportionate to the strength of a given cross section - is consistently larger in pterosaur humeri than femora, and vice versa in birds, and that these ratios are more extreme at larger body sizes (see diagram, above). We presented similar data highlighting distinction in limb bone scaling in our 2010 paper, as well as quantifying the relative weakness of azhdarchid femora - their neck vertebrae are actually stronger than this major limb bone. Their humeri, in contrast, were very strong even in giant taxa modelled at the upper limits of pterosaur mass estimates (below). This is already a strong sign of a forelimb-dominated launch strategy in giants, and there's more to consider yet.

Raw data on azhdarchid limb bone strength from Witton and Habib (2010). Note the 'avian expectation' column - pterosaur bones do not scale in the same manner as bird bones, indicating a different regime of biomechanical selection pressures, and thus different limits on parameters like size.
Secondly, the avian skeleton has two large girdles for limb muscles: an enlarged shoulder and chest region for flight muscles, and an enhanced pelvic region to anchor those powerful hindlimb launch muscles. Pterosaurs, in contrast, have only one large limb girdle - their shoulders, making this the de facto likely candidate for powering their launch cycles. Using volumetric modelling, Paul (2002) predicted that a giant azhdarchid would have space for 50 kg of muscle in their pectoral region, a value appropriate for initiating flight in 200-250 kg animals (Marden 1994). This strategy is a far more economical use of muscle mass because the same muscles that power flight can also initiate launch, thus allowing quad launchers to have smaller torsos - and thus lower masses - than bipedal launchers. For all their power, the moment birds have launched their legs are effectively useless - they're just dead weight to be hauled around until it's time to land.

Reconstructed skeletons of large and giant azhdarchids in quad-launch poses, from Naish and Witton (2017). Note how the nearly completely known Quetzalcoatlus sp.D - E - lacks a large site for hindlimb muscles - that's typical of all pterosaurs, and an important argument in favour of quad launch.
This is an critical point for giant pterosaur flight as it allows us to make hypotheses about body size maximums related to launch strategy. Because quad launch is a mass-efficient route to flight we can hypothesise that quad launchers could attain much larger overall sizes and masses than bipedal launchers (Witton and Habib 2010; Habib 2013). As everyone knows, this is borne out in our fossil record of volant birds, which max out at 5-6 m wingspans and masses of 22-40 kg (Ksepka 2014), while giant azhdarchids attained wingspans of 10 m and 200-250 kg body masses. Mass-efficient launch mechanics is almost certainly a major factor in how azhdarchids became so big, especially combined with the exceptional azhdarchoid ability for skeletal pneumaticity (Claessens et al. 2009).

Fossil birds like Pelagornis sandersi are pretty big (extant bird with the greatest wingspan, the wandering albatross, shown top right), but they wouldn't be able to poke giraffes in the face when standing next to them. 5-6 m wingspans are the known size limit for bird flight, and their inefficient launch mechanism is probably the cause. From Ksepka (2014).
A further line of evidence for quad launch concerns pterosaur trackways. Habib (2008) also notes that launch in living tetrapod fliers correlates to terrestrial gait: the number of limbs used to locomote on the ground is the same as the number used to take-off. Birds walk and launch with two legs, while bats walk and launch using all four. An extensive record of pterosaur trackways shows that pterosaurs were quadrupedal animals like bats, and it stands to reason that they also launched from four limbs: they would contrast with our living fliers if they had to shift gaits to take off. Our pterosaur footprint record includes trackways of quadrupedal giant pterosaurs (Hwang et al. 2002), so we can comfortably extend this observation to them, too. Incidentally, the fact that several bats take off quadrupedally is often overlooked in discussions of pterosaur launch: bird-like bipedal launches dominate our consciousness only because we see them taking off every day, but they do not represent the only way tetrapods can become airborne.

Quad launch cycle in vampire bats Desmodus rotundus, traced from video footage: this is a real, proven launch mechanic folks, not something dreamt up by pterosaur workers desperate to prove giant azhdarchids could fly. Several other bats launch in this way, too. From Schutt et al. (1997).
These points - bone strength, concentrations of muscle bulk, limb bone scaling and trackway data - are the cornerstones of the pterosaur quad launch hypothesis, an idea which explains many independently observed features of pterosaur biomechanics, bone proportions and absolute size. Crucially, all of these points can be investigated for giant azhdarchids, and there are no red flags suggesting quad launch did not apply to these pterosaurs. Ergo, we can assume that giant azhdarchids used the most efficient launch mechanism conceivable for a tetrapod, negating any need for unreasonably low mass estimates or cliff jumping to become airborne. We can understand why pterosaur humeri are so strong and their femora so (relatively) weak, as well as the impact this has on overall size. Through birds, we have as inverse proof of this relationship. Furthermore, quad launch negates the need for special assumptions about giant pterosaur flight, allowing us to cast the biggest azhdarchids as 'extreme' versions of the pterosaur bauplaun, not evolutionary weirdos that take us back to the biomechanical drawing board.

Despite the sound scientific basis to quad-launch it is sometimes dismissed out of hand, I think because many folks just can't imagine it working - this excellent video by Mike and Julia Molnar does a good job of showing the kinematics. My experience is that counter-arguments are made without knowledge of its supporting data as they focus on less knowable components of the launch cycle, such as the speed of wing action or intuitive ideas about how high pterosaurs could leap. These are poor argument because they a) are largely speculative, not based on measurable/observable phenomena like bone strength or trackways; and b) ignore the fact that any launch mechanic requires rapid deployment of wings or an ability to obtain good ground clearance. We have to assume giant pterosaurs could achieve these feats no matter what our preferred launch strategy is. Moreover, somewhat ironically, the elevated flight speeds necessitated by giant pterosaur mass actually minimises some of these concerns. Flapping amplitude scales negatively with animal size and flight speed, making ground clearance less of an issue for large fliers than smaller ones (Habib 2008).

Flying the gauntlet

Let's put all this together - congratulations if you've waded through this long, often technical post. Giant azhdarchids...
  • are poorly known, but have anatomy consistent with volant habits in every known aspect.
  • do not seem to have struggled with take-off energetics more than any other large flyer.
  • are often anatomically mischaracterised, being overly compared to extant birds or modelled in ways which distort their likely flight parameters.
  • evolved from animals with a fundamentally more efficient launch strategy than that of birds, which lifts their body mass ceiling well above that predicted for avians. Every tested aspect of giant azhdarchid anatomy points to retention of this launch strategy even at their huge sizes.
  • have flight parameters which, when modelled using conventional animal flight software in modern-grade atmosphere and gravity, equate to excellent flight performance, analogous to that of large soaring birds.
The take-home message is that interpretations of giant azhdarchids as flying animals are based on numerous corroborating lines of investigation and hypotheses which support and predict one another. Moreover, the methods used in these studies are entirely conventional techniques of palaeontological inquiry, and to disregard or ignore them requires dismissal of entire scientific fields of study. Don't buy the limb bone strength studies? Fine, then you also don't buy beam theory or structural engineering. Don't believe the flight analyses? OK, but you're also challenging software written by noted experts in animal flight, using data measured from real flying animals and a deep understanding of aerodynamics.

This is not to say that we know all there is to know about giant pterosaur flight - far from it. They remain poorly known animals and we can only guess at their variation in flight performance. Who knows, maybe a flightless species will turn up one day - this is not a ridiculous concept, we just don't have any evidence for it yet. But, for now, anyone seriously wanting to challenge this interpretation needs to discredit a robust theoretical foundation of pterosaur flight mechanics and provide a superior interpretation of the many strands of evidence we've discussed. This seems like a tall order to me, but it's the gauntlet that anyone who says giant pterosaurs were 'too big to fly' or 'they needed different gravity' has to run. Such comments reflect an ignorance or unwillingness to engage with a growing body of sound technical research on these animals, and - unlike giant pterosaurs - their arguments just don't fly.

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