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Saturday, 10 October 2015

We just can't quit you, Pterodactylus

A small flock of Pterodactylus antiquus, represented by small juveniles (left) up to big adults (right) scope out foraging options in a Jurassic marsh. The animal on the right is luring prey to the surface through paddling forefeet, a behaviour common to (at least) several modern gull species.
Pterosaur researchers are infamous for their frequent disagreements over flying reptile evolution, lifestyles and even basic anatomical interpretation. I can certainly attest that there is some truth to this: writing about pterosaurs can be frustrating because of continual need for clarification and digressions to ensure all points of view are represented. But if there's one pterosaur we must have reached a consensus over, one species we must all agree about, surely it's the Late Jurassic, Solnhofen species Pterodactylus antiquus. The holotype of Pterodactylus - below - has been known to us longer than any other pterosaur fossil, this being the specimen which kick-started flying reptile studies in 1780. Since then, it's almost a rite of passage for researchers to see this specimen along with some of the other several dozen Pterodactylus fossils in museums around the world. Pterodactylus has been looked at so much that a general agreement over what it is, and the species it is related to, has more or less been established. Characterised by a long, low skull, simple teeth, a long neck and largish feet, we consider Pterodactylus a relatively early form of pterodactyloid, and most likely a member of the ctenochasmatoid/ archaeopterodactyloid* branch of pterosaur evolution. This puts it in the same lineage as several other familiar species, such as comb-toothed Ctenochasma and twisty-jawed weirdo Cycnorhamphus. We're all basically happy with the idea that our dataset for Pterodactylus is fairly good: 30 specimens (a very conservative estimate) provide numerous complete skeletons and a growth series from small juveniles up to very large adults.

*As with many parts of the pterosaur tree, nomenclature for 'Pterodactylus-line' pterodactyloids is confused by the use of several, conflicting names and definitions. I wasn't kidding about those caveats and digressions.

All that said, some areas of Pterodactylus research remain contentious, and new insights into its anatomy and disparity are still being published more than 200 years after it was discovered. Surprisingly, unlike the way we often gain novel appreciation for familiar taxa - new specimens shedding new light on old problems - much of our recent understanding of Pterodactylus relies on the same, well-worn specimens we've been analysing for centuries. It's actually quite sobering to see specimens which have been interrogated so much still providing talking points, and it makes me wonder what we're missing from those briefly described, rarely analysed specimens comprising so much of our vertebrate fossil dataset. Here, I want to cover some of the new insights provided on Pterodactylus in just the last two years.

The specimen which started it all: the Pterodactylus antiquus holotype. The wingspan of this specimen, preserved in a 'falling forward' posture rather atypical for a Solnhofen pterodactyloid, is about 45 cm.

Anatomy and palaeobiology

Until recently, most of us have been used to seeing Pterodactylus depicted as a crestless species. A privately owned specimen described in 2003 showed that, like many other pterosaurs, this animal bore a set of soft-tissue structures associated with the top and back of the skull (Frey et al. 2003). Specifically, it seems Pterodactylus bore a soft-tissue crest along the posterior region of the head and a pointed, posteriorly-projecting 'occipital lappet' at the back of the skull. The latter, for now at least, seems unique to Pterodactylus. This information is well known to 21st century scholars, but it's less appreciated that these soft tissues were first mentioned almost 100 years ago. Pterodactylus crests were first reported in the 1920s, and the lappets in 1970 (see Bennett 2013). I find it bizarre that we didn't start restoring Pterodactylus with these interesting structures until the 2000s: this seems to be an example of artists and scientists not working together as well as they might.

Unlike other pterosaurs, the soft-tissue crest of Pterodactylus did not seem to anchor on a low, striated bony ridge. The absence of this feature, even when preservation was sublime enough to record soft-tissues and detection methods were of late 20th century quality, was likely a key factor in our general consideration of Pterodactylus as a crestless species. I always found the occurrence of soft-tissue crests without corresponding bony structures an alarming prospect, one implication being that we could be ignorant of soft-tissue crests in a huge number of pterosaur species.

It was somewhat relieving, therefore, to see Chris Bennett reporting a crest-anchoring structure for Pterodactylus in 2013. It's small, and often smooth rather than striated, but Pterodactylus definitely does have a midline ridge for crest anchorage - even on the holotype has one when we look close enough. Exactly how extensive these structures were remains unknown thanks to many historic specimens being accidentally damaged during preparation. It's easy to see how this occurred: the crests are low, extremely fragile, and only 0.2 millimetre thick. They'd be hard to detect and avoid damaging even if you were looking for them. Hopefully, preparators working on unprepared specimens can recover intact crests now we know they exist.

The most extensive example of cranial soft tissues known thus far from Pterodactylus. Unfortunately, we're still some way from knowing what shape they took in life, although this specimen indicates that almost half of the skull was covered by the crest and that the lappet was also quite large. Parts of the diagram labelled 'fa' record sediments which fluoresce under UV light - they're likely matrix contaminated by organic seepage from the decaying pterosaur head. They are unlikely to tell us much about the appearance of the animal in life. From Bennett (2013).
Pterodactylus cranial soft tissues are now known to occur in a number of specimens, but it remains unclear how large or what shape the crests were. The lappets seem to be of a consistent size and position, and many curve upwards, but whether they are joined to the rest of the crest (as suggested by Frey et al. 2003) remains to be confirmed (Bennett 2013). At least some aspect of crest and lappet development matches what we see in other pterosaurs, in that we only start picking up evidence of these structures in larger Pterodactylus specimens. There also seems to be a rough correlation between crest proportions and body size. Pterodactylus thus seems to be another pterosaur species where cranial ornament signifies entry into adulthood, suggesting a function of sexual communication (Bennett 2013).

Speaking of adulthood, it was also only recently that we've obtained a true sense of how large Pterodactylus may have grown. We typically imagine this animal as small bodied - maybe with a 50 cm wingspan - but a newly described skull and lower jaw (below) makes the first unambiguous case for Pterodactylus reaching at least 1 m across the wings (Bennett 2013). To put this in a modern context, large Pterodactylus would be of comparable size to smaller heron species, and large individuals would have been conspicuous components of the Solnhofen pterosaur fauna. A trend where skull, neck, and limb proportions increase with body size, first intimated by Peter Wellnhofer (1970), seems to hold up in modern interpretations of Pterodactylus specimens. Realising how variable this pterosaur's proportions might have been throughout life has been very informative to recent considerations of Pterodactylus taxonomy.

The mother of all Pterodactylus skulls. A preserved skull length of 142 mm indicates a skull of around 200 mm long in life, and an animal reaching a 1 m wingspan. From Bennett (2013).

One species, two species, or three genera?

This brings us to some of the more contentious recent developments in Pterodactylus studies: just how many species are represented in the Pterodactylus dataset? Many readers will be aware that the name Pterodactylus was once applied to almost any new pterosaur fossil, and around 80 'Pterodactylus' species have existed in the last 200 years (Ford 2013). The taxonomic history of Solnhofen pterodactyloids has been especially mixed up with the name Pterodactylus and, by the end of the 1800s, their taxonomy was in a real tangle. Work in the mid-20th century, particularly by Peter Wellnhofer (1970), streamlined systematic interpretations of Pterodactylus so that, by the 2000s, only two species were considered valid: P. antiquus and P. kochi. A couple of 'hangers on' were still knocking about ('P'. longicollum and 'P'. micronyx), but researchers universally agreed that these animals were not true members of Pterodactylus, and were simply awaiting new generic names (they now have them: Ardeadactylus and Aurorazhdarcho, respectively).

Distinguishing features between kochi and antiquus were subtle, being primarily aspects of tooth shape, tooth number, and proportions of the skull, neck and torso. This is not a new observation, and suggestions that they may represent the same taxon date back to the 1800s. Eventually, studies of Pterodactylus teeth was used to suggest outright synonymy of these two species (Jouve 2004). Many pterosaurs, as with most reptiles, increase their tooth counts with age and size. Jouve realised that the allegedly distinctive tooth count of P. kochi aligned perfectly with tooth numbers predicted for antiquus of comparable body size. At least in this respect, these two species could not be distinguished. More recently, Bennett (2013) bolstered this synonymy with an assessment of kochi proportions, noting that perceived distinctions in skull and body length were reliant on erroneously recorded measurements. Once corrected, kochi proportions were very similar to comparably sized antiquus individuals (there's a lesson there about the importance, and repetition, of basic data recording in this) and, along with Jouve's work, this study have eroded the foundations of the kochi/antiquus split considerably. Remaining distinguishing features between these species are rather poorly defined, and certainly not divorceable from effects of growth, preservation and preparation. Finally, after 200 years, it was looking like Pterodactylus taxonomy had finally been tidied up: we have one Pterodactylus species, not two, or 80.

Historically considered to represent Pterodactylus antiquus, recent work argues this specimen (along with some referred material) is a wholly distinct species, and distantly related to P. antiquus. It was recently christened Aerodactylus scolopaciceps. Image from Vidovic and Martill 2014 (this particular version from Steve Vidovic's Mesozoic Monsters blog).

Except... the story doesn't end there. Last year, my University of Portsmouth colleagues Steven Vidovic and David Martill suggested that not only were 'cryptic taxa' present in the Solnhofen Pterodactylus dataset, but that the traditional phylogenetic placement of some Pterodactylus-like animals might be erroneous. Using a variety of methods, Steve and Dave proposed that Pterodactylus contained at least three taxa: antiquus (which they considered the only true member of the genus), kochi (a separate genus in their interpretation, and more closely related to other pterodactyloids than antiquus), and a resurrected Pterodactylus species from the 1800s, scolopaciceps (Vidovic and Martill 2014, see image, above). Steve and Dave created the generic name Aerodactylus for this animal, providing a diagnostic combination of over 10 character states relating to skull shape and proportions, orbit shape, tooth count, neck length, humeral curvature and limb bone robustness. Attempting to establish the relationships of these three 'Pterodactylus' taxa saw Ctenochasmatoidea/Archaeopterodactyloidea dissolve into a paraphyletic spread across the base of Pterodactyloidea. In this topology, antiquus and kochi anchor the base of Pterodactyloidea (without forming an exclusive clade themselves) and scolopaciceps is at the other end of the 'ctenochasmatoid' range, in a sister clade to the rest of Pterodactyloidea.

That's quite a shake up, contradicting virtually all other recently published opinions on the taxonomy and evolution of these animals. But although different, at least some of these ideas are not be untenable. For instance, the idea that Ctenochasmatoidea/Archaeopterodactyloidea might be paraphyletic is suggested by the 'Painten pro-pterodactyloid', an unusual pterosaur specimen revealed two years ago (below, Tischlinger and Frey 2013). This taxon, which shows a Pterodactylus-like skull combined with postcranial features somewhat like those of non-pterodactyloid pterosaurs, suggests aspects of 'ctenochasmatoid' anatomy developed outside of Pterodactyloidea proper. It therefore will not be that surprising if this taxon pulled some 'basal' ctenochasmatoids of traditional lore to the root of Pterodactyloidea once it's included in phylogenetic studies. (Those interested in the influence of the 'Painten pro-pterodactyloid' animal on our understanding of pterosaur evolution might find this previous post of interest).

The 'Painten Pro-pterodactyloid' specimen, messing up our nice, neat interpretation of pterodactyloid evolution since 2013. Notice the Pterodactylus-like posterior skull morphology alongside traditional non-pterodactyloid features - a long(ish) tail and big fifth toes. From Tischlinger and Frey (2013).
But do our few dozen Pterodactylus specimens really comprise three, distantly-related species? On this, I'm less certain. We see a lot of variation across Pterodactylus specimens reflecting those aforementioned factors of ontogeny, preservation and preparation - not to mention individual variation. Having played with Pterodactylus data a little myself, and seen a fair share of specimens relevant to these discussions (though I stress not all), I find the arguments for synonymy more compelling than those for splitting Pterodactylus apart. This said, I have no horse in this race and could be persuaded otherwise. What we really need - and a number of folks in pterosaur research have been saying this for a while now - is someone to travel the world exhaustively documenting and illustrating Pterodactylus specimens, ultimately producing a modern synthesis on its anatomy. Such a study would not only be a valuable research aid (the last attempt at this was 50 years ago, which is an age ago in terms of research and publication techniques), but would pack a lot of weight in resolving ongoing, long running disputes in this animal's taxonomy.

Talking about the future of research into Pterodactylus seems like a sensible place to leave off, and I'll summarise in saying that - as with much else in pterosaur research - we're a little while off a complete consensus on Pterodactylus for now. Clearly, although the concept of Pterodactylus is over two centuries old, there's still things learn about it. Who knows what we'll be saying about this most familiar of pterosaurs in years to come?

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  • Bennett, S. C. (2013). New information on body size and cranial display structures of Pterodactylus antiquus, with a revision of the genus. Paläontologische Zeitschrift, 87(2), 269-289.
  • Ford, T. L. (2013). Is Pterodactylus monophyletic or paraphyletic? Short Communications - International Symposium on Pterosaurs, Rio Ptero 2013. 68-70.
  • Frey, E., Tischlinger, H., Buchy, M. C., & Martill, D. M. (2003). New specimens of Pterosauria (Reptilia) with soft parts with implications for pterosaurian anatomy and locomotion. Geological Society, London, Special Publications, 217(1), 233-266.
  • Jouve, S. (2004). Description of the skull of a Ctenochasma (Pterosauria) from the latest Jurassic of eastern France, with a taxonomic revision of European Tithonian Pterodactyloidea. Journal of Vertebrate Paleontology, 24(3), 542-554.
  • Tischlinger H, Frey E. 2014. Ein neuer Pterosaurier mit Mosaikmerkmalen basaler und pterodactyoider Pterosaurier aus dem Ober-Kimmeridgium von Painen (Oberpfalz, Deutschland) [A new pterosaur with moasic characters of basal and pterodactyloid Pterosauria from the Upper Kimmeridgian of Painten (Upper Palatinate, Germany)]. Archaeopteryx 31, 1-13.
  • Vidovic, S. U., & Martill, D. M. (2014). Pterodactylus scolopaciceps Meyer, 1860 (Pterosauria, Pterodactyloidea) from the Upper Jurassic of Bavaria, Germany: the problem of cryptic pterosaur taxa in early ontogeny.
  • Wellnhofer, P. (1970). Die Pterodactyloidea (Pterosauria) der Oberjura-Plattenkalke Süddeutschlands. Bayerische Akademie der Wissenschaften, Mathematisch- Wissenschaftlichen Klasse, Abhandlugen, 141, 1-133.

Tuesday, 6 October 2015

New sauropodoramas: Stormy brachiosaurs! Apatosaurine brontosmash!

Realising that Recreating an Age of Reptiles was a bit light on sauropod art, I've been beavering away on two additional sauropodoramas* to pad things out a bit. I thought I'd share them here.

*Sauropods are such special animals that they deserve their own nomenclature for most things, including artwork. See, for another example, 'shards of excellence'.

The first is a reworking of a 2013 image of the Wealden (probable) brachiosaur Pelorosaurus conybeari in hammering wind and rain. We know that Wealden climates were subject to storms and intense downpours on occasion (lightning and floods being, of course, key elements in the production of fossil-rich plant debris horizons in certain Wealden deposits) and it stands to reason that any sauropods around when those rains arrived would have got quite wet indeed. I don't say that just casually: the prospects of being a wild animal the size of a house mean that you're actually pretty exposed to just about everything weather can throw at you. When unexpected meteorological fit hits the shan, your options as a giant are pretty limited. Running away is out, because your legs are pillar-like structures adapted for supporting immense weight, not nimble escape. Seeking shelter is not an option either, because you're bigger than everything else around you. You're just too darned huge to do anything but stand there and take it. The life of a sauropod must've been spent baking in the sun, being battered by wind, and drenched in rain. I find that idea quite romantic and evocative as an artist. When painting sauropods, I often wonder how cracked, weathered and worn their skin must've been through a lifetime of battles with changing weather.

Like masts in a storm, three Pelorosaurus conybeari brave typically English weather, c. 135 million years ago. They're doing their best to look tough next to a couple of rainbows.
Second is an image inspired by a recent SVPCA talk by sauropod expert Mike Taylor and his colleagues Matt Wedel, Darren Naish and Brian Engh. Regular readers of the palaeoblogosphere will probably already know where this is going, given that Mike's talk (and the upcoming Wedel et al. paper) has been given some hefty coverage at SV:POW!. Those familiar with sauropods will know that apatosaurines (Apatosaurus, Brontosaurus and a few other taxa) have atypically proportioned, large and robust neck vertebrae, with their cervical ribs being especially elongated and reinforced. These structures possess peculiar buttresses on their underside which, it seems, are not products of muscle or ligament attachment (if they are, they have no modern analogue). Instead, they might relate to an epidermal feature like a boss or horn, as such structures sometimes leave peculiar swellings on underlying bones. Exactly what these anatomies indicate has long been puzzling, and all the more so because all apatosaurines show neck vertebrae with these features. Some (like Brontosaurus) were more extreme than others in development of these features, but even modest apatosaurines were doing crazy, mysterious stuff with their neck anatomy. Question is, what?

Matt, Mike and others have recently been outlining a first principles approach to this conundrum. They note that the reinforced construction of apatosaurine necks, the additional muscle attachment afforded by vertebral expansion, and those strange vertebral buttresses might render their necks effective clubs or wrestling appendages, particularly well suited to rapid, powerful downward motions. Summarised a little more succinctly: there is reason to think Brontosaurus and kin might've smashed the crap out of each other, or other animals...

...with their necks.

Yowsers. But outlandish as the Brontosmash hypothesis seems, it really isn't just idle speculation: a paper is in the works, the Taylor et al. SVPCA talk abstract is a preprint at PeerJ, and you can see the case explained in Mike's talk slides here. I find it pretty convincing myself: I mean, there had to be some reason apatosaurines had those crazy necks. Evolution is a sloppy craftsman at times, but the energy put into growing and maintaining such massive neck anatomy must've been substantial, and that almost certainly reflects a certain adaptive purpose. Combat might well have been that driving force. We also know from living animals - camels, giraffes and some seals - that necks are used for fighting, and that neck-based combat can promote reinforcement and restructuring of neck anatomy. It certainly sounds provisionally convincing to me, and I'm sure we'll hear a lot more about it in the future as the hypothesis is developed.

We're also sure to see this concept frequently in future palaeoart. Mike has been collecting some of the early artwork of this idea over at SV:POW!, including a wealth of coloured sketches and concepts by Brontosmash coauthor and palaeoartist Brian Engh, palaeoartist Bob Nicholls, #MikeTaylorAwesomeDinoArt (the revolution palaeoart deserves, if not the one it needs) and an alternative interpretation of apatosaurine neck data provided by myself (we secretly know I'm on the money with that one). I also decided to attempt a full on painting:

Multiple tonnes of Brontosaurus excelsus in disagreement.
There're two nods to classic palaeoartists here. There's a Knightian influence to the style (not the first time he's infected my work), as well as, via the very upright postures of the wrestling animals, a hat-tip to Robert Bakker's famous 'boxing Brontosaurus' image. The latter had a big impact on me when I first saw it as a teenager, and it's been on my mind for obvious reasons with all this talk of fighting apatosaurines. I thought it also made for a bit of a contrast to Brian's 'official' depictions as well, these showing the animals in quadrupedal or near-quadrupedal poses (I assume at least some of the postures in those artworks mimic neck combat in elephant seals, a favoured modern behavioural analogue of Team Brontosmash). The setting is meant to be in the wetter, northern parts of the Morrison Formation palaeoenvironment, alongside swollen river margins. Initial plans were to record the progression of the wrestling match in muddy footprints, but adding splashes and visual noise to proceedings was too much fun, especially with those tails whirling around everywhere. Sloshing water provided a means showing specific actions, too, the splashes from colliding brontosaur hide signifying each powerful, multi-tonne impact. This was definitely a fun image to put together, and it's certainly a favourite of my recent work. Brontosmash!

That's all for now. Coming soon (probably): The Triassic! And a boring old pterosaur that we just can't leave alone!

These sauropodoramas were brought to you by Patreon

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Friday, 25 September 2015

What pterosaurs tell us about the evolution of feathers

2011 PR image for the 2014 description of Laquintasaura venezuelae, a basal ornithischian from Venezuela. Scales were the requested integument for this reconstruction, but how does that decision hold up today?
For the last two weeks I've been revising an image of the Jurassic ornithischian Laquintasaura venezuelae. The original (above) was produced in 2011, but a request to include it in an upcoming book was impetus to tidy up the art and update the anatomy. One significant question for updating this old piece was whether the animals should stay scaly or receive a coat of filaments. The systematic placement of Laquintasaura isn't certain, but it seems to lack features allying it to major ornithischian clades and, for now, is simply considered a basal member of Ornithischia (Barrett et al. 2014). This puts it in a controversial spot as goes interpretation of dinosaur integument: scales, filaments, or a mix of both?

The origins on filamentous integuments and feathers in reptiles remains an ongoing source of fascination and investigation for palaeontologists. It has been known that filamentous reptilian integuments extend deep into geological time since the 1800s, but research into these structures exploded in the 1990s and 2000s when fossils of many non-avian theropods - seemingly all coelurosaurs - were found adorned with feathers or filamentous feather precursors. Soon after, recovery of quills, filaments and strange, fibrous scales in ornithischians made a reality of once speculative ideas about filaments being widespread across Dinosauria. For years now, palaeontologists have been discussing the possibility that theropod filaments and feathers share ancestry with those of ornithischians. One implication of this is that bodies of dinosaur ancestors would be covered in fuzz instead of, as traditionally supposed, scales. Unravelling this conundrum is of key interest to those attempting to understand ancient reptile evolution and physiology, as well as for artists wanting to know how to credibly restore early dinosaurs. However, integument preservation, and particularly filamentous hide, is rare in the fossil record. Much as we might want to, we currently have insufficient data about the skin of early dinosaurs to address this issue directly.

All is not lost, however: some insight into dinosaur filament evolution can be provided by pterosaurs. Flying reptiles and dinosaurs are largely thought to form a more or less exclusive clade, the Ornithodira, which we now recognise as being characterised by a suite of anatomies - not just hindlimb features, as originally proposed - and commonalities of interpreted anatomy: postcranial pneumaticity, upright postures, elevated metabolisms, and filamentous integument. It's the latter which makes pterosaurs potentially useful to understanding the ancestral state of dinosaur skin. It's a little surprising that it's taken us so long to capitalise on this data, since we've had conclusive evidence of pterosaur filaments (we call them pycnofibres) since the 1970s (Sharov 1971). Suggestions that pycnofibres may have been homologous to dinosaur fuzz arrived much later, in the 2000s, when the evolutionary depth of dinosaurian filaments had become apparent and new discoveries of fuzzy pterosaur fossils were being reported (Czerkas and Ji 2002; Ji and Yuan 2002). Perhaps it was the coincidence of these events, the realisation that filaments were widespread in Pterosauria, and increased confidence in the sister relationship between dinosaurs and pterosaurs which lead to this idea finally being proposed.

Late Jurassic pterosaur Sordes pilosus, described in 1971, was one of the first pterosaurs confirmed to have a filamentous body covering. But are pterosaur filaments tied to those of dinosaurs, or independently evolved?
Studies into pterosaur and dinosaur filament homology remain thin on the ground, and much of what has been said thus far is reliant on gross filament morphology. Earlier this year, a team of researchers (Barrett et al. 2015) tackled the issue of ornithodiran filament evolution quantitatively, estimating the likelihood of homology between theropod, ornithischian and pterosaur integuments via their distribution on the ornithodiran tree. Using 18 different variations in methods, calculations and data values, they predicted the likelihood of ancestral integument states in dinosaurs and ornithodirans: were they scaly, filamentous, or feathered? The result, announced in not only the paper but also a subsequent media release, was that 12 of those 18 assessments suggested scales were ancestral to ornithodirans, and the filaments seen in pterosaurs, ornithischians and theropods were derived independently from a common scaly ancestor.

This conclusion was undoubtedly surprising to some and, indeed, a clear caveat accompanies it: scaly ancestral dinosaurs are "sensitive to the outgroup condition in pterosaurs". Support for ancestrally-scaly ornithodirans relies on the assumption that pterosaur ancestors were also scaly. This condition assumed for 50% of those 18 assessments to account for uncertain ancestral condition for pterosaur integument. In the 9 analyses where pterosaurs were treated as wholly filamentous - and thus consistent with what we see in existing pterosaur fossils - six returned results indicating an ambiguous scaly/filamentous ancestral condition for ornithodirans and dinosaurs, and only 3 supported a wholly scaly interpretation. Of those six 'ambiguous' results, most reported a strong likelihood of ornithodirans being ancestrally filamentous, and many gave dinosaurs a good chance of being ancestrally filamentous too. Moreover, treating pterosaurs as filamentous has knock-on effects through the dinosaur tree: suddenly, there are reasonable, or at least equivocal, chances that ornithichians and saurischians were also ancestrally filamentous. This is a different conclusion to the straighter story of ornithodirans and dinosaurs simply being ancestrally scaly.

What influence do fuzzy pterosaurs have on dinosaur skin evolution? Seemingly, quite a bit. The tree on the left shows integument likelihoods (pie charts) where pterosaurs are considered scaly, tree on the right shows a filamentous analysis.  Modified from Barrett et al. (2015).

Clearly, the crux of all this is the assumption that pterosaur ancestors were scaly: just how defendable is this? Because we know little about pterosaur origins, it's hard to say anything conclusive about the evolution of pterosaur integument with our current fossil record. The stratigraphically oldest pterosaur fossil with pycnofibres is from Middle/Late Jurassic deposits, and thus about 50-60 million years younger than the oldest pterosaur fossils - little help in determining if the first pterosaurs were fuzzy. Ongoing disagreements over pterosaur phylogeny complicate attempts to estimate the appearance of lineages with confirmed pycnofibres. Some schemes (those derived from Kellner 2003 and Unwin 2003) suggest pycnofibres must have appeared by the Triassic, close to or at the base of pterosaur ancestry, but others (e.g. Andres et al. 2010) indicate pycnofibres reliably extend no further than the Lower Jurassic. Of course, such assessments of filament distribution might not even be meaningful at this stage, given that pycnofibres are very rare components of pterosaur fossils. They are nowhere near as common as other soft-tissues, such as wing membranes, and we should probably be cautious about any assessment of their evolutionary pathways until we have more data. Perhaps the only significant observation we can make from our current, limited dataset is that, to date, no pterosaur is known with a scaly body covering, even when regionalised scalation - foot pads - preserves in their fossils (Frey et al. 2003).

A possible pterosaur relative with scaly hide is known: the Triassic archosaur Scleromochlus taylori. Benton (1999) described structures interpreted as thin, transversely orientated scales across the back of multiple specimens of this animal. This might provide vindication of the scaled pterosaur ancestor model, but, again, there are some caveats with this idea. For one, Scleromochlus fossils are not well preserved. The scales are feint sediment impressions, visible only in strong, low angle light, such that that they are only considered 'probable' integument impressions by Benton (1999). Previous workers have interpreted them in a different way (as gastralia). Clearly, the evidence for them being scales could be more compelling, and there's certainly not much to work with if we want to test their identification. Secondly, exactly how Scleromochlus is related to pterosaurs is not precisely agreed. Some workers consider it the sister taxon of Ornithodira, others as a member of the pterosaur branch, and others see it as more closely related to dinosaurs than pterosaurs. That might seem a minor issue, but we've already seen how sensitive models of ornithodiran integument are to changes of single variables at the base of the tree. We would probably need to run many variants of the integument probability calculations to account for all the uncertainty surrounding Scleromochlus. This might give more idea of the range of possible integuments at the base of ornithodiran evolution, but that's not much of an improvement on our current situation.

Was Scleromochlus taylori scaly? Maybe - weakly preserved structures on several specimens seem to suggest so. On this diagram, from Benton (1999), possible transverse scales can be seen on the left and middle specimen.
In all, I feel like we're hitting a bit of a wall here. It seems we just don't know enough, and have too many caveats with the limited data we have, to make even a half convincing best guess on this. Thus, how much weight we put on models of ornithodiran integument using scaly pterosaurs is almost a philosophical issue. From my end, I don't think they should be used to argue for scaly ornithodiran and dinosaurian ancestors, at least not with the same weight as tests made using a filametnous pterosaur lineage. When reconstructing ancestral states, characters objectively observed in fossils have to trump assumed character states, even if we know that our dataset is full of holes. After all, the whole point of attempting to figure out an ancestral state is establishing links between character data we have, so introducing opposing character states seems a little contrary to that objective. To be clear, I'm not saying that running models with scaly pterosaur ancestors is a waste of time. To the contrary, it's a good test of model robustness, and Barrett et al. (2015) certainly demonstrate how sensitive our models of ornithodiran integument evolution are by using this approach. Their hypothetical scaly pterosaurs demonstrate that we really do need more early ornithodiran fossils to understand ornithodiran skin evolution. However, I do not think that results of the scaled pterosaur analyses are as informative as their other assessments, as we have to overlook existing data to consider them equally valid.

With all that said, do pterosaur fossils really help us understand the evolution of dinosaur filaments? Playing the conservative card here, it seems they do not provide super strong evidence for an all-fuzzy Dinosauria, but they certainly make it difficult to defend ideas of entirely scaly dinosaur ancestors. Forcibly arguing for either scales or filaments at the base of Dinosauria seems premature at this stage, and, whatever our personal hunches are, it seems sensible to accept some ambiguity in this situation for now.

I began this article with my Laquintasaura conumdrum: how did that play out when, apparently, I can't make up my mind about this scales and filaments debate? Well, I've argued elsewhere that palaeoart can do no better than illustrate credible interpretations of the past and that, so long as the hypotheses they depict are sound, they're doing OK. When we have conflicting or ambiguous hypotheses, we just have to make a judgement call based on our own opinions, gut feelings and interpretations of existing arguments. With my own leaning being towards data showing that scales may not be ancestral to ornithodirans, but also knowing that some dinosaurs are mosaics of filaments and scales, I decided to partially enfluffen my Laquintasaura, while leaving their snouts, tails and limbs scaly. I'll leave you with the revised image.

Laquintasaura venezuelae 2015 edition: basically the same picture, but a bit fluffier, and a bit greener.

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  • Andres, B., Clark, J. M., & Xing, X. (2010). A new rhamphorhynchid pterosaur from the Upper Jurassic of Xinjiang, China, and the phylogenetic relationships of basal pterosaurs. Journal of Vertebrate Paleontology, 30(1), 163-187.
  • Barrett, P. M., Butler, R. J., Mundil, R., Scheyer, T. M., Irmis, R. B., & Sánchez-Villagra, M. R. (2014). A palaeoequatorial ornithischian and new constraints on early dinosaur diversification. Proceedings of the Royal Society of London B: Biological Sciences, 281(1791), 20141147.
  • Barrett, P. M., Evans, D. C., & Campione, N. E. (2015). Evolution of dinosaur epidermal structures. Biology letters, 11(6), 20150229.
  • Czerkas, S. A., & Ji, Q. I. A. N. G. (2002). A new rhamphorhynchoid with a headcrest and complex integumentary structures. Feathered Dinosaurs and the origin of flight, 1, 15-41.
  • Frey, E., Tischlinger, H., Buchy, M. C., & Martill, D. M. (2003). New specimens of Pterosauria (Reptilia) with soft parts with implications for pterosaurian anatomy and locomotion. Geological Society, London, Special Publications, 217(1), 233-266.
  • Kellner, A. W. (2003). Pterosaur phylogeny and comments on the evolutionary history of the group. Geological Society, London, Special Publications, 217(1), 105-137.
  • Ji Q., & Yuan C. (2002) Discovery of two kinds of protofeathered pterosaurs in the Mesozoic Daohugou Biota in the Ningcheng region and its stratigraphic and biologic significances. Geol. Rev. 48, 221–224.
  • Sharov A, G. (1971). New flying reptiles from the Mesozoic of Kazakhstan and Kirghizia. - Transactions of the Paleontological Institute, Akademia Nauk, USSR, Moscow, 130: 104–113 [in Russian].
  • Unwin, D. M. (2003). On the phylogeny and evolutionary history of pterosaurs. Geological Society, London, Special Publications, 217(1), 139-190.

Friday, 18 September 2015

Humps, lumps and fatty tissues in dinosaurs, starring Camarasaurus

I like to see fossil animals restored as if they belong in the world they're depicted in. That is, not just as basic, conservative reconstructions of ancient species in an certain landscape, but instead with colours, integument and soft-tissue adaptations suited for their possible lifestyles and the environments they frequented. To this end, last year I published an illustration of the Late Jurassic, North American sauropod Camarasaurus supremus as an species well adapted for life in arid settings. As a common part of the famous Morrison Formation dinosaur fauna, dry conditions would be familiar to Camarasaurus, and especially because it occupied the drier, desert-like southern extent of the Morrison palaeoenvironment. I rendered Camarasaurus as a dinosaurian camel, complete with several common cranial adaptations to resisting dry conditions and, most obviously, a fat hump on its back.

2014 restoration of Camarasaurus supremus, published in Witton (2014). Painted to make a point about palaeoart (as well as plugging the awesomeness of All Yesterdays), here's what the caption read. "Reasoned speculation in palaeoart. The sauropod Camarasaurus supremus depicted with adaptations for living in a very dry environment: enlarged nasal cavities to aid resorption of moisture, sealable nostrils to reduce evaporation, wrinkled skin to enhance heat dissipation, white and tan colouring to resist heat soaking, and a fat hump to store energy. Such features are speculative, but do not contradict any data we have for this taxon, and are consistent with the adaptations of modern desert-dwellers."

I decided to revisit this image this week to boost the sauropod content of Recreating an Age of Reptiles (coming soon, I swear!). In doing so, I decided to conduct some more research into the likely nature of non-avian dinosaur fatty tissues. I wanted to keep the fat store on Camarasaurus, as equivalent structures provide energy and water reserves for many modern desert species, and there's no reason to think that extinct dinosaurs would not have developed fat stores for similar purposes. However, is a camel-like hump really likely in a dinosaur? Can we credibly restore any details of dinosaur fats? These were questions I sought to investigate more thoroughly before jumping into my revisions.

Yo extant diapsids so fat

If we're thinking about how to restore dinosaur fats, we need to investigate what the reptile lineage is capable of when it comes to producing and storing fatty tissues. The composition of diapsid fats is a little different to our mammalian ones, although we share functionally comparable approaches to fatty tissue makeup in many respects, including responses to endothermic demands (Goff and Stenson 1988; Saarela et al. 1991; Azeez et al. 2014). Amniotes, as a whole, have fairly similar approaches and uses for fatty tissues, which is great, because that allows us to make some reasonable inferences about fossil species.

Modern reptiles generally have lower fatty tissue fractions than mammals because of their lower energy requirements (Birsoy et al. 2013; Azeez et al. 2014). However, this is not to say that they are incapable of storing large quantities of fat, or even putting on weight rapidly. Some reptiles are indeed lean species, but some - most famously certain geckos, but also some iguanas, skinks and snakes - periodically or permanently hold large stores of fat in case of hard times, or to prepare themselves for energy-intensive feats (e.g. reproduction or long distance travel). Reptiles generally sequester fatty deposits within their torsos or in their tails, but some species also store them in their armpits and in fat pockets located at the back of the head. Individuals of many lizard species are considered healthy when these regions are literally bulging with fatty mass. To my knowledge, these masses are not directly supported by the skeleton or other tissues: it is simply the cohesive nature of fatty tissues and dermis which keeps them in place. It is known that some lizards can pack their tissues with fat rapidly when necessary, some experiments finding geckos can increase their body mass by 50% in four days (enough fuel to sustain them for over half a year!) (Mayhew 2013). Indeed, reptiles are so good at packing on fat, and maintaining it, that owners pet reptiles will know that obesity can be a real issue for captive lizards.

What about living dinosaurs? As with other diapsids, birds can rapidly generate fatty tissues in anticipation of stressful periods, and frequently do so before, for instance, migrating (Lindström and Piersma 1993). 10-15% body fat is considered low for a migrating bird, with the bodies of some species comprising 50% fatty tissues before embarking on their travels - seasoned ornithologists recognise birds as positively emaciated when they finish their journeys (Alerstam and Christie 1993). However, birds are not fully reliant on fatty tissues as energy stores, some species routinely using their muscles and organs as fuel sources during long migrations. It seems only their lungs and brains are safeguarded against being turned into energy (Battley et al. 2000): everything is fair game for fuel or other components needed to maintain a functioning body. Avian fatty tissues are, like those of lizards and crocs, deposited within their torsos but, in lieu of large tails, they also store them across the surface of the chest and abdomen. Bird skin has some transparency, and field ornithologists interested in avian fat tissue fractions can determine their extent by simply checking the amount of yellowish fat tissue visible underneath bird feathers (e.g. Rogers 1991).

The dinosaur hump controversy

Is there any direct indication of fatty tissues in Mesozoic dinosaurs? The answer is probably 'no', except for the controversial idea that the elongate dorsal neural spines if some dinosaurs are indicative of a camel-like 'hump' morphology. Spinosaurus, Ouranosaurus and Deinocheirus are key species here, these animals being depicted sometimes as humpbacked creatures. These interpretations are not the sole remit of artists, either: Bailey (1997) proposed that the tall neural spines of certain dinosaurs supported masses of tissue acting as energy stores or heat buffers - in other words, a heap of fat.

I must admit to being very sceptical that neural spine anatomy is linked to fat humps. For one,it seemingly violates what we see in the extant phylogenetic bracket for dinosaurs, where no species (to my knowledge) have substantial fat deposits on their backs. Of course, it might be queried how meaningful phylogenetic bracketing is for this issue. Fatty tissues seem quite pliable in an evolutionary sense, being chucked around animal bodies with ease as lineages adapt to new conditions (Birsoy et al. 2013). It isn't crazy to think that dinosaur bodies are different enough from those of modern diapsids that they could not have their own take on fat distribution, and there are certainly functional constraints on extant diapsid fatty tissues which are unlikely to apply to non-avian dinosaurs. However, that's only speculation, and one which conflicts with a big pool of direct data on this issue.

Another approach might be to look at animals which do have fatty humps on their backs - several types of mammal - to see if their composition is analogous to anything we see in non-avian dinosaurs. What do their humps look like internally?

A collection of animals with humpbacks and sails. Fatty humps are not directly supported by skeletons in modern species including (B) lowland gorillas (Gorilla gorilla), (C) dromedaries (Camelus dromedaries) and (D) white rhinoceros (Ceratotherium simum). Vertebral spines anchor sails in some modern lizards, such as crested chameleons (Trioceros cristatus; E), and withers anchor powerful neck muscles as in American bison (Bison bison; F). Cropped figure from Witton (2014); B–D and F from Goldfinger (2004); E historic x-ray (1896) by Josef Maria Eder.

Turns out that most mammalian humps are akin to those bulging reptile fat masses mentioned above: they tend to exist without internal support or even osteological correlates. Where humps do correlate with bone, they are comprised of powerful musculature, not fat: the shoulder humps of rhinos and bison show this well. These structures might have subcutaneous fat on them, but this is not their primary composition, nor does fat storage seem to be a principle adaptive purpose. In several species, like camels and rhinos, the longest neural spines do not align with soft-tissue humps at all, these actually being located over dorsal vertebrae with smaller neural spines (camels) or short-spined cervical vertebrae (rhinos). Taking our attention away from mammals, and turning to reptiles, we see that elongate neural spines anchor laterally compressed sail-like structures, not masses of fat. It thus seems that we have no modern correlation between fatty humps and skeletons at all, and that there is no link between elongate neural spines and fatty deposits - quite the opposite actually seems true. It was this suite of observations which led to my 2014 humped Camarasaurus image: bizarrely, it is more consistent with modern data (though still extremely speculative) to put a camel-like hump on something without long neural spines, like Camarasaurus, than it is to put one on Spinosaurus, Ouranosaurus or Deinocheirus. Sail-like structures or (at least for the lower regions of the spines) muscle attachment seem more parsimonious interpretations of their strange vertebrae - if we're being scientific (as we should be in palaeoart), we really shouldn't be looking at those tall neural spines and thinking 'fat hump correlate'.

Tying all this together

Although we may lack direct evidence of them from fossils, data from extant animals suggests it is sensible to restore dinosaurs with noticeable, prominent fatty tissues, especially if we're reconstructing animals associated with extremes of behaviour, climate or environment. Animals about to undertake migration should look well fed and bulky, and those at the other end might look leaner and less nourished. We certainly have good precedent for restoring desert-dwelling Mesozoic dinosaurs - of which there are many - with energy and water reserves, given that even energy-limited ectothermic diapsids take such precautions, as do some endotherms. We should probably not limit fatty tissues to bulky energy stores, either: as in modern lizards, some extinct reptiles may have housed pockets of fat in prominent places to serve as advertisements of health and virility.

Where should we locate those big energy stores? With no direct indication from fossils, I suggest we err on the side of caution and maintain the diapsid condition, principally locating them around the tail base and abdomen. Most Mesozoic dinosaurs had well-developed, powerfully muscled tails, and were thus likely capable of supporting a wad of adipose tissue at the tail base. We could start restoring humps in other places, but it seems sensible to speculative anatomy grounded somewhere. Besides, it's not like a fat-tailed dinosaur is boring concept!

Combining all this together, I'll leave you with the completed, revised version of my desert-adapted Camarasaurus image, now with fatty tissues fully consistent to those of modern diapsids. This meant chopping off the back hump (I'm not going to pretend I wasn't disappointed to do that), but it's worth it for a more defensible image. Note that the adult is sporting not only a fat tail, which is meant to represent sustenance for wandering through harsh desert settings, but also a pair of natty fat pockets behind the skull. It looks fairly happy with them.

Camarsaurus supremus, queen of the desert, not a member of Weight Watchers.

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  • Alerstam, T., & Christie, D. A. (1993). Bird migration. Cambridge University Press.
  • Azeez, O. I., Meintjes, R., & Chamunorwa, J. P. (2014). Fat body, fat pad and adipose tissues in invertebrates and vertebrates: the nexus. Lipids Health Dis, 13, 71.
  • Bailey, J. B. (1997). Neural spine elongation in dinosaurs: Sailbacks or buffalo-backs?. Journal of Paleontology, 1124-1146.
  • Battley, P. F., Piersma, T., Dietz, M. W., Tang, S., Dekinga, A., & Hulsman, K. (2000). Empirical evidence for differential organ reductions during trans–oceanic bird flight. Proceedings of the Royal Society of London B: Biological Sciences, 267(1439), 191-195.
  • Birsoy, K., Festuccia, W. T., & Laplante, M. (2013). A comparative perspective on lipid storage in animals. Journal of cell science, 126(7), 1541-1552.
  • Goldfinger, E. (2004). Animal Anatomy for Artists: The Elements of Form: The Elements of Form. Oxford University Press.
  • Goff, G. P., & Stenson, G. B. (1988). Brown adipose tissue in leatherback sea turtles: a thermogenic organ in an endothermic reptile?. Copeia, 1071-1075.
  • Lindström, Å., & Piersma, T. (1993). Mass changes in migrating birds: the evidence for fat and protein storage re-examined. Ibis, 135(1), 70-78.
  • Mayhew, W. W. (2013). Biology of desert amphibians and reptiles. In: Brown, G. W. (Ed.). Desert biology: special topics on the physical and biological aspects of arid regions (Vol. 1). Elsevier.
  • Rogers, C. M. (1991). An Evaluation of the Method of Estimating Body Fat in Birds by Quantifying Visible Subcutaneous Fat. Journal of Field Ornithology, 349-356.
  • Saarela, S., Keith, J. S., Hohtola, E., & Trayhurn, P. (1991). Is the “mammalian” brown fat-specific mitochondrial uncoupling protein present in adipose tissues of birds?. Comparative Biochemistry and Physiology Part B: Comparative Biochemistry, 100(1), 45-49.
  • Witton, M. P. (2014). Patterns in Palaeontology: Palaeoart-fossil fantasies or recreating lost reality. Palaeontology Online, 4, 1-14.

Friday, 11 September 2015

The life aquatic with flying reptiles

Pteranodon sternbergi dives for a school of panicked fish. So, what, pterosaurs are super good at swimming now? Read on... Reworked version of an image from Witton (2013). Click here to buy prints of this image (and join my Patreon campaign for a discount!). And yes, I'm calling this animal Pteranodon, not Geosternbergia.

Whether or not pterosaurs could swim, or how well they could swim, is a recurrent discussion among those interested in flying reptiles. For the most part, palaeontologists have seemed happy to assume that pterosaurs were aquatically capable, at least long enough to permit their escape from water, because so many pterosaur fossils occur in coastal or marine sediments. Moreover, some long-known specimens show evidence of pterosaurs feeding on aquatic prey. Odds are that pteroaurs would end up in water some of the time, even if only by accident, so it makes sense that they could at least keep themselves afloat for a while. Plus, virtually all tetrapods can swim one way or another, including bats and bird species which, on first principles, seem ill-suited to aquatic locomotion. Pterosaurs might be a bit strange, but they'd have to be very strange not to be capable of at least limited aquatic locomotion.

Proof that pterosaur workers of old thought swimming was possible: Bramwell and Whitfield's (1974) landmark paper on Pteranodon flight depicts one attempting to take off from water.
In recent years, pterosaur researchers have taken a more in depth look at pterosaur swimming, with three main lines of inquiry offering perspectives on how, and how well, pterosaurs took to water. The first concerns pterosaur swim tracks, scratch marks made in upper Jurassic sediments of North America made by pterosaurs paddling across a shallow lake. First described by Lockley and Wright (2003), these tracks record pterosaur feet scraping narrow gouges into sediment, sometimes with toe pad impressions, as buoyant pterosaurs propelled themselves over lake margins. At least locally, such tracks are not rare: Lockley and Wright (2003) report bedding planes covered with hundreds of parallel scratch marks attributed to swimming pterosaurs. Toe pad impressions are only seen occasionally, suggesting that most of these track makers were more or less entirely supported by water, only the very tips of their toes scraping the lake bed. Notably absent altogether from the same slabs are imprints from pterosaur forelimbs. Neither wingtips or walking fingers left impressions when these pterosaurs were swimming or punting about. This helps us work out what these swimming pterosaurs might have looked like, as well as how they propelled themselves: their arms must been held higher than their legs, and at least some of their locomotion was achieved through peddling feet. Quite what this means for mysterious 'manus only' pterosaur tracks (pterosaur track sites where only hand impressions are recorded) is a discussion for another day.

Select examples of pterosaur swim tracks from the Jurassic Summerville (A) and Sundance (B) formations, North America. There are many more examples of tracks like this - some bedding planes are covered in them. Traced illustrations from Lockley and Wright (2003), published in Witton (2013).
Pterosaur swim tracks provide a compelling answer to the basic question over whether pterosaurs could swim at all: clearly, some could, and swim track abundance suggests this behaviour was not unusual, in at least some species. But were all pterosaurs equally adept at swimming, and what - beyond relative positions of the limbs - was their likely floating posture? We have typically assumed that floating pterosaurs might look a bit like floating birds, sitting high on the water surface with their heads well clear of the anything wet. To test this, Hone and Henderson (2014) threw sophisticated, 3D virtual models of pterosaurs into buckets of digital water to see how they floated. These models had variable density for major body components, so account for the distribution of airsacs throughout pterosaur bodies. Some readers may be familiar with Don Henderson's digital water experiments concerning other species: we've seen sauropods and giraffes given the same treatment to understand their floating mechanics (Henderson 2004; Henderson and Naish 2010; and yes, that reads 'giraffes': as explained here, no-one really knows how well they swim!). The Hone and Henderson study included multiple pterosaur species, and grounded itself with convincingly replicating the floating postures of birds (as, indeed, other Henderson studies have done with the floating postures of other extant animals).

How did pterosaurs fare? Although they assumed a stable floating posture, it was not quite as expected. As explained by Dave Hone at The Guardian, pterosaurs were incapable of assuming a bird-like pose when floating. Playing around with postures and body component densities made little difference: the digital pterosaurs consistently floated with their heads close to, or somewhat submerged, in water. Crucially, their nostrils always ended up close to the digital water line, suggesting that anything but a motionless pterosaur in the calmest water was going to be struggling for a clear airway. The problem, it seems, is that pterosaurs are very front heavy. Pterosaurs combine large heads, necks and shoulders with comparatively slender hindquarters so that, even accounting for their pneumatic features and denser hindlimbs, they consistently pitch forward when floating. This condition is more pronounced in pterodactyloids than other pterosaurs, but the general problem applies across the group.

So, sorry, 2013 Ornithocheirus-as-a-bird-like-floater-image-that-I-have-a-little-soft-spot-for, you're out of date.
Does the inability for pterosaurs to float like birds make them unlikely to enter water? To answer that, we might need to think about bird anatomy for just a minute. Because we see swimming birds so often, we don't think their floatation skills are especially remarkable, but they actually are. Bird anatomy is ideal for stable, effortless floating: not only are their necks and heads smaller than those of pterosaurs, but their heavy shoulder regions are counterbalanced by large, strongly muscled legs. This, of course, is a reflection of the hindlimb-dominated launch strategies employed by birds. The only reason their legs are so large and heavy is because of the demands of bipedal launch. Other tetrapod fliers, like pterosaurs, launch using different strategies have no need for heavy hindquarters, which means they overbalance easily in water. Bird bodies are thus exapted for floating, their comparatively large, well-balanced and pneumatised torsos providing stable, lightweight platforms to rest on water. Birds are so good at floating that they can be entirely passive when doing so, their heads sufficiently clear to avoid issues with respiration and light enough that they can move them around freely without overbalancing. This is taken to extreme in aquatic birds like swans and ducks, which perform much of their daily activities on water surfaces. They're essentially flying barges, alighting on water to collect food with lightweight, crane-like necks and heads. The next time you see a floating swan, consider that you're looking at an evolutionary champion as goes floating and foraging from the water-surface.

What does this mean for pterosaurs? Their inability to float like birds probably rules out some behaviours, such as prolonged bouts of sitting on water to rest or forage. However, most swimming animals - even those which routinely travel or forage in water - also can't float like birds. Indeed, predicted pterosaur floating postures are actually pretty consistent with those of other non-avian tetrapods. We might therefore surmise that pterosaur floating abilities are not atypically bad, but simply not at the 'advanced' avian level. As with most tetrapods, pterosaurs might have been quite happy in water, the caveat being that it would never be a passive, restful act. Water-borne pterosaurs were likely either were there for a reason (e.g. finding food, moving through an environment) or, if they had no business there, sought to escape it as soon as possible.

This brings us to the third string of recent work on aquatic pterosaur habits: the biomechanics of entering and exiting water. Earlier this year I discussed pterosaur water launch at some length, so will only provide a brief summary here. Calculations by pterosaur biomechanicists Michael Habib and Jim Cunningham (2010) suggest that pterosaur quadrupedal launching also works on water, albeit in a modified, and slightly more energy intensive form. For some pterosaurs, the effort needed to escape water necessitates a series of hops across the water/air interface to escape surface tension and build up velocity, but some - like the big, powerful azhdarchids - could hulk smash water powerfully enough to escape in one go.

Ornithocheiroid Ornithocheirus simus achieves launch velocity from a coastal sea. Prints of this painting are available from my shop.
Pterosaur water launch becomes especially relevant to our discussion of aquatic habits when we consider the adaptations it imposed on pterosaur anatomy. Certain pterosaurs - ornithocheirids, pteranodontids, rhamphorhynchids - possess features which are unusual among pterosaurs until viewed in light of aquatic launching. Reconfiguring the shoulder muscles to optimise for aquatic launches favours warped or hatchet-shaped deltopectoral crests (the flange on the humerus which anchors flight muscle), robust shoulders, reduced hindlimbs and broad wing joints: most or all of these characters occur in these lineages (Habib and Cunningham 2010). Other pterosaurs - like the aforementioned azhdarchids - seem capable of water launching without these features, suggesting they are not strictly essential for water launch. We might therefore consider some pterosaurs as specifically adapated for aquatic takeoff, implying that some taxa were routinely entering aquatic realms rather than just casually dropping in, or suffering the odd accident.

As with terrestrially-based pterosaurs, it seems takeoff strains put a cap on the maximum size of aquatic-adapted forms. In his Flugsaurier 2015 talk, Mike Habib explained how animals larger than Pteranodon (biggest wingspans around 5-6 m) would struggle with water launching. The largest ornithocheiroids (8-9 m wingpan, c. 160kg in mass) seem to require significant energetic investment and space to take off from water, to the extent that entering aquatic settings resulted in a net loss of energy unless food was particularly plentiful (Habib 2015). This is not to say water launching was impossible for very large or giant pterosaurs, but that the energy demands make it an unlikely routine behaviour. Pterosaur aficionados will note that this size constraint is lower than those proposed for terrestrial launchers (Habib 2013): as might be expected, this reflects the complexity of launching from a fluid substrate instead of hard ground.

Nevertheless, most pterosaurs were not operating at those enormous proportions, and so could theoretically enter water with less concern. Intriguingly, early calculations suggest that some pterosaurs were well-suited to rapid water entry. Qualitative assessments of Pteranodon anatomy indicate that it might be capable of performing shallow dives because, in general construction, it is no less robust than diving birds like pelicans (Bennett 2001; note this is not advocating pelican-like feeding for Pteranodon per se, but simply that Pteranodon anatomy was robust enough to dive into water from a flighted position). Mike's Flugsaurier 2015 talk suggested that this observation is borne out in some basic assessments of skeletal strength. Diving actions would not exceed safety factors of the Pteranodon skeleton, and its streamlined head and air sacs anterior to the torso would aid force dissipation as the animal penetrated the water surface. I must admit to finding the concept of diving Pteranodon quite appealing. Pteranodon skulls are especially streamlined and pointy compared to many other marine pterosaurs (not the least because they lack teeth and anterior crests), and we know that at least some individuals predated relatively tiny fish (Bennett 2001) which may have been difficult to snag during flight. Thus, some sort of shallow diving to get Pteranodon into water where it can pursue prey makes sense to me (as depicted above, see Witton 2013 for more discussion of this concept).

Pteranodon sp. jaw specimen AMNH 5098. That mass of random crap between the manidibular rami is a heap of small, half-digested fish vertebrae. Scale bar represents 100 mm.
To tie this together, our understanding of pterosaurian aquatic locomotion has moved on a lot in just over a decade. While it would be remiss to pretend we have anything more than a basic understanding of their aquatic skills, we nevertheless have some basic hypotheses in place for further work: footprints tell us pterosaurs did swim; digital models give us some idea of likely floating postures and constraints on behaviour; and biomechanical studies hint at anatomical parameters suited to aquatic locomotion. This allows us to start asking more refined questions: which species regularly entered water, and why? How did they propel themselves? Were any pterosaurs specifically adapted for aquatic lifestyles? There are several projects in the works which have bearing on these questions, and those interested in pterosaur lifestyles will definitely want to keep an eye out for them.

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If you've enjoyed this post, and would like to see more original artwork and articles on cool fossil animals, please consider contributing to my Patreon campaign. For as little as $1 a month you can help keep this enterprise ticking over, and you'll get access to exclusive content, discount print prices and other rewards for your troubles. A huge thanks to those who have signed on in my first week of Patreon - your support is really appreciated and encouraging.


  • Bennett, S. C. (2001). The osteology and functional morphology of the Late Cretaceous pterosaur Pteranodon Part I. General description of osteology. Palaeontographica Abteilung A, 1-112.
  • Bramwell, C. D., & Whitfield, G. R. (1974). Biomechanics of Pteranodon. Philosophical Transactions of the Royal Society B: Biological Sciences, 267(890), 503-581.
  • Habib, M. (2013). Constraining the air giants: limits on size in flying animals as an example of constraint-based biomechanical theories of form. Biological Theory, 8(3), 245-252.
  • Habib, M. 2015. Size limits of marine pterosaurs and energetic considerations of plunge versus pluck feeding. Flugsaurier 2015 Portsmouth, Abstract Volume, 24-25.
  • Habib, M. B., & Cunningham, J. (2010). Capacity for water launch in Anhanguera and Quetzalcoatlus. Acta Geoscientica Sinica, 31, 24-25.
  • Henderson, D. M. (2004). Tipsy punters: sauropod dinosaur pneumaticity, buoyancy and aquatic habits. Proceedings of the Royal Society of London B: Biological Sciences, 271(Suppl 4), S180-S183.
  • Henderson, D. M., & Naish, D. (2010). Predicting the buoyancy, equilibrium and potential swimming ability of giraffes by computational analysis. Journal of theoretical biology, 265(2), 151-159.
  • Hone, D. W., & Henderson, D. M. (2014). The posture of floating pterosaurs: Ecological implications for inhabiting marine and freshwater habitats. Palaeogeography, Palaeoclimatology, Palaeoecology, 394, 89-98.
  • Lockley, M. G., & Wright, J. L. (2003). Pterosaur swim tracks and other ichnological evidence of behaviour and ecology. Geological Society, London, Special Publications, 217(1), 297-313.
  • Witton, M. P. (2013). Pterosaurs: Natural History, Evolution, Anatomy. Princeton University Press.

Tuesday, 8 September 2015

Announcing my Patreon page

Scleromochlus taylori, because a hustling palaeoartist needs mascots. The full painting can be seen at my Patreon page, and you can read more about this fantastic little animal here.
Regular readers may have noticed I've been making a bit of an effort to make a living from my art this year, setting up a print store and producing a palaeoart book which will be arriving in the next month or so. I really appreciate the enthusiasm and interest I've received about these projects but, like many artists, I find that selling specialised merchandise only goes so far when it comes to making a living. My situation has got to the point where I need to justify the time put into these projects instead of, you know, getting a proper job or something. I like doing what I do, and get the feeling that people enjoy my output, so I'm looking for ways to make my artwork and writing sustainable for the long run.

To that end, I've hopped on the Patreon bandwagon. Patreon, for those unfamiliar with it, is a site which allows followers of artists to pledge money in support of their creative output. The idea is to provide creative types with reliable income which, in other respects, can be otherwise difficult to source through online media - especially if that media is highly specialised (which I think we can all agree applies to palaeontology-based content).

Through my Patreon page, you can support my work with donations whenever I produce a new piece of artwork and/or article. Even small donations - as little as $1 per work - are appreciated, and you can cap the number of pledges you make per month so budgets are not exceeded. There's no obligation to maintain pledges over time, and you can change or stop your contribution whenever you like. Whatever your pledge, the ultimate pay off is that I can invest more time and effort into my output, which means more varied, interesting and higher quality content. As a bonus, I'm also offering reward packages to say thanks to those supporting my work. They include access to exclusive content (previews of upcoming work), access to print-quality artwork files for non-commercial use, prints, books, and commissions. The full break down is thus:

Pledge $1.00 or more per artwork and/or article
  • Access to exclusive Patreon content 

Pledge $5.00 or more per artwork and/or article

  • Access to exclusive Patreon content
  • One small print (up to 8x10") of your choosing each year

Pledge $15.00 or more per artwork and/or article

  • Access to exclusive Patreon content
  • One small print (up to 8x10") of your choosing each year
  • Access to a library of web-resolution artworks for free use (with attribution)
  • Access to print-quality artwork files, allowing you to print your own copies for personal use

Pledge $30.00 or more per artwork and/or article
  • Access to exclusive Patreon content
  • One small print (up to 8x10") of your choosing each year
  • Access to a library of web-resolution artworks for free use (with attribution)
  • Access to print-quality artwork files, allowing you to print your own copies for personal use
  • Signed copy of my upcoming book Recreating an Age of Reptiles (eta. October 2015)
  • Your own commission - I'll paint a fossil species of your choosing, you'll get a high-quality digital file for (non-commercial) use, and a signed print
That's pretty much the nuts and bolts of this plan - further details can be found at my Patreon page. A few individuals have already signed up - huge thanks to them - and, if you like what I do, please consider joining them.

Monday, 17 August 2015

A new book, Recreating an Age of Reptiles, coming this Autumn

Twitter and Facebook followers will be aware that teases of new artwork and allusions to a second book form the majority of my recent social media output. Today, the teases stop and the covers are coming off : Recreating an Age of Reptiles, a collection of my recent palaeoartworks, is due out later this year. I'm really thrilled to see enthusiasm from the online community for this project. Every time I mention this book I have someone ask a question or two about content, availability etc. With that in mind, I thought I'd provide some answers via a quick FAQ. I'll do my best to answer any further queries in the comments below.

1. So, what is this exactly?
Recreating an Age of Reptiles is a print-on-demand collection of my palaeoart from the last few years. Encouraged by a very positive social media response to the question of 'would people buy a book of my stuff?', I've been putting it together throughout the summer. The focus is on art, not text, and most of the latter focuses on the artwork more than the palaeobiology of the depicted animals. As I often attempt at this blog, it would be great to try to tackle both the scientific and artistic angles simultaneously, but there just isn't enough room for in-depth scientific discussion of each image. That said, I'm sure certain images will form the focus of articles here eventually.

2. How much new stuff is in there?
There's just over 60 images in the book, being a mix of new and old, with the bulk of it forming revised images from the last few years. Some of the revisions are substantial, but they're almost all to do with technique and colours: the compositions are very similar to the original versions. There are a bunch of completely new images in there too: giant vampire squids, the 'new look' Hatzegopteryx, Repenomammus and others. I've held back, or only partly revealed, many of those images, so hopefully there'll be plenty of surprises to even regular readers.

3. Any sketches or concept work?
Alas, no. To be honest, I don't really have any: working digitally removes a lot of need for dedicated drafting and conceptualising. I have included some older versions of concepts which have been redrafted several times where I think their evolution is particularly interesting.

4. What sort of format will this be in?
Pending some sort of formatting disaster with test versions, expect a full-colour, letter-sized (8.5 × 11", or 216 × 279 mm), soft-bound volume with 100 pages. I'm printing copies with Lulu, the same company that printed All Yesterdays and the Cryptozoologicon, so check those titles for an indication of quality (if you don't have copies of these, rest assured it's pretty good. Also, go buy those books! They're great, and All Yesterdays is a definite must-have if you're interested in my volume).

Draft cover art for what the kids are already calling RecARep.
5. Will there be a hardback version?
Sorry, no. I'd love to have a one too, but the costs are prohibitive for large, full-colour print on demand hardbacks. We're talking c. £100 for a 100 page volume - no-one should be spending that amount of money on a 100 page book. If anyone knows a way around this, I'm all ears, but I have no plans to pursue hardbacks at the moment.

6. What will this cost?
The likely pricetag is going to be £20-25 for each book. I know that's a little on the steep side, but the reality of print on demand is that each book costs nearly £20 just to produce - the profit margin here is not huge. Books published on a larger scale are made cheaper through bulk economy: alas, that's not an option here. That is, unless any publishers are reading and want to sign me up for a cushy deal...

7. Will there be a cheaper electronic version?
I expect so, although my focus is getting the physical version sorted first. An ebook should be available soon after.

8. 'Age of Reptiles'? What do you think this is, the 1950s?
A number of people have commented on the the title of this book, wondering why I've chosen the term 'Age of Reptiles' when it has connotations to more archaic views of many Mesozoic animals. There are a number of reasons I went for this title, not the least being that the world really doesn't need another tome entitled "[Something something] dinosaurs and other prehistoric creatures".

Firstly, the focus of the book is not just dinosaurs, or even Mesozoic archosaurs. These animals dominate, but there's sufficient other taxa in there to warrant a title which doesn't overtly emphasise specific groups of animals. Secondly, the term 'Age of Reptiles' accurately describes the time period covered in the book, it being popular parlance for 'Mesozoic'. Given the dream that a book like this might sell a few copies outside a hardcore palaeontology demographic, it seemed sensible to use phraseology which is widely understood. Thirdly, 'Age of Reptiles' resonates within palaeoart, it being the title of Zallinger's seminal 1947 Peabody Museum mural as well as Ricardo Delgado's Age of Reptiles graphic novels. The latter was a big influence on my childhood art, a fact not lost on me when choosing the title. Finally, our advances in dinosaur palaeontology in the last few decades have not stopped dinosaurs being members of Reptilia (the turtle, lizard + archosaur clade): ergo, the title is scientifically sound. I'm sticking with it.

9. Will there be signed copies?
Possibly. I'll figure that out later.

10. When is it out?
There's not a specific date yet, and the honest answer is 'when it's done'! All being well, that won't be very long off: there's some text to finish and proofing to do, and then we're good. I'm aiming for copies to be available mid-late Autumn.

Right, that should do for now - I'm happy to field any additional questions in the comments below, or on Twitter, Facebook etc. Thanks to all who've given their support thus far, and needless to say, there'll be updates soon.