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Thursday, 27 April 2017

Walking with ichthyosaurs: the amphibious ichthyosaur hypothesis

Benjamin Waterhouse Hawkin's (1858?) sketch of amphibious marine reptiles, including a large shambling ichthyosaur. Image borrowed from Frank T. Zumbach's Mysterious World.
One of the most charming aspects of mid-19th century palaeoart are those amphibious marine reptiles: depictions of ichthyosaurs and plesiosaurs that hauled themselves onto rocks or beaches to rest, or lunge with open jaws at passers by (above). To modern eyes these images look naive and quaint, a clear reminder of how far our understanding of fossil animals has progressed in the last two centuries.

Of course, art has a habit of imitating life and, a good 150 years after amphibious marine reptiles became unfashionable in palaeoartworks, Ryosuke Motani and colleagues (2014) published a new marine reptile suggested to be capable of locomotion on land as well as in water: the ichthyosauriform Cartorhynchus lenticarpus. This Chinese, Early Triassic species is anatomically remarkable in several respects. Although reminiscent of early ichthyosaurs in overall shape, it has a considerably reduced snout, seems to lack teeth, is just 20 cm from snout to vent despite indications of osteological maturity, and bears enormously long forelimbs. Though unique when first discovered, another, much larger Cartorhynchus-like species has since been found in the same deposits, Sclerocormus parviceps. Together, these animals form a clade at the base of Ichthyosauriformes known as Nasorostra, the 'nose beaks', referring to a defining feature where their nasal bones reach the jaw tip (Jiang et al. 2016).

Holotype specimen of Cartorhynchus lenticarpus. Note the enormous forelimbs with their expansive unossified wrists, indicated by the distal phalanges being well posteriorly displaced from the upper arm bones. From Motani et al. (2014).
The amphibious habits of Cartorhynchus are primarily based on its unusually large forelimbs and small body size, it being reasoned that Cartorhynchus could drag or propel itself over exposed sediments like a mudskipper, turtle or pinniped. I find this idea fascinating: an ichthyosauriform that was at home outside of water? Cartorhynchus certainly deviates from ichthyosaur anatomy and evolutionary trends enough to inspire inquiry about its weird bauplan - if it was not amphibious, it might be doing something else equally unexpected. The amphibious Cartorhynchus hypothesis has received surprisingly little detailed attention online, save for coverage of a 2014 press release and this excellent primer article at Tetrapod Zoology, so there's scope for a closer look at this idea. What is the evidence for amphibious habits in Cartorhynchus, and how does this concept fit models of early ichthyosaur evolution?

The functional basis for an amphibious lifestyle in Cartorhynchus

Motani et al. (2014) present a fairly detailed argument in favour of amphibious habits in Cartorhynchus. The chief lines of evidence are those expansive forelimbs, but it's not just their size that matters: their enormous, unossified carpal regions are also significant. Several early ichthyosauriforms have poorly ossified carpal bones but the unossfied region in Cartorhynchus flippers is proportionally bigger by some margin. This would allow these ordinarily-rigid marine reptile flippers an unusual degree of flexibility and optimise them for terrestrial locomotion. Flipper-based terrestrial motion is surprisingly tricky because its users tend to be suboptimally designed for movement out of water and they almost always have to overcome drag forces acting on the body as well as shove themselves around. Moreover, substrates associated with coasts and waterways tend to be unstable, yielding under pressure and being challenging for even proficient terrestrial animals. These factors mean flippers can easily dig into substrate or slip across it rather than propel their owners about, and it's easy to see why beaching is fatal for so many specialised aquatic species.

Studies (using robot turtles!) suggest that rigid flippers are generally poor at terrestrial locomotion and may even be incapable of moving animals over some surfaces (Mazouchova et al. 2013). A bendy flipper, in contrast, works well, allowing the forelimb to flex before the substrate moves, spreading the weight of the animal over the distal limb and allowing the proximal flipper region to elevate and support the body (Mazouchova et al. 2013; Motani et al. 2014). The unusually expanded flexion zone in Cartorhynchus forelimbs would be well suited to this purpose, and certainly much better at this task than those of other ichthyosaurs. We might note, as an aside, that the lack of flexion zones in other marine reptile flippers, such as those of plesiosaurs, might be good reason to doubt their ability to crawl over land.

video
Did I mention the robot turtles? There are robot turtles. Supplementary video data from Mazouchova et al. (2013).

The downside of having lots of cartilage in a long flipper is that they are weaker against bending than a more ossified one, so their utility as a walking limb lessen as the forces involved in moving the body increase. It's here where the small size of Cartorhynchus comes into play. Small size equates to low body masses and smaller forces associated with lifting the body, less structural demand on the flipper, and reduced drag effects from the sliding belly. As is so often the case in evolution, small body size might be an enabler for evolutionary experimentation in Cartorhynchus, allowing it to perform feats that its bigger relatives just couldn't even if they were also equipped with giant, bendy fins.

The tail of Cartorhynchus is incompletely known but it's anatomical and phylogenetic proximity to the completely-known Sclerocormus suggests that its tail was long, flexible, and lacked any sort of fin or fluke (Jiang et al. 2016). A relatively simple tail lessens the risk of it dredging sediment or catching on debris during terrestrial locomotion and its flexibility might have permitted its use as a prop or even propulsive organ: fish such as the Pacific leaping blenny show how a long, bendy tail can be used to powerful effects in semi-terrestrial locomotion (Heish 2010, also below). Combinations of fin and axial motion in land-crawling fish can be surprisingly effective over a range of substrates (Standen et al. 2016) and we might assume similar options were available to Cartorhynchus.

 
Leaping blennies, robot turtles... is this the best blog post ever? From Wikipedia, source: Hsieh (2010).

The torso of Cartorhynchus is also of interest for this hypothesis. In contrast to some other Triassic ichthyosaurs, Cartorhynchus has a broad, stout torso rather than a long, laterally-compressed one (Carrol and Dong 1991). Though a wider torso would impart more drag during terrestrial crawling, it would aid stability when crawling over land. Moreover, torso drag can be lessened by shortening the body overall, giving new significance to the low Cartorhynchus pre-sacral vertebral count of 31 vertebrae, instead of a more typical ichthyosaurian count of 40-80 (Motani et al. 2014). Short, narrow hindlimbs, rather than the broad pelvic flippers of some other early ichthyosaurs, might have further aided drag reduction.

Cartorhynchus in context

It seems there's a prima facie argument for considering Cartorhynchus as equipped with some amphibious features. However, we should not get carried away - a suite of evidence for an aquatic lifestyle suggests it wasn't it a specialist denizen of shallow, partly-exposed habitats, but more of an animal able to exploit two realms. It has pachyostotic bones, true flippers rather than webbed walking limbs, and is adapted for suction-feeding: a mechanism where the combination of a small mouth and a large oral cavity creates a pressure differential during feeding, literally sucking small prey into the mouth if it's opened quickly (Motani et al. 2014). This foraging strategy cannot work outside of water so is strong support for Cartorhynchus foraging in fully aquatic settings.

Cartorhynchus also stems from the Nanlinghu Formation, a mudrock and limestone marine deposit rich in fossils of aquatic reptiles and marine invertebrates: ammonoids, bivalves and conodonts. We might take these data as signs that Cartorhynchus was quite happy in water and maybe spent most of its time there, visiting coastlines and beaches on occassion, rather than living there permanently. We should also regard it as a marine animal, not an inhabitant of rivers or swamps (though it would be extremely cool if one turned up in such deposits!).

Holotype of Hupehsuchus nanchangensis, a marine reptile seemingly more closely related to the ancestor of ichthyosaurs than Cartorhynchus. These guys surely deserve their own blog post and painting at some point. From Carroll and Dong (1991).
The relationships of Cartorhynchus to other marine reptiles is also interesting in light of the amphibious hypothesis. You could be forgiven for interpreting Cartorhynchus as some sort of bridge between ichthyosaurs and terrestrial reptiles, but, no, the nasorostran clade seems to nest above the root of the ichthyosaur line between 'true' ichthyosaurs and the fully marine, ichthyosaur-like hupehsuchians (Motani et al. 2014; Jiang et al. 2016). The ichthyosaur + hupehsuchian clade, Ichthyosauromorpha, may be further allied to another group of marine reptiles, the amphibious thalattosaurs (Motani et al. 2014 - Darren Naish has an excellent overview of this topic here). This surrounds Cartorhynchus with lineages that had taken to water in a significant way and we should conclude that any amphibious adaptations of Cartorhynchus do not represent an ichthyosaurian invasion of the sea, but ichthyosaurs returning to land.

Some might consider this surprising evolutionary scenario evidence against the amphibious hypothesis - why would a lineage of marine reptiles start retracing their adaptive steps to become landworthy, when the rest of the group is pressing ahead with more specialised aquatic lifestyles? In response, perhaps we should ask if a potentially amphibious marine reptile is really that surprising. A huge number of vertebrates have transferred between terrestrial and aquatic lifestyles in the last 400 million years, sometimes contrasting with wider adaptive trends taking place in closely related species. Well-understood evolutionary 'transitions' also show that large-scale adaptive phases are often complex with all manner of evolutionary experimentation and dead-end offshoots. We know that bridging aquatic and terrestrial realms can be advantageous to aquatic species - refuge from predators or rough seas, access to food off-limits to other marine species, access to safe habitats for rest or reproduction, etc. - and there's no reason to think ichthyosaurs were incapable of capitalising on these advantages, or immune to their selective draws. With all this in mind, the concept of a marine reptile exploiting semi-exposed habitats isn't really that radical. Maybe the key question here isn't 'why would a marine reptile go rouge and turn landward?' but is 'why aren't we seeing more of this sort of thing?'.

What about Sclerocormus?

A question currently unaddressed in technical literature is whether the other currently known nasorostran, Sclerocormus, might have also bear amphibious hallmarks. It has virtually all the same features that we likened to amphibious adaptations above, the only distinctions being marginally enhanced ossification of the forelimb (though it still retains a comparatively enormous unossified carpal region) and greater size overall (body length of 160 cm, representing an animal about 3.3 times larger than Cartorhynchus). In lieu of a detailed, quantified assessment it's difficult to say whether Sclerocormus was too heavy to pull itself along on land, but we can note that it is not especially big compared to the truly massive aquatic animals we have scampering over beaches today - leatherback turtles, giant pinnipeds, the odd manatee (Motani et al. 2014) and so on. Some of these animals weigh several tonnes and, if they can haul themselves out of water, maybe Sclerocormus could too.

Holotype specimen of the larger nasorostran species, Sclerocormus parviceps. From Jiang et al. (2016).
I find this question particularly interesting given how similar Sclerocormus and Cartorhynchus are in virtually all aspects (above). Is nasorostra a clade of potentially amphibious ichthyosaurs, or are we actually looking at growth stages of one oddball species? Their proportions are near identical, and they are only separated by fine details of anatomy (Jiang et al. 2016). Many proposed differences might be attributable to intraspecific variation, too. For instance, the significance of their slightly different vertebral counts is questioned through populations of living snakes, limbless lizards and fish with variable numbers of axial elements (Tibblin et al. 2016). Individually variable vertebral counts seem common in species with large numbers of axial elements, and this might have been true for ichthyosaurs. Ontogeny and scaling effects could explain other differences, including overall size, greater ossification of the postcranial skeleton, and subtle arrangements of skull bones. It can't be overlooked that these near identical species, unique in morphology in the grand scheme of ichthyosaur evolution, also happen to occur in the same member of the same formation, separated by only 14 m of strata (Jiang et al. 2016). For the time being, the identification of 'adult' skull fusion and textures in Cartorhynchus suggests they aren't the same species, but the marine reptile trait of retaining poorly fused skeletons into adulthood makes identifying adult forms especially tricky, especially with so few specimens to look at (Motani et al. 2014). It also seems worryingly difficult to tease fossil adults from juveniles without histological assessments, even with large sample sizes and good growth series (e.g. Prondvai et al. 2009). Perhaps we're waiting on histological examinations and more specimens to make a call on this.

So, walking with ichthyosaurs?

And finally, a painting: Cartorhynchus goes for a drag around a Triassic lagoon.
Putting all the strands of the amphibious Cartorhynchus hypothesis together, I don't see reason for excessive suspicion about the idea of beach hauling nasorostrans. At the core of the pro-amphibious argument is that Cartorhynchus (and perhaps, by extension, Sclerocormus) has weird anatomy that requires an explanation - it's just too different from other ichthyosauromorphs to pretend it wasn't doing something unusual, maybe even unexpected. Amphibious behaviours are an explanation that seem to chime well with provisional form-function investigations and seem a sensible hypothesis at this time. That said, we should be appropriately cautious in our interpretations of these animals: our understanding of nasorostrans is in its infancy and alternative, currently-unexplored functional hypotheses could explain their anatomy as well, or better, than the amphibious concept in future. Fingers crossed that these animals receive more dedicated functional investgiations in years to come.

Or maybe more robot turtles. Either is good with me.

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References

  • Carroll, R. L., & Zhi-Ming, D. (1991). Hupehsuchus, an enigmatic aquatic reptile from the Triassic of China, and the problem of establishing relationships. Philosophical Transactions of the Royal Society of London B: Biological Sciences, 331(1260), 131-153.
  • Hsieh, S. T. T. (2010). A locomotor innovation enables water-land transition in a marine fish. PloS one, 5(6), e11197.
  • Jiang, D. Y., Motani, R., Huang, J. D., Tintori, A., Hu, Y. C., Rieppel, O., ... & Zhang, R. (2016). A large aberrant stem ichthyosauriform indicating early rise and demise of ichthyosauromorphs in the wake of the end-Permian extinction. Scientific reports, 6, 26372.
  • Mazouchova, N., Umbanhowar, P. B., & Goldman, D. I. (2013). Flipper-driven terrestrial locomotion of a sea turtle-inspired robot. Bioinspiration & biomimetics, 8(2), 026007.
  • Motani, R., Jiang, D. Y., Chen, G. B., Tintori, A., Rieppel, O., Ji, C., & Huang, J. D. (2015). A basal ichthyosauriform with a short snout from the Lower Triassic of China. Nature, 517(7535), 485-488.
  • Prondvai, E., Stein, K., Ősi, A., & Sander, M. P. (2012). Life history of Rhamphorhynchus inferred from bone histology and the diversity of pterosaurian growth strategies. PLoS One, 7(2), e31392.
  • Standen, E. M., Du, T. Y., Laroche, P., & Larsson, H. C. (2016). Locomotor flexibility of Polypterus senegalus across various aquatic and terrestrial substrates. Zoology, 119(5), 447-454.
  • Tibblin, P., Berggren, H., Nordahl, O., Larsson, P., & Forsman, A. (2016). Causes and consequences of intra-specific variation in vertebral number. Scientific reports, 6, 26372.

Friday, 14 April 2017

New paper: pterosaur palaeoecology, as told by the fossil record

A female Pteranodon tries to explain the new Silverstone et al. (2017) paper on Pteranodon taxonomy to the Cretaceous shark Squalicorax. Unfortunately for her, the sharks quite liked the 'Dawndraco' hypothesis.
Last year I posted a couple of overviews of the better parts of the pterosaur palaeoecological record, discussing what we know was eaten by Rhamphorhynchus and azhdarchid pterosaurs, as well as what species ate them. These reviews were tied to a peer-reviewed paper on the same subject which, at the end of Febuary 2017, was published as part of an upcoming collection of pterosaur papers (Witton 2017). This collection, edited by David Hone, myself, and David Martill, is the proceedings of the Flugsaurier 2015 pterosaur meeting and will, when finished, contain over a dozen new insights into pterosaur research, with an emphasis on their palaeobiology. You can check out the existing content here - keep an eye on that site, as there are more papers to come.

With my paper now out (though sadly not open access, but I will eventually be able to post an unformatted version online next year) I thought it would be a good time to take a holistic look at direct fossil evidence of pterosaur lifestyles. What are some of the most interesting examples of pterosaurs interacting with other species? Which purported interactions stand up to scrutiny, and which ones are a little tenuous? And what do they tell us about the all important Big Picture of pterosaur palaeobiology?

Yes, some pterosaurs may well have been seabird mimics

A number of pterosaur specimens have been reported as being associated with the remains of their last meals. Several of these have been lost, found to be erroneously interpreted, or are simply too poorly preserved to interpret their gut content. However, examples of the Jurassic non-pterodactyloids Rhamphorhynchus and Scaphognathus, the Triassic Eudimorphodon, and the famous Cretaceous taxon Pteranodon show reliable insights into their dietary preferences (below). These are virtually all remains of aquatic animals - mostly fish - preserved in intimate association with pterosaur skeletons, either between their jaws, aligned with their throats or within the torso skeleton. One example of a coprolite is known, though it's difficult to say exactly what it contains.


Pterosaurs and their last meals (shaded grey). A, torso of Eudimorphodon; B-D, various Rhamphorhynchus with gut content and coprolite (C), E, Scaphognathus; F, Ludodactylus; and G, Pteranodon. From Witton (2017).
Many of these specimens have been known for several decades, and their evidence of aquatic feeding probably played some part in the stereotyping of pterosaurs as seabird analogues (e.g. Wellnhofer 1991). Nowadays, we need to be a little more circumspect about what they tell us. Yes, they do show that some pterosaurs ate fish and other pelagic prey and, along with results from detailed studies into functional morphology, they help portray certain pterosaur species in the 'classic' seabird niche. RhamphorhynchusScaphognathusEudimorphodon and Pteranodon have at least some adaptations consistent with foraging for pelagic prey, such as long wings ideal for marine soaring, 'fish-grab' jaws and adaptations for launching from aquatic settings, as well as occurrences in coastal or marine settings. It would be a little odd if these aquatic-adapted species weren't catching aquatic animals from time to time.

But we can't maintain the older view that these specimens, on their own, undermine the increasingly diverse and nuanced takes on pterosaur palaeoecology hinted at by form-function studies, biomechanics, and modern understandings of pterosaur habitats. We have thousands of pterosaur specimens in museums around the world, of which gut content is known from less than a dozen examples, and in four species. That's not even enough to demonstrate the full dietary range of the species in question, let alone tell us about the ecology of all pterosaurs. Indeed, the scarcity of pterosaur gut content agrees with some new predictions of pterosaur lifestyles in that non-aquatic food sources now suggested for pterosaurs - insects, wormy things, fruits, small tetrapods - have limited preservation potential, particularly outside of Lagerstätten. When factored against common agents of taphonomy and preservation, these hypotheses predict empty bellies in many pterosaur fossils, which is what we find virtually all of the time. It is, of course, difficult to be certain of anything concerning negative evidence, but it's nevertheless useful to note this predicted match between modern ideas and fossil data.

A selection of pterosaur foraging traces - beak tip impressions and scrape marks - from Jurassic and Cretaceous sites. The black-filled elements are the feeding traces, dark grey are manus prints, and light grey are footprints. From Witton (2017).
Evidence that not all pterosaurs were obtaining their food out to sea comes in the form of feeding traces - small, paired impressions and scratch marks created by beak tips (above). These were likely formed by pterosaurs wandering over water margins in pursuit of invertebrates and other small prey, much like extant shorebirds and waders. Indeed, if you walk across a mudflat on a falling tide you can find near identical traces made by living avians mimicking this pterosaur strategy. Somewhat frustratingly, the identities of the pterosaurs that made these tracks remain mysterious. That said, in my new paper, I have - finally - formalised a case for a Late Cretaceous Mexican set of tracks and possible feeding traces (panel D, above) having an azhdarchid trace maker.

Pterosaur feeding evidence: the 'close, but no biscuit' specimens

Inferring palaeoecological details from fossils can be tricky, and it is unsurprising that some purported insights into pterosaur diets and lifestyles are contentious. One of these is the famous and perhaps darkly comic circumstances surrounding the holotype skull and mandible of Ludodactylus sibbicki, a Cretaceous, likely fish-eating Brazilian ornithocheirid found with a sharp, pointed leaf between its lower jaw rami (panel F in the image above). Much of the 2003 description of this specimen (Frey et al. 2003) discusses this association and concludes that ingestion of these plant remains led to the death of the pterosaur. According to this story, the pterosaur accidentally scooped up the leaf, having mistaking it for its usual prey, stabbed the plant material on its throat tissues, frayed the end of the leaf trying to work it loose, but starved to death before it could dislodge it.

I must admit a little scepticism about this scenario. This is not because animals getting things stuck in their mouths is implausible, but because the story presented by Frey and colleagues is pretty presumptive. It infers a lot about pterosaur behaviour, foraging strategies, throat tissue strength and so on that we can't confirm at present. Moreover, the hyoid apparatus - the skeletal support for much of the throat and tongue tissue - is preserved lying on top of the leaf, despite the suggestion that the plant matter was deeply imbedded in the throat tissues. How did that work itself loose with the leaf fatally stabbed between the jaws? The answer to that question - as with a lot of questions about this association - would easily fall into speculation and special pleading about all manner of unknown quantities, and thus has little value to understanding fossil animal palaeobiology. Boring and po-faced as it is, I don't think the unusual Ludodactylus holotype provides enough information to tell us much about pterosaur behaviour, or how this unlikely fossil association came to be.

A similar observation might be made about insect specimens - a dragonfly and lacewing - from the Jurassic Solnhofen Limestone that have torn wings, allegedly from a pterosaur attack (Tischlinger 2000). The logic goes that these otherwise perfectly preserved insects cannot have been attacked by aquatic predators, or else they would have been eaten after their wings were damaged. Failed attack from an airborne predator that would not pursue the injured insects into water is suggested as more likely. Solnhofen deposits do hold pterosaurs that were almost certainly aerial insect hawkers - such as Anurognathus (below, see Bennett 2007 and Witton 2013) - and these might be ideal perpetrators in this scenario.

Anurognathus ammoni was an insect-hawking pterosaur that lived over the Solnhofen lagoon. Has it left feeding traces on fossil insect wings after a failed attack?
 As with Ludodactylus, this set of circumstances is quite elaborate to base purely on damaged insect wings. The extent of their wing damage is considerably greater than we might expect under general 'wear and tear' and foul play was probably involved, but whether it was a pterosaur, a conspecific, or even those disregarded aquatic predators is difficult to say. I appreciate the logic that aquatic predators would eat disabled insects after a failed strike, but animals are not predictable, logic-driven machines: they make mistakes, strike at things they have no intention of eating, get bored, distracted and so on. In all, other than the fact that these insects were almost certainly attacked by something, it might be difficult to say anything more substantial about their final moments.

Pterosaurs vs. dinosaurs, crocodyliforms and... the revenge of the fish

The fossil record gives us an insight on the question "did pterosaurs taste good?", and that answer seems to be "yes". Bite marks, embedded teeth and vomited pterosaur remains indicate that dinosaurs, crocodyliforms and fish all ate pterosaur flesh, at least on occasion (below). Among the more impressive examples of these interactions is a spinosaurid tooth, likely from the Brazilian spinosaurine Irritator challengeri, embedded in the cervical vertebra of an ornithocheirid (Buffetaut et al. 2004). Alas, no other evidence of their interaction was evident on the specimen (a series of pterosaur vertebrae) and it's not possible to ascertain much about circumstances that brought these species together.
Evidence of many, many things that ate pterosaurs. A, ornithocheirid cervical vertebrae with embedded spinosaurid tooth; B, azhdarchid tibia with tooth gouges and embedded dromaeosaur tooth; C, ornithocheiroid wing metacarpal with unidentified puncture marks; D, Quetzalcoatlus sp. skull with puncture marks; E, Eurazhdarcho langendorfensis cervical vertebrae with crocodyliform puncture marks; F, Pteranodon sp. cervical vertebra with intimately associated Cretoxyrhina mantelli tooth; G, Velociraptor mongoliensis torso with possible azhdarchid pterosaur gut content; H, probable fish gut regurgitate including Rhamphorhynchus bones; I, associated Rhamphorhynchus muensteri and Aspidorhynchus acutirostris skeletons. Images drawn and borrowed from many sources - see Witton 2017 for details.
The fossil record's most common purveyors of pterosaur murder, however, are not dinosaurs or crocodyliforms, but fish. Apparently out for revenge after learning of all that fishy pterosaur gut content, we've got evidence of fish eating and spitting out pterosaurs, of pterosaurs getting entangled with piscine predators, and even fish bite marks on pterosaur bones. A lot of these pertain to specimens of Rhamphorhynchus and you can read more about them in this post - some of the specimens are exceptional and there's lots to say about them. One of the more famous examples of piscine-pterosaur consumption -  an Italian, Triassic pellet composed of alleged pterosaur bones (Dalla Vecchia et al. 1989) - has recently been reappraised. It's now more reliably interpreted as vomit ball made of bones from the tanystropheid Langobardisaurus (Holgado et al. 2015).

Lesser known, but pretty darned awesome examples of fishes eating pterosaurs are Pteranodon specimens that found themselves at the wrong end of Cretaceous sharks. Several Pteranodon bones reveal bite marks and even embedded teeth from two genera of sharks, the 2-3 m long 'crow shark' Squalicorax and the larger, 6 m long 'ginsu shark', Cretoxyrhina. The former seems to have eaten Pteranodon flesh on several occasions, while evidence of the latter is only currently known from a tooth closely associated with a cervical vertebra (panel F, above). Further work on the latter specimen is currently underway.

 Feeding traces from these sharks are common in Western Interior Seaway fossils and those of Squalicorax are particularly abundant and taxonomically indiscriminate. Given that even giant marine reptiles are among the species consumed by this mid-sized shark, it's often assumed that this animal was a scavenger, biting into whatever free meat floated about America's continental sea. However, it is less certain that Pteranodon was scavenged by Squalicorax, as even a 2 m long specimen would vastly outweigh the largest Pteranodon. It is not inconceivable that an unwary Pteranodon could be grabbed and killed by a stealthy Squalicorax, though I stress this scenario is no better supported than the shark simply chancing across a Pteranodon carcass. Whatever the scenario, it's somewhat grounding to think of a weird extinct creature like a pterosaur being devoured by a fairly conventional-looking shark. It's a reminder, perhaps, that Mesozoic life was not a pantomime of exotic, giant reptiles and weirdo evolutionary experiments, and that much of our modern ecosystem was in place many millions of years ago.

The big picture

Looking at the pterosaur palaeoecological record holistically, what patterns emerge? If we look at where the record focuses phylogenetically (below), it's obvious that our records are significantly biased towards certain taxa - Pteranodon, Rhamphorhynchus, and azhdarchids. Even their close relatives, with similar anatomy and adaptations, preservational conditions and so on, don't get much of a look in. There's a few data points scattered here and there, but tumbleweeds run though the palaeoecological data stores for the majority of the group.

Attempting to make sense of the pterosaur palaeoecological record in a holistic way mainly shows how paltry this record remains. It's improved a lot in recent years, but we await evidence of diet and consumer-consumed relationships in virtually all major pterosaur clades. The images at the bottom of this figure are takes on known examples of pterosaur ecology: Rhamphorhynchus ingesting fish, and azhdarchids being devoured by dromaeosaurs. From Witton (2017).
We wouldn't be scientists if we didn't ask ourselves why this is. I don't think it's simply a sampling issue. The pterosaur record is not great, but we are talking about several thousand specimens now - enough that we might start looking at what we don't have as well as what we do. So why does Rhamphorhynchus show 10 palaeoecologically-relevant fossils, but other Solnhofen species only preserve one confirmed piece of gut content? Why do azhdarchids, which are never found in sites of exception preservation and are generally only known from bits and pieces, have a better record than those lineages which are abundant, represented by dozens of complete skeletons, and often found in sites of exceptional preservation? Interestingly, there's no obvious correlation between factors like abundance, preservation quality and palaeoecological data. Several lineages - the ctenochasmatoids (wading pterodactyloids), the rhamphorhynchids (excluding Rhamphorhynchus) and ornithocheiroids (excluding Pteranodon) - have everything going for them in terms of abundant fossils, occurrences in sites of exceptional preservation, and yet they turn up very little in the way of gut content, or evidence of being consumed by other Mesozoic animals.

My take on all this is that there must other factors at play here. We don't get evidence of pterosaur palaeoecology just by throwing more fossils, or better quality fossils, into the mix. I'm sure these factors have some role, but perhaps only in concert with special traits of certain pterosaur groups - maybe behaviours and anatomies - that allow them to have good records. We might have a good record of azhdarchids being consumed by dinosaurs and crocs, for instance, because their bones are often quite big and allow predators to bite them without destroying them. Perhaps we have good palaeoecological insights for Rhamphorhynchus and Pternanodon because of their habits and behaviour - both have strong aquatic adaptations (see this blog post for ideas on that), and there is a bias towards preservation of aquatic animals in the fossil record. Perhaps this aids preservation of not only palaeoecological data, but also explains why these taxa are our most abundant pterosaurs (>100 Rhamphorhynchus fossils are known, >1000 Pteranodon).

The pterosaur palaeoecological record, then, is perhaps in a transformative state. Though vastly improved over its condition a few decades ago, it requires further augmentation to provide us with significant insights into pterosaur lifestyles, and to explain its biased nature. However, we should not be too pessimistic about the insight it offers into pterosaur palaeobiology: it still provides useful datapoints that can shape our interpretation of flying reptile ecology for several species. Cliched as it is, the take-home message of this project is that any palaeoecologically-relevant pterosaur fossils are worth putting on record. We still have a lot to learn about how these animals lived and behaved, and direct insights are the most reliable ways to do that.

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References


  • Bennett, S. C. (2007). A second specimen of the pterosaur Anurognathus ammoni. Paläontologische Zeitschrift, 81(4), 376-398.
  • Buffetaut, E., Martill, D., & Escuillié, F. (2004). Pterosaurs as part of a spinosaur diet. Nature, 430(6995), 33-33.
  • Dalla Vecchia, F. M., Muscio, G., & Wild, R. (1989). Pterosaur remains in a gastric pellet from the Upper Triassic (Norian) of Rio Seazza valley (Udine, Italy). Gortania, 10(1988), 121-132.
  • Frey, E., Martill, D. M., & Buchy, M. C. (2003). A new crested ornithocheirid from the Lower Cretaceous of northeastern Brazil and the unusual death of an unusual pterosaur. Geological Society, London, Special Publications, 217(1), 55-63.
  • Holgado, B., Dalla Vecchia, F. M., Fortuny, J., Bernardini, F., & Tuniz, C. (2015). A reappraisal of the purported gastric pellet with pterosaurian bones from the Upper Triassic of Italy. PloS one, 10(11), e0141275.
  • Martin-Silverstone, E., Glasier, J. R. N., Acorn, J. H., Mohr, S. & Currie, P. J. (2017). Reassesment of Dawndraco kanzai Kellner, 2010 and reassignment of the type specimen to Pteranodon sternbergi Harksen, 1966. Vertebrate Anatomy Morphology Palaeontology, 3, 47–59.
  • Tischlinger, H. (2001). Bemerkungen zur Insekten-Taphonomie der Solnhofen Plattenkalke. Archaeopteryx, 19, 29-44.
  • Wellnhofer, P. (1991). The illustrated encyclopedia of pterosaurs. Salamander Books.
  • Witton, M. P. (2013). Pterosaurs: natural history, evolution, anatomy. Princeton University Press.
  • Witton, M. P. (2017). Pterosaurs in Mesozoic food webs: a review of fossil evidence. Geological Society, London, Special Publications, 455, SP455-3.

Friday, 31 March 2017

Scientist-palaeoartist collaborations – what palaeontologists can, and probably should, critique when reviewing palaeoart

Last month I posted a complaint about poor scientist-led palaeoart - those artworks of extinct animals produced under direct control of scientists to promote research, without any interference from TV companies or book publishers, and yet still end up being objectively flawed at a scientific level. I focused a lot of my criticisms at scientists themselves, as they have final authority over factual aspects of palaeoartworks. That doesn't necessarily clear artists from all blame, but it's naive to think that every artist tackling palaeoart has specific training in palaeontological matters, unrestricted access to technical literature, or the anatomical knowledge required to restore fossil animals without instruction. A scientist's main role in a palaeoart collaboration is bringing rigour and information to the table and, when science-led pieces are objectively poor, we have to wonder what happened to those guiding hands.

Our instinct might be to assume that lack of scientific rigour reflects flippancy towards palaeoart and its impact, and I think that is true - in at least some cases. But in a comment posted after my article, Matt Bonnan proposed that scientifically poor artwork might reflect scientists struggling with their role in the palaeoart process - that is, not really knowing how to instruct an artist, or where the line between scientific and artistic considerations lies. I can believe this is true, too. Lots of people - including many scientists and artists - view science and art as incompatible concepts, and are unsure how to approach projects blending the two. For some folks the idea of contributing to an art project is pretty terrifying, perhaps because they're afraid of being seen as naive, or of their contribution letting a project down.

Whatever the reason, I want to follow my previous criticism with something more constructive: some pointers for how scientists might approach their role in producing palaeoartworks. The take-home message is that scientists concerned about getting paint on their fingers shouldn't worry about the artistic aspects of paleaoartworks. The primary role of a scientist is not to understand colour hues, choice of media, composition and so on, but to comment on objective, factual components of the work. Is the restored species the right size in relation to other species and the environment? Is its head the right shape? Has it been restored with the right soft-tissue anatomy? Answering these and other questions does not require artistic training, or any special training at all for that matter, just application of knowledge that most palaeontologists will already have.

This discussion mainly considers scientists involved in producing a novel palaeoartwork, but will also apply to those reviewing artwork before publication or exhibition. This includes peer review of papers with palaeoartworks, and I encourage editors to make sure these aspects are checked alongside other parts of a paper. After all, if an artwork is being presented as scientifically-credible enough to be included in a peer-reviewed publication, it should be held to the same standards as the rest of the paper. I don't want to beat a dead horse by bringing this up again, but we should recall that palaeoart components can remain in use long after the context of its genesis has been consigned to history, and artworks associated with papers can be especially prone to long-term use. We should take all opportunities to steer depictions of the past in the most credible directions, hopefully influencing subsequent generations of artists in the best way possible.

So, what should scientists look to critique in artworks, and how might they go about assessing credibility? A perquisite of guiding palaeoart processes is having a concept of what the subject species looked like. This might seem like a patronising comment, with experts replying 'of course I know what [subject species] looks like!', but really think about it - do you really know what the proportions of your subject are, and what they look like when reconstructed against one another? As in, to the point where you could render a reasonable stick-figure version of it? Could you describe what it looks like outside of plain lateral view, and are these interpretations based on modern technical data, not treatments of the same subject by previous artists? Palaeoart is highly varied in scientific credibility and even those works produced by masters of palaeoart may have dated or contain errors. Ergo, palaeoart consultants should form their basic concepts of appearance from images of fossils, tables of measurements and other primary resources, not previous artistic interpretations. This need for caution applies to skeletal reconstructions too, as these become dated and require modernisation as much as any other reconstructions of prehistoric life. So before dusting off that copy of Romer's Vertebrate Palaeontology for another round of consultancy work, or digging out a skeletal reconstruction from a century ago, consider how kind the last few decades of research have been to those familiar images. Anyone needing an example of how a well-known, seemingly 'safe' skeletal can become dated should check out Scott Hartman's new Dimetrodon skeletal. The animal we all 'know' as Dimetrodon is really Romer's 1927 skeletal - 90 years on, it's looking pretty different.

Role 1 of a palaeoart consultant: know what the basic anatomy of a subject looks like when pinned together. A great poster child for this requirement are pterosaurs, familar animals but with very unfamiliar proportions. It's continually clear from scientist-led pterosaur palaeoartworks that their proportions remain poorly appreciated, even to specialists, a fact made especially obvious by the continued depiction of large torsos in many species. Spend an afternoon piecing together pterosaur fossils, or even just measurements of their bones, and their tiny bodies - shown here via Quetzalcoatlus sp. - become inescapable.
Once good contemporary and credible references have been amassed to account for the technical side of a project, they can be used as core reference material for all parties on the project, giving a common goal to work towards. A useful shopping list for 'core references' might be a skeletal reconstruction of the subject species (either drawings or a good museum mount; a closely related species might do if the specific skeletal is unavailable, and especially if the subject is poorly known or only subtly different from available reference material), lists of bone measurements and ratios, and literature providing a detailed insight into the anatomy of the subject. Good photos of referred specimens, from as many angles as possible, are always helpful too.

Is palaeoart a reliable source of information about the appearance of a fossil animal? Sometimes yes, but oftentimes, no. Talk slide from my 2014 TetZooCon presentation on the cultural evolution of azhdarchids pterosaurs, showing some of the earlier, zanier attempts to restore these animals. There's so much incredulous anatomy here that artists and scientists should steer well clear of these as reference material and go back to primary sources - fossils, descriptions, measurements - to form the foundation of their artwork. (Psst - TetZooCon is happening again soon, details here)
With these references in hand, regular checks on developing artwork can begin. A rule of thumb in palaeoart is that aspects of an artwork should be justifiable one way or another ("these proportions are because of that, this pose seems OK because of this, this speculation reflects this..."). If they aren't, or the defence for that aspect is suspect, the artwork should be modified until a superior interpretation is presented. We can go a long way to bringing palaeoart credibility up to speed by appraising the following, fact-based elements:
  • Anything to do with basic measurements, including the size of the subject relative to its environment and other species, or the proportions of its body. Obtaining metrics from 2D art can be difficult if a subject is obliquely posed or foreshortened, but their rough proportions can be estimated based on their relationship to other body parts. If in doubt, it’s better to get the artist to check their work than to ignore it. Pay particular attention to the proportions of the head to the rest of the body, the size of the torso, and the ratios of the limb bones, as these are prone to errors.
  • Whether the skeleton of the subject fits within the restored soft-tissue volumes. Especially notice the shape of the head and teeth, the cross-section and length of the torso, and the bulk of the appendages, as these are often problem areas. Also make sure the position of the shoulders is correct – it is often more challenging to reconstruct the pectoral region than the pelvic, so the forelimb attachment region can end up in strange places.
  • Whether the chosen pose breaches predictions of joint articulation. Over-stretched limbs, as well as exaggerated neck and tail poses, are key to look at here.
  • Whether appropriate fossil soft-tissues have been factored into the painting. This includes tissue types (e.g. correct integument) and aspects of tissue bulk. Where tissue types are unknown, check that the predicted substitute is based on sensible use of phylogenetic bracketing and comparative anatomy.
  • Finally, note whether the species depicted in the artwork were actually contemporaneous, and that the restored environments and climates are appropriate. 
If, via aid of flux capacitored DeLorean, I was consulting for my own azhdarchid art from 2008, I could make lots of suggestions for improvement on purely scientific grounds. The comments here - concerning proportions, limb bone orientations, bone shapes and so on - could be made from any scientist familiar with recent work and interpretations of pterosaur anatomy, and do not require any forays into the artistic side of palaeoart.
Note that none of these aspects stray into areas of artistry, except - sometimes - a need to interpret 3D shapes in 2D art. Moreover, virtually all of these elements relate to commonly studied aspects of fossil forms. All we're doing is taking the same bone shapes and proportions that inform taxonomic or systematic studies, or the ratios and metrics that underlie functional analyses, and applying them to a different project. We're using information that most scientists already know or have immediately to hand, just set to a different tune.

Because of this, palaeoart consultancy is not as arduous a task as it first appears, nor a total time sink. I'm not going to pretend that good palaeoart consultancy is a job you can do in seconds but, once you have basic references established, most comments simply pertain to nudging the reconstruction in the right direction. As with many academic projects, advising on palaeoart requires the most time investment up front, and then relatively little after. Needless to say, the more prepared you are at the start, the less time investment is needed down the line.

And these points - basic as they might seem - will see just about any palaeontologist able to guide and shape palaeoart production. It should be stressed how continued checking along these lines can make an amazing difference to a palaeoartwork, and thus its success at capturing a hypothesis and future legacy. Correcting a scientific goof not only makes a picture more credible, but it often marks a division between a picture being artistically lacking and coming together. There's a reason artists of living creatures (including humans) are so obsessed with the anatomy of their subjects, and that's because it's essential to producing good artwork. Palaeoart is no different, so don't be shy: help your artist get the information and understanding they need to make your science look great.


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Friday, 24 February 2017

Plesiosaur palaeoart: thoughts for artists

Jurassic plesiosauroid Plesiosaurus dolichodeirus with a controversially dipped left hindfin. Nothing like a little drama to start a blog post.
Among the first animals to feature prominently in palaeoart were plesiosaurs, those four-flippered marine sauropterygians that need no introduction to anyone who's reading a blog focused on prehistoric life. Some plesiosaur depictions are among the most spectacular palaeoart of all: their arcing spinal columns, toothy faces and the moodiness intrinsic to seascapes are wonderful ingredients for palaeoartists to play with, leading to two centuries of plesiosaurs as dependably gripping art subjects.

Despite their popularity among artists, the theory we apply to our plesiosaur reconstructions has not been significantly 'modernised' in the way that it has for other prehistoric species, most obviously Mesozoic dinosaurs, pterosaurs or fossil mammals. A number of authors and artists have produced solid foundations for the reconstruction of the latter animals - libraries of skeletal references, assessments of gait and stance, heightened awareness of common soft-tissues, etc. - and their life appearances are now more uniformly reconstructed and prone to fewer obvious errors. This has yet to happen for plesiosaurs, however. Modern skeletal reconstructions are few, references for muscle layout and soft-tissue data are fewer, and discussions over aspects of their life appearance are rare.

I was recently commissioned to produce two studies of two Early Jurassic plesiosaurs - one of the plesiosauroid Plesiosaurus dolichodeirus (above) and another of the pliosaurid Attenborosaurus conybeari (below). I cannot claim any expertise in plesiosaur science, but when reviewing art-relevant literature on these animals it struck me that many familiar elements of plesiosaur palaeoart oppose our soft-tissue data, modern muscle studies and flipper arthrology, as well as the generalities of vertebrate anatomy. I'm sure others have noticed these issues before me, but their prevalence in contemporary plesiosaur art suggests they are not as widely known as they could be. In the interests of stirring conversation on restoring plesiosaurs, I thought I'd share my findings and thoughts here.

Flipper shape and motion

One of the ‘classic’ elements of plesiosaur reconstruction is their distinctive flipper shape: a tight, oar-like profile which hugs the contours of the fin skeletons. However, both muscle studies and soft-tissue data indicate that their limb morphology was quite different to the underlying osteology, and our 'oar-like' depictions are problematic.

Firstly, reconstructions of plesiosaur forelimb musculature show that they were likely powerfully muscled around the shoulders, especially ventrally. Reconstructions of plesiosaur forelimb musculature have been around for almost 100 years and several alternative ideas on the exact configuration are available. They vary from sparingly muscled reconstructions where those massive, plate-like pectoral elements are left mostly free of muscle anchorage (e.g. Carpenter et al. 2010), to models where the entire girdle is swathed in huge muscle attachment sites (Araújo and Correia 2015). The latter seems to reflect the most phylogenetically-informed hypothesis (using data from lizards, crocs and turtles, which seems sensible given on-going uncertainty about plesiosaur ancestry) and - from a purely intuitive perspective - an extensively muscled limb girdle seems more likely than a lightly muscled one. Why develop those huge coracoids if they aren't going to anchor anything?

If the more extensive models of pectoral musculature are correct, we need to consider how the proximal regions of plesiosaur forelimbs would have looked like in life. One key consequence is that, once we link all the pectoral muscles to their insertions on the limb and body, the 'shaft' of the 'oar-shaped' flipper disappears: muscles running along the anterior and posterior region of the humerus fill the pinched, concave regions so that the proximal region is almost as thick as the bony paddle. Much of the proximal humerus becomes buried in muscle dorsally and ventrally too, to the extent that we might imagine the shoulder region was quite bulky in life.

Summary diagrams of plesiosaur pectoral musculature based on Araújo and Correia (2015), with some of my own input on the body outlines (middle and right). Left shows a schematic plesiosaur skeleton (based on Rhomaleosaurus) and a 'traditional' soft-tissue outline, traced from Araújo and Correia (2015). Middle shows the superficial dorsal pectoral musculature predicted by their study - note that it embiggens the pinched proximal region of the flipper by bulking out the anterior and posterior humeral regions. Right shows how data from plesiosaur soft-tissues - see below - changes the flipper shape even further.
In this respect their limb anatomy might look more similar to that of modern tetrapod swimmers – such as whales, seals and turtles – than we typically reconstruct it. We might draw particular comparison to pinnipeds, where a noticeable bulge can be seen at the junction between the forelimb and the torso. The size of plesiosaur pelvic girdles probably indicate a similar muscular condition for the hindlimb and we might assume that they weren't slender-necked, 'oar-shaped' fins either.

Holotype specimen of Seeleysaurus guilelmiimperatoris. Note soft-tissue outlines behind the right forelimb and tail. If you'd like to see these tissues in person, you're too late - the body outlines of this specimen were painted over years ago. Bummer. From Dames (1895).
But these are not the only tissues which distort the outline of the flippers. Fossils of plesiosaur body outlines are very rare, but three specimens (the holotypes of Seeleyosaurus, Hydrorion and Mauriciosaurus - see Dames 1895, von Huene 1923 and Frey et al. 2017) preserve soft-tissues that considerably augment their flipper shape. All three show deep wedges of soft-tissues tapering along the back of the fin skeleton to the flipper tip, with Mauriciosaurus showing tissues - though their shape isn't entirely clear - also present behind the proximal limb regions. There is sufficient consistency across these specimens to suggest expanded paddle tissues were common, and maybe even widespread, in plesiosaurs and, for artists, augmenting our plesiosaur flipper skeletons with these trailing edge tissues should be our standard approach to their restoration.

Hydrorion brachypterygius and its soft-tissue forelimb impressions (the dark, grainy textures behind the fins). From von Huene (1923).
Moving on, artists might also want to note that ideas about highly restricted motion of plesiosaur flippers are being revised. Traditionally, authors such as Carpenter et al. (2010) have argued for limited motion at both the shoulder and hip limb joints, resulting in what I like to call the 'sinking rowing boat' pose: depictions of plesiosaurs with limbs projecting just a little off the horizontal, regardless of what they're up to. Restricted fore- and aft motion seems likely given the elongate shape of limb girdle joints, but whether the vertical movement of the limbs was restricted to tight arcs - perhaps as shallow as a 54° total range - is being challenged (e.g. Liu et al. 2015). Plesiosaur limb girdles were evidently highly cartilaginous in life and estimating their joint motion challenging - most of the information we desire to determine some sense of joint mobility is long gone. But if we assume they had more than the slimmest covering of cartilage in the girdle limb joints - which seems sensible, given the huge size of the girdle joints and their poor match for the limb bone shape - we can assume wide arcs of motion to both limb sets before disarticulation. The exact range of movement remains an open question - unpublished studies hint at even greater motion than other 'wide arc' research, such as Liu et al. (2015) (thanks to Darren Naish for advance word on this) - but artists should not feel confined to the 'rowing boat' pose that we've seen plesiosaurs depicted in for decades. With my artist hat on, I find this very welcome news. Plesiosaurs with limbs perpetually stuck out sideways can look a little static even in the hands of great artists, and their limited poseability has not made them the most interesting subjects to reconstruct. Wider arcs of motion allow plesiosaurs to be depicted in more complex and dynamic poses, and to convey a greater range of behaviours - pirouetting around corners with dipped fins, beating their flippers to attain high speeds, dropping their limbs because they're being lazy... all sorts of stuff. Well done, science, you've made at least one artist a happy person.

Aspects of the neck

My experience with the mass-economising, lightweight long necks of terrestrial or volant tetrapods means the extensively developed vertebrae of longer necked plesiosaurs are of great personal interest. Freed of the constraints of mass reduction, their numerous neck vertebrae are short, highly developed elements with long, robust processes - the exact opposite of the long, simplified structures I'm used to dealing with. Assuming plesiosaur necks were constructed like those of other amniotes (below), they likely anchored powerful muscles along their lengths. In particular, their neural spines are very tall and we can assume they bore enhanced musculature associated with lifting and turning the neck - useful features for long necked animals living in a dense fluid medium. Myological reconstructions suggest that the axial column would bear muscles connecting to the pectoral girdle, producing a deep set of tissues at the neck-torso junction (Araújo and Correia 2015, see pectoral myology diagram above). Artists should equip these animals with chunky, powerful 'reptilian' necks rather than svelte, bird-like variants. I do wonder if thick muscles along the neck might have impacted their neck mobility somewhat - another reason to assume long-necked plesiosaurs were only capable of bending their necks into simple curves (e.g. Zammit et al. 2008).

Amniote neck muscle groups and functionality, modelled by the American alligator Alligator mississippiensis. If the same basic rules apply to plesiosaurs, we should expect many species to have huge muscles and very powerful necks. Diagram concept and muscle layout after Snively and Russell (2007).
The neck/skull articulation of plesiosaurs is also of interest. In many taxa, including Plesiosaurus itself, the posterior face of the skull is displaced anteriorly to the jaw joints. This condition is not unique to plesiosaurs, also being found in some other reptiles including living crocodylians. This 'staggering' of the posterior skull margins might minimise any obvious topographic demarcation between head and neck tissues (the head/neck junction is less obvious in crocodylians than it is in many birds and mammals, for instance) as as well as complicate motion at the head-neck joint. The anteriormost cervical vertebrae and their articulation with the skull would be buried by bone laterally and throat tissues (including muscles and hyoid cartilages) ventrally, and we have to wonder if this envelope of material would limit how far the skull could pivot on the neck. The analogous condition in modern crocodylians seems to bear out this prediction, so perhaps we should not be restoring plesioaurs with mammal- or bird-like cocked heads.

Trunk shape - cross section and lateral profile

Plesiosaurs are often restored with a generic, 'barrel-shaped’ trunks. This is appropriate for some taxa, but not all. It must be said that plesiosaur torso shape is an area of on-going research. I recently spoke with a number of plesiosaur experts on this matter and found aspects like rib and gastralia articulation, the vertical position of the pectoral girdle and so on were somewhat contentious (thanks to Richard Forrest, Aubrey Roberts and Mark Evans for their thoughts). The crux of the issue is that, unlike some reptiles (such as birds or pterosaurs), plesiosaur torso skeletons don't slot neatly together in a single, incontrovertible manner, as is evident to anyone who's seen more than one plesiosaur mount in a museum. Understanding their torsos requires precise appreciation of their vertebral rib articulations, knowing their rib and gastralia curvature in three dimensions, and the benefit of fully articulated fossils for reference. This is quite a list of requirements, and one that is only currently met by a fraction of plesiosaur taxa.

Despite this, detailed reconstruction attempts provide reason to think not all plesiosaurs had tubby, barrel-shaped torsos. Close inspection of vertebral rib articulations and the shape of three-dimensionally preserved plesiosaur torso skeletons allowed O’Keefe et al. (2011) to reconstruct some cryptoclidids with tall, barrel-shaped bodies, and others with dorsoventrally compressed ones (below). In some genera, like Tatanectes, this is augmented further by almost flat dorsal ribs. It is difficult to gauge torso cross sectional shapes from just looking at a typical, half-prepared and flattened plesiosaur fossil, but artists should be mindful that not all species will have circular torso sections. Given how important torso shapes are to a reconstruction, we should check research literature carefully to make the most informed call we can on this aspect of restoring their life appearance.

Cryptoclidid torsos in cross section, with (over conservative) soft-tissue outlines. Modified from O'Keefe et al. (2011).
It is not only the cross section of plesiosaur trunks which are of artistic interest. Neural spine height is not always consistent along the dorsal column, with genera like Attenborosaurus having much taller vertebrae towards the anterior end of the torso. I don't think we know much about the torso cross section of this animal yet, but its vertebral proportions alone imply a proportionally deep shoulder region and a ‘tear-drop’ profile in lateral aspect. This may have been translated into soft-tissue depth in life: deep neural spines over the shoulder might betray a well developed m. latissimus dorsi, a forelimb elevator muscle that could be beneficially augmented for a swimming animal. Interestingly, Attenborosaurus has larger forelimbs than hindlimbs, and it's not entirely daft to wonder if its big shoulder vertebrae and their possible role in beefing out the shoulder muscles reflect forelimb-dominated swimming (see Liu et al. 2015). That's a discussion for another day, of course: the take home for artists here is to pay attention to those trunk vertebrae, and think about how they might influence the long-axis trunk symmetry.
Attenborosaurus conybeari, Jurassic equivalent of those top-heavy gym users who forget about working their legs.


And finally... so long, shrink-wrapping

A recurrent theme in this post has been the idea of plesiosaur skeletons being deeply buried in soft-tissues of varying kinds. One of the most amazing plesiosaur fossils known to date, recently described from Cretaceous deposits of Mexico (Frey and Stinnesbeck 2014; Frey et al. 2017), clearly vindicates this theory. This specimen is the holotype of Mauriciosaurus fernandezi, which preserves a near-continuous body outline to give us an unprecedented glimpse of its life appearance. Much of the soft-tissue includes belly and lateral body wall skin impressions (tiny, 12 x 2 mm rectangular scales arranged in rows along the animal), but even more surprising is how much soft-tissue there is: by gum, this was a tubby creature, particularly around the tail. Even the thinnest regions of the outline are a good 50 mm wide, and some parts are considerably deeper. Frey et al. (2017) ascribe much of this depth to fatty, subdermal adipose tissue, including the caudal mass. Many living reptiles have extensive fat deposits around their tails (as discussed for prehistoric animals in this post) and it would not be surprising if plesiosaurs used this adaptation to streamline their shape. As noted by Frey et al. (2017), the preserved torso shape is not dissimilar to those of highly pelagic turtles or penguins.
Line drawing of Mauriciosaurus fernandezi holotype, redrawn from Frey et al. (2017). This specimen is extra special for reminding us of the finest Queen song of all time.
Whether these plump tails were the case for all plesiosaurs remains to be seen. Frey et al. (2017) note that the caudal vertebrae of Mauriciosaurus has small processes for muscle attachment, and may have been weakly muscled in life. This might be predicted, as a tail encased inside a deep cone of fat is unlikely to have been capable of much movement even if it was strongly muscled (although, that said, some living marine mammals are very flexible despite their deep fatty tissues). However, other plesiosaurs - including, for easy reference, the Hydrorion depicted above - do have large caudal sites for muscle attachment - might they have moved their tails about more freely? Given the compelling evidence for caudal fins or rudders in several plesiosaur species (Dames 1895; Wilhem 2010; Smith 2013 - check out Brian Switek's post if you need a quick primer) it might make sense for some species to maintain mobile tails to aid steering. We should note that the partially preserved tail tissues of Seeleyosaurus are not as chunky as those of Mauriciosaurus: they're thick, sure, but not obviously part of a wide, wedge-shaped mass, perhaps suggesting a more easily moved structure. Hopefully, more plesiosaur soft-tissues will turn up soon to give us more insight on this matter.

As a final point on the Mauriciosaurus fossil, we can now add plesiosaurs to the list of fossil taxa with specimens directly opposing 'shrink-wrapping' palaeoartistic conventions. It joins fossils of dinosaurs (Mesozoic and beyond), pterosaurs, mammals, early archosauromorphs and many others in suggesting the soft-tissues of long extinct creatures were no less extensive than those of modern species. As with living taxa, their skeletons were mostly placed well inside their bodies, not just under the surface of a thin skin. There's no doubt that soft-tissue depth is going to vary across animal bodies and between species, but it's increasingly difficult to defend reconstructions where bodies tightly hug skeletal contours, where facial tissues are sucked into every skull cavity, and where the depth of fats and integuments are not factored into the restorative process. 'Shrink-wrapping' is one of the few aspects of palaeoart that is testable against fossil data, and it is not winning out.

And that's that, then

I'm sure there's a lot more we could say on restoring plesiosaurs, but this is where we'll have to leave this discussion for now - hopefully this post helps fill the deficit of detailed discussion on plesiosaur life appearance. I must admit that these recent efforts at restoring plesiosaurs have given me a newfound interest in the group, and I wouldn't be surprised if artwork these chaps and their relatives turn up around here soon.

Next time: sharks vs. pterosaurs - who will win? (Spoiler: not the pterosaurs)

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References

  • Carpenter, K., Sanders, F., Reed, B., Reed, J., & Larson, P. (2010). Plesiosaur swimming as interpreted from skeletal analysis and experimental results. Transactions of the Kansas Academy of Science, 113(1/2), 1-34.
  • Dames, W. B. (1895). Die plesiosaurier der süddeutschen Liasformation. Verlag d. Kgl. Akad. d. Wissenschaften.Frey, E., & Stinnesbeck, W. (2014). Plesiosaurs, reptiles between grace and awe. In Dinosaurs and Other Reptiles from the Mesozoic of Mexico (pp. 79-98). Indiana University Press.
  • Frey, E., Mulder, E., Stinnesbeck, W., Rivera-Sylva, H., Padilla-Gutiérrez, J., González-González, A. 2017. A new polycotylid plesiosaur from the early Late Cretaceous of northeast Mexico. Boletín de la Sociedad Geológica Mexicana. 69 (1): 87-134
  • Liu, S., Smith, A. S., Gu, Y., Tan, J., Liu, C. K., & Turk, G. (2015). Computer simulations imply forelimb-dominated underwater flight in plesiosaurs. PLoS Comput Biol, 11(12), e1004605.
  • O’Keefe, F. R., Street, H. P., Wilhelm, B. C., Richards, C. D., & Zhu, H. (2011). A new skeleton of the cryptoclidid plesiosaur Tatenectes laramiensis reveals a novel body shape among plesiosaurs. Journal of Vertebrate Paleontology, 31(2), 330-339.
  • von Huene, F. (1923). Ein neuer Plesiosaurier aus dem oberen Lias Württembergs. Jahreschefte des Vereins für vaterländische Naturkunde in Württemberg, 1923, 3-23.
  • Wilhelm, B.C. 2010. Novel anatomy of cryptoclidid plesiosaurs with comments on axial locomotion. Ph.D thesis, Marshall University, Huntington, WV. USA
  • Zammit, M., Daniels, C. B., & Kear, B. P. (2008). Elasmosaur (Reptilia: Sauropterygia) neck flexibility: Implications for feeding strategies. Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology, 150(2), 124-130.

Friday, 17 February 2017

Scientists: please pay more attention to palaeoart

A few years ago I wrote about how the 21st century is a terrific interval for palaeoart because of the wealth of information, discussion of palaeoart theory and diversity of talent we presently enjoy. Never before has so much data on this topic been available to practitioners of the trade, to scientists and historians, or to curious members of the public. The increase in palaeoart talent, a movement of artists exploring the outer regions of palaeoart science and artistry, and hints of wider interest in art of fossil animals are all traceable to these recent developments.

Any palaeoartist you speak to will tell you that there are honed practises and processes that should be applied when executing a palaeoart study. Several decades worth of influential artwork and writing by the likes of Knight, Hallett, Paul, Antón, Conway et al. and others send a unified message: reconstructions of fossil animals should be produced through close study of animal anatomy, both fossil and modern. They demonstrate that palaeoartworks must pass some basic, almost objective tests to be considered scientifically credible, successful examples of the medium.
  • The proportions of the subject should be accurate to those indicated by its fossil remains
  • The skeleton of the subject should fit within the restored soft-tissue volume
  • Depicted poses should conform to predictions of joint motion
  • The soft-tissue volume should - at minimum - be informed by predictions of muscle bulk derived from fossil remains as well as relevant data from modern, related species
  • Soft-tissues restoration should be informed where possible by fossil data, or else via robust predictive techniques, such as phylogenetic bracketing
These points are not controversial. I'm sure any number of modern palaeoartists would list these as their baseline, entry level requirements for bona fide, scientifically-credible palaeoartwork. Further considerations of posing, composition and behaviour become more subjective and debatable - ideas of what we think 'looks right' or is 'most likely' in these areas are influenced by personal preferences, artistic styles and intent of the artwork. Successful palaeoartists can have contrasting ideas about these latter points, and that's fine: so long as our work remains within the realms of scientific plausibility, we are free to experiment and develop unique styles. But at the core of any palaeoartwork must be a reconstruction of a species that conforms to those fundamental aspects listed above.

Although it feels like we live in an enlightened age for palaeoart, some artworks associated with the very people who should be sticklers for scientific precision and reconstruction plausibility fall well short of the most elementary aspects of fossil animal reconstruction. These are not reconstructions for TV shows or films, where creative forces override scientific input. They are not illustrations for books where an overworked generalist illustrator is given a few hours to render an animal they'd not heard of until that morning. These are artworks produced for papers and press releases where scientists - researching palaeontologists with direct access to fossil material and technical literature - have every opportunity to guide and shape the artistic process. And these works are sometimes so scientifically awful that they're almost insulting to those of us who strive to produce credible prehistoric imagery, being critically flawed at the most basic level.

The level of failure in some artworks (not linked to here out of politeness) is sufficient to question whether those involved knew anything about reconstructing anatomy or if they really cared about the artwork at all. And yes, I think we do have to look at the scientists and researchers as being ultimately responsible here. These are artworks produced directly under their control to be associated with their work, and without pressure from publishers or media producers to be fantastic, weird or sensational. Scientists made the decision to produce these palaeoartworks, chose the artist to execute it, chose the level of input to have during its production, and signed off the final product.

In these artworks, even errors which scientists can objectively veto (such as proportions and bone articulations, elements of form such as skull or tooth shape, well-studied soft-tissue anatomies like feathers arrangements on dinosaur wings) are ignored. The result is that these pieces perpetuate errors that were realised as problematic years ago: under-muscled, 'shrink-wrapped' animals; 'bunny handed' theropods; feathered maniraptorans with three free fingers extending from their wings; ichthyosaurs with visible giant eyes; pterosaurs with enormous torsos and so on. The worst offenders show no grasp of basic aspects of animal anatomy, fossil or modern, with outlandish ideas of muscle distribution or proportions which are falsified by the most cursory glance at reference material. It is no exaggeration to say that some recent scientist-led palaeoartworks would not look out of place if produced in the 1830s. 

And we - educators, scientists, palaeoartists - should feel ticked off about this. Scientist-led palaeoart should be the best there is: carefully-executed, evidence-led syntheses of research conclusions in compelling artworks. It should convey to people how the subject appeared and behaved based on both new, cutting-edge research and the best of the science which preceded it. There is no reason not to take the same attitude to our palaeoart that we do to the rest of our studies. It is frowned upon to take half-measure approaches to descriptions, statistical analyses or cladistic methodology, so why is palaeoart exempt? Making crass, basic errors in animal reconstruction is no different to executing a flawed study or analysis. Both ignore data, advice and theory documented in palaeontological literature, and both show little regard for the techniques developed by pioneers of the process. Moreover, when they make it to publication, both imply that half-baked approaches are worthy of equal consideration to more carefully executed examples. Scientists will know the feeling of frustration when work directly relevant to a paper is not cited: lousy palaeoart is guilty of ignoring the theory and development of an entire field

A baffling aspect to this problem is that scientists routinely seek expertise lacking on research teams. Need a fossil prepared but lack the skulls? Seek assistance from a preparator. Need to crunch some stats but not sure how? Contact a colleague with statistical expertise. What we don't do with our science is assume our intuition and instincts about a topic are enough to guide us alone: we defer to those with the training, specific knowledge and experience to do the jobs we can't. And we would never employ an equally inexperienced individual and guide them through a process we lack all experience of ourselves.

But this is exactly what we do with palaeoartistry. Executing a palaeoart study requires a grasp of anatomy (and not just bones!), an ability to reconstruct/interpret fossil remains, a healthy grasp of living animal form, and an ability to translate all this into an artistic creation. These are not skills that everyone has, or that even all palaeontologists have. There are numerous specialisms in palaeontology, and not all of them are associated with the expertise ideal for consulting on palaeoartworks. Fossil bones do not exude a radiation which means those who work with them automatically know everything about palaeoart methods and theory. And yet it seems some scientists think it does, resulting in ill-founded advice for naive artists and approval of poor, flawed work. I am not the first person to raise this point (it has been mentioned in palaeoart literature since the 1980s), but it seems to fall on deaf ears.

Some readers may be wondering if this matters - so what if we have the odd wobbly looking reconstruction every now and then? Consider these points. Firstly, if scientists are so relaxed about palaeoart that they have no regard for even getting fundamental aspects correct, then what, really, is the point of the art in the first place? What can art of that quality really add to our field? It can't be held up as an accurate representation of the animal itself, and knowledgeable educators will avoid it or abandon it the moment a superior alternative becomes available (which, given the popularity of palaeoart online, is normally a couple of days after a new discovery. Indeed, awful PR palaeoart normally spurs more alternative versions, and with faster turnaround times). Badly produced palaeoart is basically destined to be ignored by those in the know, reflects poorly on those involved in its production, and ends up being an embarrassing aspect of the publication.

Secondly, scientist-led palaeoart is often the basis for derivative artwork, whether it's good or bad. Whereas people might expect prehistoric animals seen in film and TV to be embellished and enhanced, scientist-endorsed artwork carries the weight of expert approval. For non-specialist illustrators, they're an obvious source of information and errors are carried over into next generation work. Scientists need to realise that the half-lives of palaeoart are often much longer than any press articles or even scientific papers: they have long-lasting impacts on public perception and even inform scientific hypotheses. Darren Naish recently wrote more about these issues at length here.

Thirdly, there are scores of competent palaeoartists awaiting opportunities to work with scientists, and their prior knowledge of reconstruction processes and anatomy would fill knowledge gaps in some teams. Not only do these individuals have the skills needed to understand a fossil specimen and technical paper, and are thus able to produce credible artwork without constant academic input, but their experience means they can converse with scientists at (or close to) a technical level. This allows for detailed conversations about the specifics of the reconstruction and development of new ideas and insights into the life appearance of the subject organism. Experienced palaeoartists are more than just people who make pretty pictures: they're peers and colleagues of scientists, and able to augment research when given the opportunity.

Lastly, it is widely known that the palaeoart industry has a problem maintaining employment for even its most talented individuals, and in this context hiring non-specialists, especially if the research team is not palaeoart savvy, is ludicrous - why not hire the right people for the job? There are many early-career palaeoartists available if tight budgets are a concern, as well as numerous veterans who can offer highly polished art and rapid turnaround times if time is tight. Finding these people is as easy as opening modern palaeoart books, asking colleagues for recommendations or even a Google search. The wealth of easily-accessed palaeoart talent makes it inexcusable not to bring specialist artists on board for palaeoart projects.

And 'inexcusable' sums up my feeling on this topic pretty well. The fact that many scientist-led artworks are really amazing shows that high quality palaeoart of this nature is achievable if scientists care enough about its production. But the availability of palaeoart-relevant information, the growing body of literature on palaeoart theory, the willingness and accessibility of talented artists, and the demands of modern scientific standards make academically-driven, scientifically-rotten palaeoart inexcusable in the modern day. I'm not arguing that scientist-led palaeoart has to be perfect. I'm not arguing that scientist-led palaeoart has to conform to specific conventions of style, or to constrained ideas of life appearance. But I am arguing that scientist-led palaeoart should look like someone gave a damn about the final product.