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Sunday, 23 December 2018

No, Cretaceous sharks did not leap from water to eat flying pterosaurs

How the heavy hitters covered Hone et al. 2018: a short, open-access and easy-to-read paper about a shark tooth preserved with Pteranodon. The Sun gets bonus stupid points for making their own graphic. 

Major news outlets have been reporting this week that a new study evidences prehistoric sharks predating pterosaurs by leaping from the water to snatch them in mid-air. It would be an awesome discovery, if it were true. In reality, these headlines are nothing but a construct by journalists based on a misread palaeoart image, ignorance of some basic facts of animal biology, and lazy science reporting. I'm particularly angry because the study being misreported, and the art being misread, stems from a paper that I recently published with with my friends and colleagues Dave Hone and Mike Habib (Hone et al. 2018). The paper is in PeerJ, and is thus open-access. Anyone - including our dear media - can fact check what we have to say without hitting a paywall.

The nature of modern news is that stories can spread like wildfire, and it's effectively impossible to correct major gaffes once a story gains momentum. With that in mind, the best I can do is outline here what our new paper actually says, what our artwork actually shows, and hope that readers link to this post wherever they see this ridiculous press story being shared. It must be said that a number our outlets are reporting the story more accurately, but enough have mangled our findings that I feel I need to do something. I genuinely care about the accurate conveyance of science, and I've found this distortion of our work and my painting very distressing.

What our paper actually says

OK, first up: a summary of our paper. Mike, Dave and I have documented a series of neck vertebrae from the famous Late Cretaceous pterosaur Pteranodon associated with the tooth of a lamniform shark, Cretoxyrhina mantelli. The vertebrae and their teeth were found in 1965 but our records about their discovery are confused and we don't know whether they were part of a larger discovery of bones, or just a few isolated remains. There are hints that they may have been part of a more impressive skeleton, but it's pretty hard to tell. In any case, today this string of vertebrae is on display at the Los Angeles County Museum as part of a composite skeleton.

Vertebrae are not diagnostic for different Pteranodon species so we refrain from identifying the pterosaur beyond Pteranodon sp. The specimen was found in Niobrara Formation rocks that traditionally yield P. longiceps however, and this is probably the likely species identity- we just can't verify it*. The identification of the shark is better constrained as the tooth is a perfect match for C. mantelli. Indeed, we can even tell which part of the mouth it came from thanks to Cretoxyrhina mantelli being exceptionally well known: even complete skulls and skeletons have been found. This allows us to roughly gauge the size of the Cretoxyrhina as c. 2.5 m long, which makes it a small individual compared to the 6-7 m specimens known from other remains. Our Pteranodon was on the small size as well at c. 5 m across the wings. This is within the upper size range of Pteranodon fossils, but still 1-2 m off the full wingspread of this species.

*If anyone's wondering, yes, we follow the traditional Bennettian concept of Pteranodon taxonomy. It was actually writing the manuscript for this paper that prompted my blog post on Pteranodon taxonomy.

Pteranodon sp. specimen LACM 50926 as mounted as part of a composite skeleton in the Los Angeles County Museum, and in more detail with their hitchhiking Cretoxyrhina mantelli tooth (arrowed). Scale bar is 50 mm. From Hone et al. 2018.
The Cretoxyrhina tooth does not actually penetrate the pterosaur bone, but is wedged beneath a vertebral process in a complex, intimate manner. We assume this evidences the shark biting into the pterosaur neck and shedding a tooth into its soft-tissues. An alternative - that the specimens became associated through actions of currents or storms - is less plausible given the strange position of the tooth, its tightness to the specimen, as well as the gentle, low-energy marine conditions of the Niobrara Formation.

But what sort of circumstances brought these animals together? It's here that our questions go beyond what the fossils can tell us. There's only so much a single tooth and string of vertebrae can objectively reveal about an ancient animal interaction, and we conclude that either a predatory and scavenging act could have produced the association - there's just no way to tell. While a scavenging story explains itself (short version: pterosaur dies over water; shark gets a meal), we explored how a predatory scenario may have played out given the known fish-eating habits of Pteranodon and hypotheses that this pterosaur regularly dived or swam to catch its prey (e.g. Bennett 2001; Hone and Henderson 2014; Witton 2013, 2018). Swimming pterosaurs are a new idea to many but substantiated by several lines of evidence, including swimming tracks, modelling of their aquatic launch strategy, and studies of their floating capability (Lockley et al. 2003; Habib and Cunningham 2010; Hone and Henderson 2014). We propose that if Pteranodon was a regular swimmer it would be vulnerable to sharks and other large predators, and its flight muscles would surely be a decent meal for many carnivores. Pterosaurs were probably pretty sinewy across their limbs, but there'd be some sizeable steaks to carve from their shoulders and chests. As big, powerful predators, it seems entirely plausible that even a half-size Cretoxyrhina would be capable of subduing a large Pteranodon, assuming they could catch one.

And that is as far as we go in our paper - it's pretty conservative stuff. You can read another summary of the paper at Dave Hone's Archosaur Musings.

The illustration

Rocket shark eats flying pterosaur? No. Artwork of a small Cretoxyrhina mantelli attacking a group of floating Pteranondon longiceps, erupting from the sea with one in its jaws. Note the other swimming pterosaurs in this picture - it's almost like pterosaurs weren't always flying, or something. From Hone et al. 2018.
Mike, Dave and I are palaeoart fans and we all - perhaps myself especially - enjoy well-illustrated papers, so we decided to include a reconstruction of Cretoxyrhina vs. Pteranodon in our paper (above). Having recently drawn an image of Pteranodon being hounded by another Cretaceous shark, Squalicorax, at the water surface, I wanted to do something different with this illustration and decided on a more exciting breaching scene: a shark leaping from the water with a Pteranodon in its mouth. Anyone who's watched even a few wildlife documentaries will know this behaviour is far from speculative: it's a widely-filmed, photographed and documented behaviour of South African white sharks (see Planet Earth excerpt from BBC Earth, below). These sharks strike floating prey with such speed that they leap entirely from the water. It's very dramatic, very awesome and seemed like great inspiration for a palaeoartwork. And sure, we have no idea that Cretoxyrhina did this, but a breaching attack is no more or less speculative than any other means of depicting a shark tooth lodging in a Pteranodon neck. I won't bore you with some additional practical factors that influenced the composition (in short, this image is being featured in an upcoming book where I have some firm ideas about visual narrative, and this strongly influenced choices of posture and colour).

In my mind, shark breaching is a widely known, instantly recognisable behaviour that shouldn't be foreign to folks writing science articles. It's routinely covered in major documentary programmes like Planet Earth, as shown here in this YouTube clip from BBC Earth. It's weird to me that folks are assuming my artwork has to be something more awesome than this, just because a pterosaur is involved.

A number of swimming and water-launching pterosaurs were added in the background of the image to make it clear that the Pteranodon was caught from a floating position. In my mind, the image shows a flock of pterosaurs busying themselves in the sea before Cretoxyrhina crashes their party - you can invent your own reasons for all the Pteranodon milling about. Maybe they're foraging, maybe they're congregating for another reason - it doesn't matter too much, what matters is that the sea has six or so Pteranodon either floating or launching from the water. Also note that water is shedding from the main pterosaur's wings, as if it's just left the water along with the shark. I carefully referenced how much water should be flying about using footage and photos of breaching sharks around the peak of their arcs: we often overdo water dynamics in palaeoart, and I was keen to make this image grounded in spite of its spectacular action.

So what's gone wrong?

What we've ended up with, then, is a pretty simple, conservative paper with an illustration that is pretty bleeding edge in terms of pterosaur science (flying reptiles as swimmers) and radical in terms of shark behaviour (breaching). But the two are entirely compatible with one another and the image is appropriately grounded in zoology and palaeontology. It looks extreme, but it's not ridiculous compared to what happens in modern natural history.

Unfortunately, this broader face of our project has been neglected in the media. Instead, our artwork - or rather a knee-jerk, lunkhead interpretation of it - has become the media story and we now have 'science-endorsed' headlines stating that sharks shot out of the water to grab pterosaurs (or even 'flying dinosaurs' in several articles) when they soared overhead. As I've outlined here, this is entirely false, and not representative of our ideas at all. Any hope that our paper could be used to broadly communicate some cool ideas about pterosaur ecology, the role of sharks as important predators throughout the Mesozoic, or even the simple fact that pterosaurs could likely swim has been lost behind ridiculous headlines based on erroneous readings of my picture.

So that's basically that - this is essentially a tale of how quickly false information can spread when it's attached to a pretty image, and why we shouldn't believe everything we read. Perhaps there's a lesson here in the power of imagery, and I am certainly now wondering if I erred in my reconstruction being too complex (to my defence, I did not contribute to the PR campaign for this and did not get the opportunity to sign off on an appropriate description for the picture). Perhaps we're looking at the reality of naive audiences assuming that anything to do with sharks or prehistoric creatures must automatically be the most awesome, badass thing. Maybe I erred in my assumption that people would be familiar with the basics of breaching shark behaviour.

But what this hits home hardest for me is the reality of science reporting in our digital, content-fuelled age. Anyone who led with the stupid shark vs. flying pterosaur headline went nowhere near our actual paper to check what we said, even though it was literally just a link click away. They simply parroted one another to make sure their outlets have the same stories as everyone else. Only a few thought to double check what our conclusions were, and I find that distressing. Remember folks, these are the same guys who're reporting news about far more important things than pterosaurs: vaccinations, climate change, health and environmental issues, and so on: these pterosaur-devouring rocket sharks are stark reminders of how they work. If you've seen accurately reported examples of this story (and they do exist) then add those news services to your bookmarks: they are rare examples of media outlets that aren't jumping our shark.

If understanding pterosaur ecology through fossil associations is of interest, be sure to check out this series of three blog posts: part 1, part 2, and part 3.

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  • Bennett, S. C. (2001). The Osteology and Functional Morphology of the Late Cretaceous Pterosaur Pteranodon Part II. Size and Functional Morphology. Palaeontographica Abteilung A, 113-153.
  • Habib, M. B., & Cunningham, J. (2010). Capacity for water launch in Anhanguera and Quetzalcoatlus. Acta Geoscientica Sinica, 31, 24-25.
  • 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.
  • Hone, D. W., Witton, M. P., & Habib, M. B. (2018). Evidence for the Cretaceous shark Cretoxyrhina mantelli feeding on the pterosaur Pteranodon from the Niobrara Formation. PeerJ, 6, e6031.
  • 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.
  • Witton, M. P. (2018). Pterosaurs in Mesozoic food webs: a review of fossil evidence. Geological Society, London, Special Publications, 455(1), 7-23.