|Pteranodon sternbergi dives for a school of panicked fish. So, what, pterosaurs are super good at swimming now? Read on... Reworked version of an image from Witton (2013). Click here to buy prints of this image (and join my Patreon campaign for a discount!). And yes, I'm calling this animal Pteranodon, not Geosternbergia.|
Whether or not pterosaurs could swim, or how well they could swim, is a recurrent discussion among those interested in flying reptiles. For the most part, palaeontologists have seemed happy to assume that pterosaurs were aquatically capable, at least long enough to permit their escape from water, because so many pterosaur fossils occur in coastal or marine sediments. Moreover, some long-known specimens show evidence of pterosaurs feeding on aquatic prey. Odds are that pteroaurs would end up in water some of the time, even if only by accident, so it makes sense that they could at least keep themselves afloat for a while. Plus, virtually all tetrapods can swim one way or another, including bats and bird species which, on first principles, seem ill-suited to aquatic locomotion. Pterosaurs might be a bit strange, but they'd have to be very strange not to be capable of at least limited aquatic locomotion.
|Proof that pterosaur workers of old thought swimming was possible: Bramwell and Whitfield's (1974) landmark paper on Pteranodon flight depicts one attempting to take off from water.|
as explained here, no-one really knows how well they swim!). The Hone and Henderson study included multiple pterosaur species, and grounded itself with convincingly replicating the floating postures of birds (as, indeed, other Henderson studies have done with the floating postures of other extant animals).
How did pterosaurs fare? Although they assumed a stable floating posture, it was not quite as expected. As explained by Dave Hone at The Guardian, pterosaurs were incapable of assuming a bird-like pose when floating. Playing around with postures and body component densities made little difference: the digital pterosaurs consistently floated with their heads close to, or somewhat submerged, in water. Crucially, their nostrils always ended up close to the digital water line, suggesting that anything but a motionless pterosaur in the calmest water was going to be struggling for a clear airway. The problem, it seems, is that pterosaurs are very front heavy. Pterosaurs combine large heads, necks and shoulders with comparatively slender hindquarters so that, even accounting for their pneumatic features and denser hindlimbs, they consistently pitch forward when floating. This condition is more pronounced in pterodactyloids than other pterosaurs, but the general problem applies across the group.
|So, sorry, 2013 Ornithocheirus-as-a-bird-like-floater-image-that-I-have-a-little-soft-spot-for, you're out of date.|
What does this mean for pterosaurs? Their inability to float like birds probably rules out some behaviours, such as prolonged bouts of sitting on water to rest or forage. However, most swimming animals - even those which routinely travel or forage in water - also can't float like birds. Indeed, predicted pterosaur floating postures are actually pretty consistent with those of other non-avian tetrapods. We might therefore surmise that pterosaur floating abilities are not atypically bad, but simply not at the 'advanced' avian level. As with most tetrapods, pterosaurs might have been quite happy in water, the caveat being that it would never be a passive, restful act. Water-borne pterosaurs were likely either were there for a reason (e.g. finding food, moving through an environment) or, if they had no business there, sought to escape it as soon as possible.
This brings us to the third string of recent work on aquatic pterosaur habits: the biomechanics of entering and exiting water. Earlier this year I discussed pterosaur water launch at some length, so will only provide a brief summary here. Calculations by pterosaur biomechanicists Michael Habib and Jim Cunningham (2010) suggest that pterosaur quadrupedal launching also works on water, albeit in a modified, and slightly more energy intensive form. For some pterosaurs, the effort needed to escape water necessitates a series of hops across the water/air interface to escape surface tension and build up velocity, but some - like the big, powerful azhdarchids - could hulk smash water powerfully enough to escape in one go.
|Ornithocheiroid Ornithocheirus simus achieves launch velocity from a coastal sea. Prints of this painting are available from my shop.|
As with terrestrially-based pterosaurs, it seems takeoff strains put a cap on the maximum size of aquatic-adapted forms. In his Flugsaurier 2015 talk, Mike Habib explained how animals larger than Pteranodon (biggest wingspans around 5-6 m) would struggle with water launching. The largest ornithocheiroids (8-9 m wingpan, c. 160kg in mass) seem to require significant energetic investment and space to take off from water, to the extent that entering aquatic settings resulted in a net loss of energy unless food was particularly plentiful (Habib 2015). This is not to say water launching was impossible for very large or giant pterosaurs, but that the energy demands make it an unlikely routine behaviour. Pterosaur aficionados will note that this size constraint is lower than those proposed for terrestrial launchers (Habib 2013): as might be expected, this reflects the complexity of launching from a fluid substrate instead of hard ground.
Nevertheless, most pterosaurs were not operating at those enormous proportions, and so could theoretically enter water with less concern. Intriguingly, early calculations suggest that some pterosaurs were well-suited to rapid water entry. Qualitative assessments of Pteranodon anatomy indicate that it might be capable of performing shallow dives because, in general construction, it is no less robust than diving birds like pelicans (Bennett 2001; note this is not advocating pelican-like feeding for Pteranodon per se, but simply that Pteranodon anatomy was robust enough to dive into water from a flighted position). Mike's Flugsaurier 2015 talk suggested that this observation is borne out in some basic assessments of skeletal strength. Diving actions would not exceed safety factors of the Pteranodon skeleton, and its streamlined head and air sacs anterior to the torso would aid force dissipation as the animal penetrated the water surface. I must admit to finding the concept of diving Pteranodon quite appealing. Pteranodon skulls are especially streamlined and pointy compared to many other marine pterosaurs (not the least because they lack teeth and anterior crests), and we know that at least some individuals predated relatively tiny fish (Bennett 2001) which may have been difficult to snag during flight. Thus, some sort of shallow diving to get Pteranodon into water where it can pursue prey makes sense to me (as depicted above, see Witton 2013 for more discussion of this concept).
|Pteranodon sp. jaw specimen AMNH 5098. That mass of random crap between the manidibular rami is a heap of small, half-digested fish vertebrae. Scale bar represents 100 mm.|
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- Bennett, S. C. (2001). The osteology and functional morphology of the Late Cretaceous pterosaur Pteranodon Part I. General description of osteology. Palaeontographica Abteilung A, 1-112.
- Bramwell, C. D., & Whitfield, G. R. (1974). Biomechanics of Pteranodon. Philosophical Transactions of the Royal Society B: Biological Sciences, 267(890), 503-581.
- Habib, M. (2013). Constraining the air giants: limits on size in flying animals as an example of constraint-based biomechanical theories of form. Biological Theory, 8(3), 245-252.
- Habib, M. 2015. Size limits of marine pterosaurs and energetic considerations of plunge versus pluck feeding. Flugsaurier 2015 Portsmouth, Abstract Volume, 24-25.
- Habib, M. B., & Cunningham, J. (2010). Capacity for water launch in Anhanguera and Quetzalcoatlus. Acta Geoscientica Sinica, 31, 24-25.
- Henderson, D. M. (2004). Tipsy punters: sauropod dinosaur pneumaticity, buoyancy and aquatic habits. Proceedings of the Royal Society of London B: Biological Sciences, 271(Suppl 4), S180-S183.
- Henderson, D. M., & Naish, D. (2010). Predicting the buoyancy, equilibrium and potential swimming ability of giraffes by computational analysis. Journal of theoretical biology, 265(2), 151-159.
- Hone, D. W., & Henderson, D. M. (2014). The posture of floating pterosaurs: Ecological implications for inhabiting marine and freshwater habitats. Palaeogeography, Palaeoclimatology, Palaeoecology, 394, 89-98.
- Lockley, M. G., & Wright, J. L. (2003). Pterosaur swim tracks and other ichnological evidence of behaviour and ecology. Geological Society, London, Special Publications, 217(1), 297-313.
- Witton, M. P. (2013). Pterosaurs: Natural History, Evolution, Anatomy. Princeton University Press.