The life aquatic with flying reptiles

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

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

Proof that pterosaur workers of old thought swimming was possible: Bramwell and Whitfield's (1974) landmark paper on Pteranodon flight depicts one attempting to take off from water.

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

Select examples of pterosaur swim tracks from the Jurassic Summerville (A) and Sundance (B) formations, North America. There are many more examples of tracks like this - some bedding planes are covered in them. Traced illustrations from Lockley and Wright (2003), published in Witton (2013).

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

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

So, sorry, 2013 Ornithocheirus-as-a-bird-like-floater-image-that-I-have-a-little-soft-spot-for, you're out of date.

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