The posture an animal may adopt when dunked into the water may not seem like an obvious lifestyle problem – after all pretty much all animals can swim to at least some degree – but it appears to have been a real issue for the pterosaurs. A new study published by myself and a colleague suggests that their floating posture was awkward and pterosaurs could not have spent long periods of time on the surface.

This is rather a surprise as many of these extinct flying reptiles (often incorrectly considered dinosaurs, or the ancestors of birds) were regular ocean-going animals that would forage around, over, and probably also in, the water. In addition to finding skeletons in rocks formed in lakes and seas, specimens have been found tens of miles from the coastline of the time, and a number of pterosaur fossils have even been recovered with fish preserved inside them. They certainly were spending plenty of time at the seaside (or further out) so to be so limited when in the drink might seem a paradox.

While pterosaurs could fly well, and some studies suggest they could swim, and even take off from the surface of the water, very little thought has been applied to how they might sit on the surface. Some palaeontologists have suggested pterosaurs floated like ducks or gulls, with the head high up and clear of the water and the wings tucked alongside, but this typically came with little consideration of how this might work. Pterosaurs, especially the later forms, have rather large heads, rather stiff necks that won’t apparently fold into a bird-like S-shape, and proportionally smaller bodies. In short, their anatomy is so different to that of a bird, it’s far from obvious if they could be twisted into a bird-like posture, let alone maintain it when on the water. This was the conundrum I had been thinking about when I teamed up with biomechanics expert Don Henderson of the Royal Tyrrell Museum of Palaeontology in Alberta to tackle the problem and that led to this new paper.

The issue may sound trivial, but it has important implications for our understanding of pterosaurs. If they float awkwardly, this would limit their ability to rest on water after long flights or after diving, and suggest they would be forced to spend more time on the wing when over the open ocean (or large lakes). We also have tracks attributed to pterosaurs that are swimming or wading in shallow water, and so it would be useful to confirm if this matches the expected posture. But how exactly do you float a 100 million year old animal known mostly from fossil bones?

Fortunately, we do have a pretty good idea of what a pterosaur weighs and how that weight is distributed across the body. Research has determined the mass of the bones, the mass of muscles, the size of the lungs and their position in the animal and other details that would give a good idea of how they were ‘constructed’. Don in particular has been one of a number of researchers who has tackled this issue in some detail, and has also published a series of papers on the posture of animals when floating. This has included dinosaurs and marine reptiles, but also giraffes – it had been suggested that they sank when placed in water and it’s neither easy nor ethically responsible to find out by simply chucking a few of them into the sea and waiting to see if they surface.

Creating a digital model of the animal in question and dipping it into digital water, it is allowed to come to equilibrium where the centre of mass is aligned with the centre of buoyancy. This can be checked and verified with living animals – of course we know exactly how they float and so we can model them and compare the results of the digital models to reality. Don has in the past checked his methods with alligators, sea turtles, and even elephants, but here he added a goose to include something with a higher position on the water – something more like a pterosaur was thought to adopt.

Facebook Twitter Pinterest A selection of different possible wing and body positions for Pteranodon, but all leave the head at the surface. Photograph: /Donald Henderson

Sticking the pterosaurs into the water immediately suggested that the bird position is incorrect. The large head with the stiff neck, against a small body, tipped them forwards with the head almost on, or even partly below, the surface, and not held well clear. Larger pterosaurs, like the famous Pteranodon, were even worse with the snout below the surface to a point where the nostrils might even be submerged. To try and correct this, we then tried a series of alternate versions to see if it was possible to get the heads up and free of the surface. We looked at alternate positions of the wings, we threw the head back to lift it up, and even experimented with alternate densities (both higher and lower) to check the overall strength of the model. In short, nothing made much difference. Pterosaurs don’t float like birds, and nor can we really make them do it, no matter how much we manipulate the model.

Facebook Twitter Pinterest Three additional pterosaurs were modeled for the project, and despite some differences, all have a floating posture with the head on the surface of the water and the legs hanging down. Photograph: /Donald Henderson

There were some variations in the postures and positions between the four different pterosaur species we looked at, but they span quite a range of sizes and anatomical body plans, but these were close enough to one another that it is likely this was a problem for all pterosaurs. It seems this posture, or something close to it, was indeed the one they adopted when sat on the surface. In the positions we recovered, the feet were often low in the water and this matches some of the suggested swimming tracks for pterosaurs, where just the feet left marks on the bottom. However, it does land in stark contrast to the bird-like postures that have been suggested and mean that any pterosaur residing on the surface would have their snouts at or under the surface.

This wouldn’t affect their ability to swim or dive, and certainly didn’t limit their forays out above major bodies of water, but it is hard to see pterosaurs hanging around on the surface for long periods. Unlike seabirds or waterfowl, they would not appear to be in a position to easily ride out a storm on the surface, or simply take a rest to interrupt a long flight. Any pterosaur that ended up in the sea (by accident or design) would have been advised to take off again shortly afterwards or risk more than a mouthful of brine. That may sound like pterosaurs were rather doomed as ocean-going animals, but actually some seabirds are rather limited when on the surface and struggle to take off again (frigate birds and shearwaters for example), and so do their best to avoid landing in the first place. Plenty more can cover huge distances between landing, and feed safely on the wing, so pterosaurs may simply have avoided coming down onto the surface. In short, it’s not impossible to be a successful open-ocean, flying animal that avoids the water, so such an issue for pterosaurs would not be fatal, and they at least could apparently take off from the surface easily.

This does then give us a rather different picture of pterosaur ecology. Illustrators over the years have put whole flocks of pterosaurs resting calmly on the surface of the sea or starting from this position to bob below the surface and dive for fish. Such behaviours now seem improbable for pterosaurs, even those which may have some diving adaptations and I hope there will be some new treatments appearing in the future to take advantage of our ideas.

Hone, D.W.E. & Henderson, D.M. in press. The posture of floating pterosaurs: ecological implications for inhabiting marine and freshwater habitats. Palaeogeography, Palaeoclimatology, Palaeoecology, doi: 10.1016/j.palaeo.2013.11.022