Oh yes, they float, Henderson (and Hone)
Nowadays, we know pterosaurs were a diverse group of animals. Even pterosaurs that do fulfill the “seabird niche” seemingly hunted their prey in a variety of feeding strategies rarely considered, such as boreopterid filter feeding and possible pteranodontid aquaflying. Within these aquatic pterosaurs, foraging while swimming appears to have been the favoured method of foraging: nearly all aquatic ornithocheiroids have adaptations thought to correlate to water based taking off, rhamphorhynchids appear to have lacked adaptations for dip feeding, and ctenochasmatoids have long, duck-like bodies and powerful hindlimbs thought to have evolved to float better and to propell themselves, respectively.
A recent study, however, casts some doubts about this: Hone & Henderson (2013) have provided equisite computer simulation models in order to predict the floating gaits of several different pterosaur species.
As you can see, the floating positions are at various levels from “reasonable” to “giraffes float better”. Rhamphorhynchus – an animal thought to be a pelagic forager akin to modern tropicbirds – is the best floater of the bunch, followed by Dimorphodon – a terrestrial carnivore -, while Dsungaripterus and Pteranodon – a terrestrial omnivore and marine piscivore, respectively – each have their heads being too heavy in porportion to their bodies in order to be effective floaters, in the former’s case with the inferred position of the nostrils dipped underwater. The first two animals could have swam relatively well in calm waters, but the latter two would seriously risk drowning if they indeed floated like that, and would have done best if they only swam for a few minutes.
Utilising these three models, David Hone and Donald M. Henderson concluded that pterosaurs, unlike birds, were not accomplished swimmers or floaters, and indeed aside from Rhamphorhynchus one could very easily arrive to this conclusion; no matter how many times the models were rearranged, the results were pretty much still the same.
For starters, the pterosaur sample is somewhat inapropriate. Neither Dimorphodon nor Dsungaripterus were aquatic animals, so it is perhaps unfair to examine the floating capacities of Pterosauria with animals that did not evolve to be effective swimmers, anymore than ostriches and eagles have. Given that the point of the exercise was to test the floating capacities of pterosaurs, period, I suppose it is reasonable to use terrestrial animals as samples, but ultimately this leaves us with only two animals by which we can examine how supposedly aquatic pterosaurs moved in the water, and when the known aquatic pterosaurs come in a relatively wide variety of body types, this isn’t very helpful.
Of these two aquatic pterosaurs, Rhamphorhynchus is modelled fairly accurately. Accordingly, as one can see, it could float reasonably well, the large torso submerged and the relatively small head rising above the surface, a floating position that while less elegant than that of waterbirds, it is nonetheless expected for a swimming quadruped like an otter or a crocodile, and actually not that different from the swimming gait of a penguin. If Rhamphorhynchus wasn’t most likely an aerial piscivore, it would be reasonable to see it as an active underwater hunter.
The other pterosaur, Pteranodon, fares much worse at floating, the head being so much heavier than the body that at best it could only use the mandible’s boat-like shape to keep it floating, and most likely it would just sink. This is quite unexpected, to say the least: Pteranodon lacked adaptations for aerial piscivory, but was quite adapted for water based launching and possibly aquaflying.
Unfortunately, the Pteranodon model was seemingly somewhat outdated. As you can see, the animal’s torso has a distinctive extreme ventral curvature, rapidly losing vertical width as one moves towards the caudal area, resulting in a vaguely tear drop like shape. This shape has been described in Morphology of the Late Cretaceous pterosaur Pterandon and systematics of the Pterodactyloidea (Bennett 1991), and has been the most accepted arrangement among researchers. However, Claessens 2009 proposes a different morphology to Pteranodon‘s torso: due to the short length of the animal’s scapulocoracoid and the long ribs, it would have been virtually impossible for the animal’s sternum to have the extreme dorsal curvature as seen in the floating model. Rather, the sternum was slightly tilted ventrally, resulting in a rather distinctive “pot belly”. A small refinement to the torso’s model was conducted by Conway in 2010, but the basic jist remains the same: Pteranodon‘s body wasn’t tear drop shaped, but considerably rotund, almost-heavy looking and almost certainly considerably heavier than suggested.
Whereas this actually mattered much in regards to the animal’s actual floating performance is not clear, largely because it has not been tested yet. However, should this be a trait exclusive to pteranodontids (something I am very doubtful of, considering that similar examinations of the scapulocoracoid and ribs haven’t been performed on other ornithocheiroids), it could imply an adaptation towards aquatic foraging, rendering the torso sufficiently heavy in order to prevent the head from sinking.
With this said, we can’t therefore judge aquatic pterosaur floating capacities, since one model suggests a decent performance and the other is inaccurate.
A good test subject for further experiments are ctenochasmatoid pterosaurs. Among all pterosaurs, these seem to have been the ones most adapted to forage in the water, having developed large feet and powerful hindlimbs, large torsos akin to those of ducks and geese, and have actual swim tracts in their name. A study performed on ctenochasmatoid models would be most definitive for the swimming and floating capacities of pterosaurs.
Hone, D. W. E. & Henderson, D. M. 2013. The posture of floating pterosaurs: ecological implications for inhabiting marine and freshwater habitats. Palaeogeography, Palaeoclimatology, Palaeoecology doi: 10.1016/j.palaeo.2013.11.022
Claessens, L.P.A.M., O’Connor, P.M., and Unwin, D.M., 2009. Respiratory Evolution Facilitated the Origin of Pterosaur Flight and Aerial Gigantism. PLoS ONE vol. 4 (2) pp. e4497
Wilton, Mark P. (2013). Pterosaurs: Natural History, Evolution, Anatomy. Princeton University Press.
John Conway 2010