One of the classic vignettes from “Deep Time” is that of a pterosaur gliding over the surface of a lake, river, or ocean, perhaps even swooping down to skim some fish from just underneath the surface. While I am not well-versed enough in the history of paleontological art to know when this trend started or why, this scene has a lot in common with the feeding habits of modern shorebirds called Skimmers (the Black Skimmer, Rynchops niger, being one of the species found along the New Jersey shore, although so far I have not seen one myself). The trouble is, however, that this feeding strategy probably didn’t actually work for many pterosaurs and their kin.
The case for skimming pterosaurs gained a fair amount of support and attention in February of 2002, with the discovery of Thalassodromeus sethi seeming to provide some evidence in terms of morphological convergence to suggest that it caught its food the same way that modern Black Skimmers do. Thalassodromeus is much larger than Black Skimmer’s however, and it’s unlikely that it would be able to dip any more than the distal third of its jaws into the water without experiencing so much drag that it may not have been able to keep flying (or, I would imagine it would topple end over end if it were going fast enough). This limitation is clearly makes the pterosaur different from the Black Skimmer in that the birds can stick their entire lower bill into the water, being specially adapted to have narrow enough jaws that they are not slowed down or stopped by drag.
Enter a new study by Humphries, et al entitled “Did Pterosaurs Feed by Skimming? Physical Modelling and Anatomical Evaluation of an Unusual Feeding Method” (available for FREE via PLoS Biology). Using models, the researchers were able to determine drag forces on the jaws of a subspecies of Black Skimmer and the pterosaurs Tupuxuara (the outgroup) and Thalassodromeus sethi, the results showing that the pterosaurs seemed to be ill-adapted and the energetic costs of skimming for food is actually much higher than previously thought. Turned to morphology, regarding the head and neck as a whole, the researchers also found little convergence between modern Skimmers and the pterosaurs outside of a superficial resemblance. They write;
Almost without exception, pterosaur anatomy appears poorly adapted for skim-feeding. Comparisons between pterosaurs and Rynchops reveal little of the convergence expected between animals postulated to have such similar, specialist lifestyles, with the relatively wide mandibular symphyses, apparent absences of elongate, abradable mandibular sheaths (at least observable in Rhamphorhynchus), and lack of a suitably reinforced jaw joint particularly pertinent arguments against pterosaur skimming.
Does this mean that no pterosaur ever skimmed for food? Of course not; the authors note that small pterosaurs (2 kg or less), maybe even Rhamphorhynchus, could have skimmed for food, but the large size and energy costs of the larger pterosaurs (not to mention lack of adaptation) probably precluded them from using skimming as a way to acquire food. Likewise, this study does not shut the door to pterosaurs feeding on fish either; they could have acquired fish some other way, and there are plenty of ways to do it. During my recent trip to Cape May, I noticed several strategies being used other than the skimming method mentioned here. Terns seem to fly about 8-15 feet above the water, looking for fish. When they spotted one, they drew their bodies upright (perpendicular to the water’s surface), hovered while flapping their wings for a moment, and dove down to catch the fish. An osprey took a somewhat different approach, flying higher and then folding in its wings in a sort of slow-motion freefall to catch the fish with its beak. Other “fish eagles” have been known to employ similar techniques, but using their talons instead. Seagulls, on the other hand, seemed to primarily sit on the surface and go after whatever came up, although there are birds that dive down to catch fish. While unlikely analogies to ancient pterosaurs because of the differences in body shape, herons and egrets wading in a salt marsh and stabbed into the water after fish. There are plenty of ways to catch fish at sea or in clamer marshes and lagoons, and its probable that some pterosaurs may have developed a method of acquiring fish that we do not have a modern analogy for. For the moment, however, we can at least be fairly confident that any but the smallest of pterosaurs probably did not skim for food, especially not on a regular basis, and we will have to look elsewhere if we’re to determine their habits.