Charles Knight painting image courtesy of the Field Museum of Natural History
It has been a tradition to illustrate certain pterosaurs, like Pteranodon and Thalassodromeus (Kellner and Campos 2002) flying low over the waters, skimming for subsurface prey or floating carrion. This behavior would have been similar to living black skimmers (Rynchops niger), feeding while on the wing, occasionally dipping a mandible into the water to snare small fish. At first glance, some pterosaurs seem to have been perfectly suited for skimming. They sported extremely sharp jaw tips, either with a set of procumbent teeth, as in Rhamphorhynchus, or largely without, as in Nyctosaurus (Figure 1). Some pterosaurs were preserved with fish bones beneath their ribcages.
Figure 1. Click to enlarge. Nyctosaurus KUVP 66130 showing in two views the extremely sharp mandible tipped by a single tooth. This is the only known genus of pterosaur with a mandible that extended further than the rostrum.
The Anti-Skimming Hypothesis
A few years ago, Humphries et al. (2007) reported: “Our results refute the hypothesis that some pterosaurs commonly used skimming as a foraging method and illustrate the pitfalls involved in extrapolating from limited morphological convergence.” But they also conceded, “It also leaves the theoretical possibility that smaller (2-m wingspan) pterosaurs such as Rhamphorhynchus may have been able to skim-feed occasionally without particularly specialised jaw morphology due to their lower flight costs.” Good. Apparently someone reminded them there FISH inside certain pterosaurs, including Rhamphorhynchus.
To determine the amount of drag a dipped mandible would experience, Humphries et al. (2007) modeled several mandible shapes based on the black skimmer and several pterosaurs, including a jaw fragment they assigned to Thalassodromeus sethi (DGM 1476-M). Unfortunately this specimen had a robust upturned tip with a tear-drop shaped cross-section and so probably belonged to a dsungaripterid. Even so, that misidentification may not have been a deal killer. As a point of reference, a sister to Thalassodromeus, Tupuxuara, had a boat-hull-shaped mandible cross-section, as Humphries et al. (2007) illustrated in their figure 1. More on this shape below.
Let’s Look at the Anti-Skimming Methods and Assumptions
Humphries et al. (2007) employed a number of velocities (1.8 to 6.8 m/s or 4 to 15 mph) on the skimmer bill model and found that range produced a 14-fold increase in drag at the higher speeds. The larger pterosaur mandible model experienced a magnitude more drag at all speeds. Humphries et al. (2007) reported the typical speed of a black skimmer was 10 m/s (22 mph), or a third faster than their highest tested speed. Hmm. Wonder why they didn’t test the skimmer model at its observed speed?
After reporting, “The added costs of flight due to hydrodynamic drag range from 20% of total costs in Rynchops [the black skimmer], up to 68% in the low mass estimate for Pteranodon,” they wrote, “It is clear that in all species, hydrodynamic drag constitutes a major component of total flight costs.” Here, unfortunately, Humphries et al. (2007) assumed that a high-speed skimmer-like penetration of the surface was the only means by which a pterosaur might skim. But is this so?
Alternative Methods for Skimming
Black skimmers prefer windless conditions over still bodies of water. Humphries et al. (2007) apparently did not take into account that 1) a head wind produced by an oceanic breeze could have produced a much lower water (ground) speed and 2) the pterosaur skimming technique was likely different (see below). Pterosaurs may have employed a little or a lot of headwind (Figure 4) often present near larger bodies of water. This could have greatly reduced their hydrodynamic speed to a hover, obviating the need for a skimmer-like cut through the water with its attendant drag problems.
Humphries et al. (2007) reported that skimmers have some 30 adaptations for skimming that pterosaur lacked, including an ever growing mandible tip. This alone indicates that if pterosaurs skimmed, they would have done so using a less damaging technique.
Figure 2. Heron damage from spearing a fish. Perhaps this was the preferred technique for the sharp billed germanodactylids, including the nyctosaurs and pterandontids.
Not All Pterosaurs Would Have Fed by Skimming
Ctenochasmatids had too many fragile teeth at odd angles, hence they would have acted more like modern spoonbills, dipping their beak occasionally while slowly wading in shallows. Azhdarchids had bills like yardsticks. They would have acted more like storks and probably fed the same way, not on the wing. Pterodactylus was also a hunt-and-peck beachcomber. Germanodactylus had a sharp rostrum that was capable of stabbing prey like a heron does on occasion (Figure 2).
The Best Candidates for Skimming Pterosaurs appear to be Nyctosaurus (Figure 1) and Pteranodon (Figure 3). Like the black skimmer, Nyctosaurus had a mandible longer than the rostrum (Figure 1). The jaws were needle-sharp tipped by a single tooth (so nyctosaurs were not totally toothless). According to cladistic analysis, Nyctosaurus was derived from certain Germanodactylus and so may have retained a stabbing ability and behavior. Pteranodon was similar to Nyctosaurus in having a sharp rostrum and mandible, but in all cases the mandible was shorter.
Figure 3. The shorter, but just as sharp mandible of the Triebold specimen of Pteranodon.
Black skimmers don’t have a sharp mandible. It was tipped with a keratinous sheath that occasionally suffered damage. While skimming placid waters with their open jaws partly submerged, skimmers bang into prey with the dorsal rim of their mandible (not the tip). The force pulls their head down and the rostrum snaps on the captured fish.
By contrast, Nyctosaurus and Pteranodon would have speared their prey, probably at a much lower speed and a much lower angle relative to the water. Perhaps that is why the cross section of the deep mandibles of Pteranodon and Tupuxuara look like boat hulls. Nyctosaurus (Figure 1) did not have such a deep mandible. In these three pterosaurs a quick flick of the mandible would have dislodged the fish and flipped it into the throat. So is this still skimming? Or is it dipping and stabbing?
Figure 4. Click to Play. Hovering albatross video
A video of a hovering albatross (Figure 4) demonstrates the ability of ocean breezes to provide sufficient airspeed over the wings to permit flight at very low water speeds, down to zero.
The Black Skimmer (Rynchops niger) is Not a Good Analog
The skimming speeds attained by black skimmers over placid waters were not necessary in skimming pterosaurs over breezy oceanic waters, hence drag would not have been the limiting factor, as Humphries et al. (2007) proposed. Moreover, the technique for snaring fish was dissimilar in skimmers and pterosaurs due to beak shape. The extremely pointed mandibles of Nyctosaurus and Pteranodon were hydrodynamically streamlined, likely to reduce drag but also to increase prey penetration.
As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.
References:
Humphries S, Bonser RHC, Witton MP and Martill DM 2007. Did pterosaurs feed by skimming? Physical modelling and anatomical evaluation of an unusual feeding method. PLoS Biol 5(8): e204. doi:10.1371/journal.pbio.0050204 online
Kellner AWA and Campos DA 2002. The function of the cranial crest and jaw of a unique pterosaur from the Early Cretaceous of Brazil. Science 297: 389–392.
The Pterosaur Heresies 