What a strange animal, I think, is the Rhamphorhynchus muensteri.
The Solnhofen seems to refer to a lagoon, a sheltered cove of sorts, with a slow-moving, oxygen-deprived bottom that becomes excellent for preserving the skeletons of animals that die in its waters.
In those waters, in an archipelago that most of Europe was during the Late Jurassic, pterosaurs as we know them are so extraordinarily diverse as to defy our imagination. Many seem to have fed on fish. Rhamphorhynchus specimens are known to have remains of fish (or what seem like fish) in their guts, a clear indication as any that they, amongst all pterosaurs at Solnhofen, were piscivores.
This fuzzy animal seems quite suited to the task, with its short, triangular skull but long, anglerfish-like jaws. A real trap for fishy prey. We might imagine these pterosaurs as sleek hunters of prey, dashing about on the wing to sight a fish near the surface and then—SPLASH!–a dive to grab prey. Soaring up through the waves, prey in jaw, these pterosaurs’ sleek appearance and robust jaws would have helped them support this lifestyle.
Maybe. Unlike many piscivores, Rhamphorhynchus doesn’t have vertically interlocking teeth: they are pointed forward, slightly curved but not angled to the closing of the jaw. An animal like this trying to spear prey on its teeth might find its jaws caught in the fishy innards, and would have to pry the food off else it couldn’t close its mouth. Rather than a long, slender jaw, with a serrated margin that seems keen for catching wriggly, small prey, much like Pterodactylus antiquus (a soft-bodied specialist?) Rhamphorhynchus has the jaw that evokes the skimmer Rhynchops nigra, with a long-keel-like jaw that would help it “feel” prey below the water, to be quickly snatched up. But of course, skimmers don’t have teeth, and those teeth make for a strange bedfellow. Many other soaring, seafaring birds that catch fishy prey on the wing also don’t have teeth–or serrated jaw margins–but some do. Gulls and albatrosses are suited surface fishers, with narrow-gauge and high-aspect wings that help them soar with minimal energy output, and what beautiful soaring sights they make. Quite likely, the pseudodont birds, or pelagornithids, were very similar, yet those had such wicked serrated teeth we might easily imagine them as spearing food on their sharp “teeth.” But these birds also have hooked beaks, and in the gulls and albatrosses, and many pelacaniform birds like cormorants, the upper jaw features a very large hook, while the lower is shorter, down-curved. For these birds, the hooked bill is to help grab prey, quickly caught in the jaws and swallow, on the wing if need be.
I doubt Rhamphorhynchus had such help: It has no hooked upper jaw, and its teeth seem ill-suited for spearing. But they do seem useful for trapping prey, a comparison one might make for anglerfish, as alluded to. I sort of imagine a tidal forager, though of course this is an imperfect comparison. The long, toothless prow of the beak could be useful for pushing around through muck and water, to find juicy soft prey. The teeth, for grabbing jellyfish to be swallowed whole. It would help segregate the pterosaurs of Solnhofen, who each seem unique in some way, a helpful way to discriminate their diets when in competition with one another.
Piscivory is an odd type of feeding, and one of the things that I’ve noticed is that animals that specialize in fish tend to have different adaptations depending on their method of locomotion, their habitat, or even how they eat fishy prey. For the jaws, there aren’t really “one special set” of adaptations that say “Look, here’s a piscivore!” You make inferences based on similar animals, but those require close, in-group specializations. Broad, across-group features are mostly absent. Except for teeth. If the predator’s jaw anatomy is suited for catching fishy prey, lest it escape the teeth must impale the prey, and they work best when vertically oriented. Fish-eaters will also tend to have a bit of an overbite or upper jaw that curves downward: the front teeth of the jaw are angled into the mouth. This is seen in mostly aquatic predators, such as other fish, crocodilians that specialize on fish, etc. Rhamphorhynchus just seems to lack these features. Indeed, amongst all Solnhofen pterosaurs, only Scaphognathus seems to suggest it had this type of fishy-predator look, even though others have been treated as generalists or piscivore-generalists (like Pterodactylus). They can’t all have been swooping around the lagoon or out in the open ocean diving for or snatching fish from the surface.
But that’s a casual reading of the data on what it is to be a piscivore. Like I said, it depends on what kind of animal you are: birds have different adaptations from crocs, from lizards, from other fish. Turtles have no teeth, and those that have serrated margins of their jaws (many tortoises) tend to be vegetarians.
No matter. Pterosaurs are odd. And collections of them from primarily aquatic environments as odd. Deposits in the Yixian, Jiufotang, Solnhofen, Niobrara, Crato, and Romualdo formations suggest that our biases to seeing pterosaurs as piscivores is an entrenched one. Clearly, many were not. Azhdarchids were terrestrial stalkers, but some must have focused on more aerial (anurognathids) or terrestrial prey. It’s just a matter of being a lot more judicious and plenty analytical about the features we ascribe to these animals to assume their diet.