Did pterosaurs have lips?
The oral tissue configurations of dibnosaurs have been debated ad nauseaum with various levels of civility – see the countless posts about it in The Bite Stuff -, but much less work has been done in other ancient sauropsids, such as pterosaurs. Considering that modern sauropsids have such divergent oral tissues that they can’t be used as models for dinosaurs, just imagine the situation in animals with no clear close living relatives.
We do have some acknowledgement of pterosaurian rhamphothecae: toothless pterosaurs like pteranodontians, azhdarchids and thalassodromedids not only have bird-like jaw anatomy like deep blood vessel channels indicating horny coverings, but we actually specimens with preserved rhamphothecae, most notably in the exquisitely preserved tapejarids fossils. In the toothless pterosaurs, these sheets seem to extend over much of the jaws, and actually form a continuous sheet with the headcrests in at least thalassodromedids and tapejarids, and likely nearly all other examples as well. There’s no evidence that beaked pterosaurs had their rhamphothecae divided into different plates, the basal condition in birds.
We also have rhamphothecae from some toothed pterosaurs: Rhamphorhynchus and Pterodactylus preserved small, keratinous hooks in their jaw tips that essentially act as extra teeth, and Cycnorhamphus has strange rhamphothecae sheets on the sides of the upper jaw, filling the gap of the jaw curvatures – most notably, said sheets appear to be mineralised. Istiodactylids don’t have preserved rhamphothecae, but they do have a triangular hook in their lower jaws that could have supported a hook similar to that of rhamphorhynchines and Pterodactylus. Dsungaripteroids seem to have converted these hooks into genuine beaks, as more derived forms have toothless dystal jaws, having shoved the teeth backwards. However, we don’t know how rhamphothecae evolved in actual toothless pterosaurs, and unless we come across hooks in ornithocheirids, boreopterids and lonchodectids, chances are that toothloss and subsequent keratin replacement was rather rapid, which seems to coincide with the sudden appearence of these toothless pterosaurs in the fossil reccord.
Other than rhamphothecae, little has been achieved in the understanding of other soft tissues in pterosaur muzzles. We do know that anurognathids had fuzzy mouths, completly covered in pycnofibrils, and that other pterosaurs had naked jaws, but no studies seem to have been done in determining the nature of the skin in the jaws. We don’t know if pterosaurs had lips, or skin-tight facial tissues like crocodiles, or semi-cornified skin like the jaw tips of some maniraptors.
The general tendency at least seems to be depictions of crocodillian-like oral tissues, and in at least some cases this might be correct: ornithocheirids and rhamphorhynchines have tusk-like teeth that probably didn’t go well with lips, and it would explain how toothless pterosaurs got their beaks so quickly, simply by keratinising this skulltight suit. However, other pterosaurs imply much different arrangements: “campylognathoidids” have clear molariforme teeth, and shows signs of mastication, something that would probably require some form of lips. Likewise, the strange ridges in lonchodectid and ctenochasmatoid skulls might imply some rather complex fleshy structures.
As we have learned so much about the stranger anatomical adaptations of these incredible animals, it’s bitterly ironic that the simplest parts of their anatomy still elude us.