In some pterosaurs, basal forms primarily, the metatarsals were appressed to one another. In many other clades the metatarsals separated distally, spreading the bases of the toes apart from one another. Between the toes of all pterosaurs (in which soft tissues like this are preserved) there is webbing, like duck feet. Toe webbing in pterosaurs goes back at least as far as Sharovipteryx. Toe webbing most often appears in Pterodactylus (Figs. 1, 2) and Rhamphorhynchus.
Figure 1. Pterosaur toe webbing on the Vienna specimen of Pterodactylus from Wellnhofer (1991).
Figure 2. Click to enlarge. Toe webbing on the Frey and Tischlinger private specimen of Pterodactylus.
Was toe webbing present on all pterosaurs?
It’s hard to tell whether webbing was present on more pterosaurs without better preservation over a wider range of taxa. Anurognathus had webbing, but apparently PAL 3830 did not. At present, toe webbing is less common than wing membrane preservation, but some of that is due to the angle of preservation, as in Sordes, in which webbing cannot be ascertained due to the medial view exposure.
Figure 3. Dorsal view of n42 (BSPG 1911 I 31) in flight position. The dorsal side of the foot would have pointed laterally. The curve of the toes would have produced a vertical stabilizer with lateral lift if the toes were webbed. Such lateral lift would have extended the legs without so much of the effort to do so, counteracting the natural drag forces on the leading edge working to flex the knee during flight.
Useful for?
Swimming (paddling the feet) and flying (establishing an aerodynamic surface that lifts the foot dorsally (laterally in flight, Fig. 3) while the uropatagium provides lift to the entire hind limb.
References
Wellnhofer P 1991. The Illustrated Encyclopedia of Pterosaurs. London, Salamander Books, Limited: 1-192.
