Earlier we looked at terrestrial locomotion in pterosaurs, discriminating a basal bipedal taxon from the quadrupedal track makers that can be matched to tracks attributed to ctenochasmatid, pterodactyloid (check out the animation!) and maybe even ornithocheirid pterosaurs (Peters 2000, 2010, 2011). We also looked at the many potential problems that surround the wing launch hypothesis and presented an alternative or two.
Pteranodon and especially Nyctosaurus (Fig. 1) were two special cases united by extremely long metacarpals coupled with relatively short hind limbs that prevented them from walking in the same manner as pterosaurs having shorter metacarpals.
Bennett’s Take on Nyctosaurus
Bennett (1997) provided a great illustration of Nyctosaurus “essentially bipedal” (Fig. 1) because the forelimbs could only touch the substrate on the “back” of the folded wing finger, so far in front of the jaw tips that they were unable to provide a thrust vector to the elbow and shoulder. The fingers were greatly reduced, perhaps because they were no longer in use. See Muzquizopteryx and Nyctosaurus bonneri for extreme proportions within this clade. Even shorter metacarpals on pterosaurs don’t appear to contribute thrust, only support, especially when nosing around for food items buried in the substrate or swimming around their submerged ankles in the shallows.
The Triebold specimen of Pteranodon NMC41-358 is the most complete one known (Fig. 2). Others had larger wings and shorter legs. In the Triebold specimen it appears difficult for the free fingers (especially fingers 1 and 2) to contact the substrate as in other pterosaurs due to the great length of the metacarpus relative to the hind legs.
If the above animation was configured too erect, then imagine it with lower shoulders (Fig. 3). That moves the free fingers even further forward, further unable to contact the substrate (despite the cheating on finger placement here by ignoring the configuration of the metacarpals). In any configuration the forelimbs were more like adult crutches on a little kid, my friends: very awkward on land. And, obviously, secondarily evolved, interrupted by a bipedal phase in pre-pterosaurs and basal pterosaurs.
Send alternatives if you have them!
Bennett SC 1997. Terrestrial locomotion of pterosaurs: a reconstruction based on Pteraichnus trackways. Journal of Vertebrate Paleontology, 17: 104–113. online pdf
Peters D 2000a. Description and Interpretation of Interphalangeal Lines in Tetrapods. Ichnos 7:11-41.
Peters D. 2010. In defence of parallel interphalangeal lines. Historical Biology 22:437-442.
Peters David 2011. A Catalog of Pterosaur Pedes for Trackmaker Identification. Ichnos, 18: 2, 114 —141