They could be walking either with their arms held off the ground or with their hands being placed on the ground (walking on all fours).
Would the footprint of the feet be the same in both cases?
Let's look at this diagram:
http://www.nature.com/nature/journal/v391/n6670/full/391886a0.html
It would seem possible that the plantigrade footprint is an artifact of all-fours walking.
It could well have been digitigrade when the arms were held off the ground.
This would explain the plantigrade footprints of a pterosaur found (based on an instance of all-fours walking). And explain the digitigrade footprints of birds which never walk on all fours.
An interesting summary:
ReplyDeletehttp://pterosaurheresies.wordpress.com/2011/08/09/pterosaurs-bipedal-quadrupedal-or-both/
Pterosaurs were both plantigrade and digitigrade. Pterosaurs were both bipedal and quadrupedal. Pedal digit 5 was useful for basal pterosaurs, but not for derived flatfoots. All of these traits are like those of living lizards, the ones capable of standing, walking and running bipedally. At such times, these lizards turn from plantigrady to digitigrady without overextending the metatarsophalangeal joints, without having symmetrical pedes and without having all of the various morphological advantages that pterosaurs enjoyed, such as an anteriorly elongated ilium, an expanded sacral series for balance and prepubes to help elevate their femora. Pterosaurs likely took off bipedally, NOT with their forelimbs as described here. They certainly had to land bipedally.
ReplyDeletehttp://www.newscientist.com/article/dn17633-pterosaur-tracks-show-it-touched-down-like-a-bird.html
Pterosaur tracks show it touched down like a bird
Pterosaurs may have been furry rather than feathery, but they may not have been so very different from birds in other respects. A set of footprints unearthed in France is the first to show one of the winged reptiles coming into land – and suggests they did so in much the same way as most modern birds.