Hi Sebastian!
Welcome to this land before time Forum
Petrie's species is kind of disputable. His uncle Pterano looks very much like a Pteranodon. Petrie, his mother, and sibilings look more like Pterodactylus if it wasn't for that tiny crest on the back of their head.
I'm quite sure that there wasn't any species of Pterosaurs that was a pure herbivore. Most of them, as you pointed out, were fish or insect eaters, or scavengers.
There is also the theory that there were omnivore kinds who could feed on anything that stood still long enough. Perhaps LBT supposes the Great Valley Pterosaurs to be omnivores who deliberately chose to restrict themselves to plants.
LBT never goes into such questions and science is sometimes abandoned for the sake of the story.
