A related problem is the origin of pterosaur flight. Like with birds, hypotheses can be ordered into two main varieties: "ground up" or "tree down". Climbing a tree would cause height and gravity provide both the energy and a strong selection pressure for incipient flight. Rupert Wild in 1983 proposed a hypothetical "propterosaurus": a lizard-like arboreal animal developing a membrane between its limbs, first to safely parachute and then, gradually elongating the fourth finger, to glide. However, subsequent cladistic results did not fit this model well. Neither protorosaurs nor ornithodirans are biologically equivalent to lizards. Furthermore, the transition between gliding and flapping flight is not well-understood. More recent studies on basal pterosaur hindlimb morphology seem to vindicate a connection to "Scleromochlus". Like this archosaur, basal pterosaur lineages have plantigrade hindlimbs that show adaptations for saltation.