Because pterosaur anatomy has been so heavily modified for flight, and immediate transitional fossil predecessors have not so far been described, the ancestry of pterosaurs is not fully understood. The oldest known pterosaurs were already fully adapted to a flying lifestyle. Since Seeley, it was recognised that pterosaurs were likely to have had their origin in the "archosaurs", what today would be called the Archosauromorpha. In the 1980s, early cladistic analyses found that they were Avemetatarsalians (archosaurs closer to dinosaurs than to crocodilians). As this would make them also rather close relatives of the dinosaurs, these results were seen by Kevin Padian as confirming his interpretation of pterosaurs as bipedal warm-blooded animals. Because these early analyses were based on a limited number of taxa and characters, their results were inherently uncertain. Several influential researchers who rejected Padian's conclusions offered alternative hypotheses. David Unwin proposed an ancestry among the basal Archosauromorpha, specifically long-necked forms ("protorosaurs") such as tanystropheids. A placement among basal archosauriforms like "Euparkeria" was also suggested. Some basal archosauromorphs seem at first glance to be good candidates for close pterosaur relatives due to their long-limbed anatomy; one example is "Sharovipteryx", a "protorosaur" with skin membranes on its hindlimbs likely used for gliding. A 1999 study by Michael Benton found that pterosaurs were avemetatarsalians closely related to "Scleromochlus," and named the group Ornithodira to encompass pterosaurs and dinosaurs"."