In phylogenetic taxonomy, the clade Pterosauria has usually been defined as node-based and anchored to several extensively studied taxa as well as those thought to be primitive. One 2003 study defined Pterosauria as "The most recent common ancestor of the Anurognathidae, "Preondactylus" and "Quetzalcoatlus" and all their descendants." However, these types of definition would inevitably leave any related species that are slightly more primitive out of the Pterosauria. To remedy this, a new definition was proposed that would anchor the name not to any particular species but to an anatomical feature, the presence of an enlarged fourth finger that supports a wing membrane. This "apomorophy-based" definition was adopted by the PhyloCode in 2020 as "he clade characterized by the apomorphy fourth manual digit hypertrophied to support a wing membrane, as inherited by "Pterodactylus" (originally "Ornithocephalus") "antiquus" (Sömmerring 1812)". A broader clade, Pterosauromorpha, has been defined as all ornithodirans more closely related to pterosaurs than to dinosaurs.