 A hypothesis, credited to Gregory Paul and propounded in his books "Predatory Dinosaurs of the World" (1988) and "Dinosaurs of the Air" (2002), suggests that some groups of non-flying carnivorous dinosaurs - especially deinonychosaurs, but perhaps others such as oviraptorosaurs, therizinosaurs, alvarezsaurids and ornithomimosaurs - actually descend from birds.Paul also proposed that the bird ancestor of these groups was more advanced in its flight adaptations than "Archaeopteryx".The hypothesis would mean that "Archaeopteryx" is less closely related to extant birds than these dinosaurs are.