In phylogenetic systematics they are variously defined as the common ancestor and all its descendants of Younginiforms and "crown diapsids" (the common ancestor of lizards, crocodilians and birds, and all their descendants) Callaway 1997, or all diapsids that are more closely related to Sauria than to Araeoscelidia (Laurin and Gauthier 2000).
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Early or basal Permian neodiaspids were lizard-like, but already include specialised swimming Claudiosaurus and gliding Coelurosauravidae forms, as well as more conventional lizard-like forms (Youngina etc.).