The basic structure of archosaurian phylogeny is understood to include two primary crown-group lineages—one leading to living crocodiles and including a broad diversity of Triassic animals (e.g., phytosaurs, rauisuchians, aetosaurs), and another leading to dinosaurs (living and extinct). These lineages were established by the middle Triassic. A few extinct groups remain controversial, such as the pterosaurs, and debate persists over the phylogenetic relationships among extant bird lineages, which have proved difficult to resolve, and divergence timing estimates within Aves and Crocodylia remain the source of contention. A few analyses support a close relationship between archosaurs and turtles, or even a nesting of turtles within Archosauria. All sources of information used to resolve these issues have weaknesses, and these problems all involve highly derived lineages when they first appear in the fossil record.