Benton, Forth and Langer (2014) present a summary of current consensus regarding the origin of dinosaurs (Fig. 1).
From the abstract: Dinosaurs arose in the early Triassic in the aftermath of the greatest mass extinction ever and became hugely successful in the Mesozoic. Their initial diversification is a classic example of a large-scale macroevolutionary change. Diversifications at such deep-time scales can now be dissected, modelled and tested. New fossils suggest that dinosaurs originated early in the Middle Triassic, during the recovery of life from the devastating Permo-Triassic mass extinction. Improvements in stratigraphic dating and a new suite of morphometric and comparative evolutionary numerical methods now allow a forensic dissection of one of the greatest turnovers in the history of life. Such studies mark a move from the narrative to the analytical in macroevolutionary research, and they allow us to begin to answer the proposal of George Gaylord Simpson, to explore adaptive radiations using numerical methods.
Unfortunately this study includes pterosaurs and lagerpetids (Fig. 1), both unrelated to the origin of dinosaurs in the large reptile tree (Fig. 2). Pterosaurs nest with lepidosaurs. Lagerpeton nests with proterochampsids in the large reptile tree.
Putting specimens in phylogenetic order and chronological order has been done for decades. Following the lineage of basal taxa here, the origin of dinosaurs should follow this lineage: Thadeosaurus > Boreopricia > Youngina BPI 3859 > Younginoides UC 1528 > Proterosuchus > Fugusuchus > Garjainia > Euparkeria > Osmolskina > Vjushkovia > Arizonasaurus > Decuriasuchus> Turfanosuchus > SMNS 12591 > PVL 4597 > Trialestes > Herrerasaurus. Many of the above are definite splinters off the main line, but remain closer than any other known taxa to that undiscovered closer sister. There are also several rises and falls in overall body size in this list, so Cope’s Rule is not in effect here.
Protorosaurs are basal diapsids that give rise to younginids and archosauriforms. They are known from Late Permian (Protorosaurus) to Middle Triassic (Pamelaria) sediments. In the second wave of dinosaur ancestors, the erythrosuchids and rauisuchids, preside with basal taxa in the early and middle Triassic. In the third wave of dinosaur ancestors we find basal archosaurs, like Decuriasuchus, Lewisuchus and Turfanosuchus. The latter gave rise to basal crocs, basal dinos and poposaurs. In the large reptile tree no taxa appear between basal crocs and basal dinos, so the term “archosaur” is restricted to them alone.
Nyasasaurus, once touted at perhaps the most basal dinosaur/dinosauriform, is more likely a big poposaur derived from Turfanosuchus (Fig. 1), from what I can tell from the sparse remains and it’s age.
When you find so many archosauriforms in the Early Triassic, one wonders if the origin of this clade occurred in the Late Permian, then found refuge in one corner of the globe during the Permo-Triassic extinction event, then radiated thereafter, leaving fossils in fossiliferous areas where and when these taxa were once again widespread and discoverable.
Benton MJ, Forth J and Langer MC 2014. Models for the Rise of the Dinosaurs. Current Biology 24:R87-R95.