Dr. Sterling Nesbitt (2011) recently published his chronological family tree of archosaurs and their ancestors (Fig. 1). It’s too small to read, so please click on it to enlarge it.
Taking the same chronological data, I rearranged and edited the taxa according to the results of the large reptile tree (Fig. 2).
The evolution of dinosaurs and their ancestors is an intriguing question made more puzzling by the early appearance of Lotosaurus and Asilisaurus, two highly derived forms appearing in the early Middle Triassic, much sooner than the rest of the known fossils.
The question of poposaur nesting is the big difference here. Nesbitt finds that Ticinosuchus is basal to poposaurids. The large reptile tree found that Ticinosuchus was basal to aetosaurs and Sacisaurus was basal to poposaurids, despite its late appearance in the fossil record. The addition of Daemonosaurus (Fig. 2) provides a short path toward the Ornithischia. Nesbitt (Fig. 1) has a much longer path to the Ornithischia, close to Pisanosaurus, but running to the Early Triassic.
Poposaurids have been described as extremely convergent with dinosaurs, but their croc-like ankle gives them away. The large reptile tree found the development of a croc-like ankle was convergent in crocs and poposaurids, both developing from ankles without a calcaneal tuber or much of one.
The clade of Eoraptor, Panphagia and Pampadromaeus turns out to be the clade from which all phytodinosaurs (sauropodomorphs, ornithischians and poposaurids) evolved.
As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.
Nesbitt SJ 2011. The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History 352: 292 pp.