The large reptile tree recovers the following taxa at the base of the Archosauria leading toward the Dinosauria (Fig. 1). This is an update of prior posts on dinosaur origins with PVL 4597 moving from closer to Trialestes to between Gracilisuchus and Lewisuchus.
- PVL 4597 (the Tucuman specimen attributed to Gracilisuchus)
- Pseudhesperosuchus, Carnufex and Junggarsuchus
- and finally, the basal dinosaur, Herrerasaurus.
Here (Fig.1) crocs and dinos have a last common ancestor
close to Gracilisuchus, probably in the Middle Triassic. Both started small and bipedal. Crocs had a wider skull. Dinos had a narrower skull. The reduction of the calcaneal tuber occurred in parallel. The tuber redeveloped in extant crocs.
Prior to these taxa
are larger forms, including Decuriasuchus and the basal poposaur, Turfanosuchus. So once again, phylogenetic miniaturization is key to the origin of both crocs and dinos (together, the Archosauria).
Wikipedia reports, “Paleontologists think that Eoraptor (Fig. 2) resembles the common ancestor of all dinosaurs;[ if this is true, its traits suggest that the first dinosaurs were small, bipedal predators. The discovery of primitive, dinosaur-like ornithodirans such as Marasuchus and Lagerpeton in Argentinian Middle Triassic strata supports this view; analysis of recovered fossils suggests that these animals were indeed small, bipedal predators. Dinosaurs may have appeared as early as 243 million years ago, as evidenced by remains of the genus Nyasasaurus from that period, though known fossils of these animals are too fragmentary to tell if they are dinosaurs or very close dinosaurian relatives.”
Too bad they are so tentative at Wikipedia when the large reptile tree lays it out pretty clearly. The purported and popular clade, “Ornithodira,” is, of course, not supported by the large reptile tree.
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