ORIGIN AND EARLY EVOLUTION OF DINOSAURS: NEW ECOSYSTEMS, NOT THE SAME OLD "EXTINCTIONS"

Although often overshadowed by research on dinosaur extinction, the timing, tempo, and mode of dinosaur origins remains a contentious topic and an important part of the Late Triassic evolutionary story. The trend in recent years is for researchers to identify new taxa from exotic (non-North American) locales as the "oldest dinosaur." For example, recent claims of Ladinian dinosaurs from the "Isalo II Beds" of Madagascar are based on cladistic biostratigraphy but at odds with tectonic, palynostratigraphic, and vertebrate biochronologic data that indicate a maximum age of Otischalkian (early?-late Carnian), and more likely Adamanian, (latest Carnian) for the Malagasy dinosaurs.

Indeed, Upper Triassic dinosaur occurrences are readily organized into four temporally successive faunachrons. The oldest dinosaurs are therefore not from Argentina, Brazil, South Africa, or Madagascar, as commonly claimed, but instead include theropods, prosauropods and ornithischians from rocks of Otischalkian age in the USA, North Africa, India, and, possibly, southern Africa/Madagascar. These clades diversify rapidly, with several representatives each by Adamanian time, thus predating the putative Carnian-Norian tetrapod extinction event. Dinosaurs, including relatively derived groups (coelophysoids, plateosaurids) are locally abundant in a few Revueltian (mid-Norian) localities. Dinosaurs first dominate Triassic tetrapod assemblages during the Apachean (late Norian/Rhaetian), when they become the numerically most common elements of Late Triassic faunas in Argentina, Europe, and South Africa. These faunas are established well after the Carnian-Norian extinction event (which is probably an artifact of the compiled correlation effect) and before the putative Triassic-Jurassic event.

Triassic dinosaurs only dominate faunas that are either at moderately high paleolatitude, well inland, or both. A comparison of Triassic dinosaur chronology and key synapomorphies indicates that dinosaurian success can be tied to diverse locomotor specializations and the acquisition of herbivory in two lineages. They are thus a unique part of a diapsid-dominated radiation led by archosaurs at large body size and lepidosauromorphs (principally sphenodontians) at smaller body size during the Late Triassic.