THE ISCHIGUALASTO TETRAPOD ASSEMBLAGE (LATE TRIASSIC, ARGENTINA) AND 40AR/39AR DATING OF DINOSAUR ORIGINS

40Ar/39Ar dating of sanidine from a bentonite interbedded in the Ischigualasto Formation of northwestern Argentina yielded a plateau age of 227.8 +/- 0.3 million years ago. This middle Carnian age is a direct calibration of the Ischigualasto tetrapod assemblage, which includes some of the best known early dinosaurs. This age shifts last appearances of Ischigualasto taxa back into the middle Carnian, diminishing the magnitude of the proposed late Carnian tetrapod extinction event. By 228 million years ago, the major dinosaurian lineages were established, and theropods were already important constituents of the carnivorous tetrapod guild in the Ischigualasto-Villa Union Basin. Dinosaurs as a whole remained minor components of tetrapod faunas for at least another 10 million years.