Red beds are common in Upper Triassic terrestrial sedimentary successions worldwide. Having formed in semi-arid to arid environments, these rocks are generally sparse in fossils. This is perhaps mainly because the preservation potential for fossils in such environments is poor, and not necessarily because there were no plants and animals at the time of deposition.
On the Colorado Plateau in southwestern U.S.A. the Upper Triassic red bed succession of the Chinle Formation, deposited by river systems and small lakes under an arid climatic regime, contains well preserved fossil plants and vertebrate remains; archeosaurs, dinosaurs as well as the precursors of dinosaurs, the dinosauromorphs (see e.g. Irmis et al., 2007; Nesbitt et al., 2009). However, relative datings of these fossil floras and faunas have been hampered by the fact that they occur within an otherwise relatively fossil-poor terrestrial succession. They have traditionally been assigned a Carnian to early Norian age, based on fossil spores and pollen (Litwin et al., 1991).
But now Randall Irmis and his colleagues (read the abstract here) present two new radioisotopic datings on detrital zircons that indicates that the Chinle Formation is younger than previously thought, namely between ~218 and ~212 million years old (maximum ages), i.e. mostly or wholly Norian in age.
This means that the early dinosaurs of North America are younger than their famous South American counterparts from the Ischigualasto Formation in northwestern Argentina.
This suggests that dinosaurs evolved diachronously on either side of the Equator…