The Blue Mesa locality of the Petrified Forest National Park in Arizona contains the Late Triassic continental sedimentary rocks of the Chinle Formation. Near Blue Mesa, the oldest documented dinosaur remains in the Chinle Formation have been found.
The Jurassic and Cretaceous periods were the golden age of dinosaurs, during which the prehistoric giants roamed the Earth for nearly 135 million years. Paleontologists have unearthed numerous fossils from these periods, suggesting that dinosaurs were abundant throughout the world. But where and when dinosaurs first came into existence has been difficult to ascertain. Fossils discovered in Argentina suggest that the first dinosaurs may have appeared in South America during the Late Triassic, about 230 million years ago - a period when today's continents were fused in a single landmass called Pangaea. Previously discovered fossils in North America have prompted speculation that dinosaurs didn't appear there until about 212 million years ago - significantly later than in South America. Scientists have devised multiple theories to explain dinosaurs' delayed appearance in North America, citing environmental factors or a vast desert barrier. But scientists at MIT now have a bone to pick with such theories: They precisely dated the rocks in which the earliest dinosaur fossils were discovered in the southwestern United States, and found that dinosaurs appeared there as early as 223 million years ago.
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