“In other basins, dinosaur precursors, early dinosaurs and faunas dominated by dinosaurs do not all conveniently exist in the same place. In the basin containing the Chañares Formation, you can follow hundreds of meters of sediments back through time,” Marsicano said in a statement. “Because of this, the margin of error is very narrow because you can see the complete history all in one basin.”
To get the most accurate dating for the rocks holding these fossils, Marsicano and her team took samples from the basin and crushed them to extract zircon crystals left behind by volcanic eruptions. They then measured the precise ratio of uranium to lead in the crystals. Because the uranium in these crystals decays into lead over time at a known rate, the researchers could date them exactly based on the ratio of the two elements.
And since the Chañares has been shown to be 5 to 10 million years younger than previously thought, pre-dinosaurs have as well. That gives dinosaurs a lot less time to show up, but they managed it anyway.