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Microscopic crystals of the uranium-bearing mineral zircon were separated from the rock samples and analyzed by the U-Pb isotopic technique at the MIT Isotope Lab. These zircons yielded a precise age of 221.82 ± 0.10 Ma for the upper Ischigualasto Formation. Credit: Courtesy of the researchers

The classic dinosaur family tree has two subdivisions of early dinosaurs at its base: the Ornithischians, or bird-hipped dinosaurs, which include the later Triceratops and Stegosaurus; and the Saurischians, or lizard-hipped dinosaurs, such as Brontosaurus and Tyrannosaurus.

In 2017, however, this classical view of dinosaur evolution was thrown into question with evidence that perhaps the lizard-hipped dinosaurs evolved first—a finding that dramatically rearranged the first major branches of the dinosaur family tree.

Now an MIT geochronologist, along with paleontologists from Argentina and Brazil, has found evidence to support the classical view of dinosaur evolution. The team's findings are published today in the journal Scientific Reports.