Dinosaurs have been major players in ecosystems for over 200 million years, yet we’re still not entirely sure about their early history. Now, according to researchers analyzing radioactive isotopes in crystals from volcanic ash, dinosaurs appeared on Earth relatively quickly after their evolutionary predecessors, called dinosauromorphs. The findings are published in Proceedings of the National Academy of Sciences this week. 

Our understanding of dinosaur origins and their rise to prominence has been limited by poor age constraints in the fossil record. Contrasting hypotheses suggest dinosauromorphs appeared anywhere from just after the end-Permian mass extinction event about 252 million years ago to around 231 million years ago when the first known dinosaurs showed up. 

University of Utah’s Randall Irmis and colleagues determined the ages of the mineral zircon in key stratigraphic sequences spanning the boundary between dinosauromorphs and dinosaurs. By measuring radioactive isotopes in zircon crystals from volcanic ash, the team was able to determine the precise age of fossil communities from northwest Argentina’s Chañares Formation, a quintessential assemblage of dinosaur precursors. These deposits are part of the Triassic Ischigualasto-Villa Unión Basin, and previous work uncovered unambiguous dinosaur fossils in younger rock from the overlying Ischigualasto Formation, which dates back 231.4 million years. 

They discovered that the closest relatives of dinosaurs had diversified by the start of the Late Triassic between 236 million and 234 million years ago – as represented by early dinosaur precursors like Lewisuchus (pictured above). “It was a big surprise, because previously we thought these early dinosauromorphs showed up in the fossil record around 245 million years ago,” Irmis tells IFLScience. “But this new dating throws all that into question.” 

Combining the high-precision radioisotopic age constraints of the Chañares Formation with age data from rocks in the same basin, the team show that the first clear evidence of dinosaurs appears just five million years after dinosauromorphs. That means the time gap between assemblages containing only dinosaur precursors and those with the first dinosaurs is between five million and 10 million years shorter than previously thought – making the origin of dinosaurs a relatively rapid evolutionary event.

Despite this quick transition and their rapid emergence, dinosaurs rose to prominence in a relatively smooth and gradual fashion: There was little compositional difference between assemblages containing just dinosaur precursors and those with the earliest dinosaurs. “It’s probably because, ecologically speaking, the first dinosaurs weren't much different from their close dinosauromorph relatives,” Irmis explains to IFLScience. So they didn’t cause a fundamental shift in the balance of the ecosystem.

^{Badlands of the Chañares Formation in what’s now Talampaya National Park in Argentina. Randall Irmis}