Rise of the dinosaurs

Cambridge, Massachusetts - 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. What’s more, they demonstrated that these earliest dinosaurs coexisted with close nondinosaur relatives, as well as significantly more evolved dinosaurs, for more than 12 million years. To add to the mystery, they identified a 16-million-year gap, older than the dinosaur-bearing rocks, where there is either no trace of any vertebrates, including dinosaurs, in the rock record, or the corresponding rocks have eroded.

“Right below that horizon where we find the earliest dinosaurs, there is a long gap in the fossil and rock records across the sedimentary basin,” says Jahan Ramezani, a research scientist in MIT’s Department of Earth, Atmospheric and Planetary Sciences. “If the record is not there, it doesn’t mean the dinosaurs didn’t exist. It means that either no fossils were preserved, or we haven’t found them. That tells us the theory that dinosaurs simply started in South America and spread all over the world has no firm basis.”

Ramezani details the results of his geochronological analysis in the American Journal of Science. The study’s co-authors are Sam Bowring, the Robert R. Shrock Professor of Geology at MIT, and David Fastovsky, professor of geosciences at the University of Rhode Island.

The isotope chronometer

The most complete record of early dinosaur evolution can be found in Argentina, where layers of sedimentary rock preserve a distinct evolutionary progression: During the Late Triassic period, preceding the Jurassic, dinosaur “precursors” first appeared, followed by animals that began to exhibit dinosaur-like characteristics, and then advanced, or fully evolved, dinosaurs. Each animal group is found in a distinct rock formation, with very little overlap, revealing a general evolutionary history.

In comparison, the dinosaur record in North America is a bit muddier. The most abundant fossils from the Late Triassic period have been discovered in layers of rock called the Chinle Formation, which occupies portions of Arizona, New Mexico, Utah, and Colorado, and is best exposed in Petrified Forest National Park. Scientists had previously dated isolated beds of this formation, and determined the earliest dinosaur-like animals, discovered in New Mexico, appeared by 212 million years ago.

Ramezani and Bowring sought to more precisely date the entire formation, including levels in which the earliest dinosaur fossils have been found. The team took samples from exposed layers of sedimentary rock that were derived, in large part, from volcanic debris in various sections of the Chinle Formation. In the lab, the researchers pulverized the rocks and isolated individual microscopic grains of zircon — a uranium-bearing mineral that forms in magma shortly prior to volcanic eruptions. From the moment zircon crystallizes, the decay of uranium to lead begins in the mineral and, as Ramezani explains it, “the chronometer starts.” Researchers can measure the ratio of uranium to lead isotopes to determine the age of the zircon, and, inferentially, the rock in which it was found.

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