Dinosaurs May Have Reached North America Earlier Than Previously Believed
redOrbit Staff & Wire Reports – Your Universe Online
A team of researchers led by Jahan Ramezani of MIT’s Department of Earth, Atmospheric and Planetary Sciences has discovered evidence that dinosaurs lived in North America millions of years earlier than previously suggested.
Writing in the latest edition of the American Journal of Science, Ramezani and his colleagues state that precise dating of rocks which were found in the southern US and contained dinosaur fossils, suggest the giant lizards appeared in this region as early as 223 million years ago.
This discovery is in stark contrast to popular theories which state that while fossil evidence suggests the first dinosaurs appeared in the South American portion of what was then Pangaea about 230 million years ago, they did not reach the North American region of the massive landmass until significantly later (approximately 212 million years ago).
The research also suggests that dinosaurs living in North America at this time coexisted with both close non-dinosaur relatives and significantly more evolved dinosaurs for more than 12 million years. Furthermore, Ramezani’s team discovered a 16 million year gap that predates the fossil-bearing rocks during which there is no trace of any vertebrates, suggesting the rocks that would have contained those remains might 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,” Ramezani explained in a statement Tuesday. “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, MIT geology processor Sam Bowring and University of Rhode Island geosciences professor David Fastovsky set out to complete a geochronological analysis of fossils discovered in layers of rock known as the Chinle Formation. Previous dating conducted at the Chinle Formation, which occupies portions of Arizona, New Mexico, Utah, and Colorado, determined the earliest dinosaur-like animals appeared around 212 million years ago.
However, compared to the more complete record of early dinosaur evolution present in Argentina, the North American dinosaur record is far less clear. In an attempt to bring some clarity to the situation, Ramezani and Bowring set out to more precisely date the Chinle Formation (including the portion from which previous dinosaur fossils were obtained) by taking samples from exposed sedimentary rock layers that were largely derived from volcanic debris.
They pulverized the rocks in the laboratory, isolating individual microscopic grains on the uranium-bearing mineral zircon. Zircon forms in magma shortly before volcanic eruptions, they explained, and from the moment it crystallizes, it begins the process of decaying uranium to lead. By measuring the ratio of uranium to lead isotopes, the investigative team was able to determine the age of the zircon and the rock in which it was discovered.
After analyzing individual zircon grains, Ramezani and his colleagues were able to create a precise age map for each sedimentary interval of the Chinle Formation. The National Science Foundation-funded study discovered that the fossils previously discovered in New Mexico do not actually represent the earliest dinosaurs in North America – fossils discovered in Arizona predate them by approximately 11 million years.
But even that doesn’t tell the whole story, Ramezani explained: “The fact that our record starts with advanced forms tells us there was a prior history. It’s not just that advanced dinosaurs suddenly appeared 223 million years ago. There must have been prior evolution in North America – we just haven’t identified any earlier dinosaurs yet.”
The answer to the question as to when dinosaurs actually first appeared in North America could rest in the 16 million year gap where the Chinle Formation contains no fossil evidence of dinosaurs – or any other creature, for that matter. The study authors suggest that dinosaurs may have reached the region during this period of time, and that any fossil evidence they left behind might have been erased.
“This is the kind of careful work that needs to be done before evolutionary hypotheses that relate to the origination and diversification of the dinosaurs can be addressed,” said Raymond Rogers, a professor of geology at Macalester College in Minnesota who was not involved in the study.
“This gap in the Chinle fossil record makes comparing the North American and South American dinosaur records problematic,” he added. “Existing hypotheses that relate to the timing of dinosaur evolution in North and South America arguably need to be reconsidered in light of this new study.”
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