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SALT LAKE CITY — A new study claims that dinosaurs may have been dying out long before an asteroid struck Earth 65.5 million years ago.
The asteroid that struck Earth at the end of the Cretaceous period may have been only one factor in dinosaurs' extinction, the study found. At least some species of dinosaurs saw drastic decline in the 12 million years prior to the impact, including some large- bodied herbivores — such as the Triceratops — and North American dinosaurs.
Carnivores, mid-sized herbivores and some Asian dinosaurs did not see decline, the study found.
One reason for the decline may have been that Earth's terrain was changing and certain species of dinosaurs struggled to adapt. Dinosaurs had to deal with more than just an asteroid.
"We also know there was massive volcanism and major sea level changes at this time," Stephen Brusatte, an author of the study and a Columbia University graduate student, told Discovery News.
The study examined the morphological disparity within groups of dinosaurs — i.e., the variability of their body structures. Usually, the greater the variability, the healthier the species. This is because increased variability means the dinosaurs could have been evolving into other species, according to the American Museum of Natural History. Decreased variability may foreshadow extinction.
Brusatte and his team found that large-bodied herbivores had limited variability and were dying out for millions of years before the asteroid struck that is commonly thought to have been the cause of extinction of the animals.
"Few issues in the history of paleontology have fueled as much research and popular fascination as the extinction of non-avian dinosaurs," Brusatte said. "Did sudden volcanic eruptions or an asteroid impact strike down dinosaurs during their prime? We found that it was probably much more complex than that, and maybe not the sudden catastrophe that is often portrayed."
The team tested data from 150 different species. One complication came with data from the hadrosaur, a common herbivore that was then found in what is now Asia, Europe and North America. North American hadrosaurs were found to be in decline in the years preceding the asteroid's impact, while Asian hadrosaurs were actually increasing in vitality during that time.
Mark Norell, chair of the museum's paleontology division, warned that the data did not conclusively mean the dinosaurs were doomed to extinction.
"Even if the disparity of some dinosaur clades or regional faunas were in decline, this does not automatically mean that dinosaurs were doomed to extinction," Norell said. "Dinosaur diversity fluctuated throughout the Mesozoic, and small increases or decreases between two or three time intervals may not be noteworthy within the context of the entire 150-million-year history of the group."
