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Some dinosaurs were declining before asteroid struck, say scientists (+video)

By the time that giant meteor collided with our planet at the end of the Cretaceous, some dinosaur species were already heading toward extinction, new research indicates.

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"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."

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Location matters

A number of factors in North America might have influenced the evolution of dinosaurs there as compared with other continents, including mountain formation and extreme fluctuations in size and sea level of the Western Interior Seaway, a vast inland sea that divided what is now North America in half.

"The mountain-building and changes in the sea would have meant the land area in North America was constantly growing and shrinking, and so it would make sense that animals living on that land would change in an evolutionary sense as well," Brusatte told LiveScience. "It also makes sense that you would see declines in large plant-eaters such as hadrosaurs and ceratopsids first. They were distant relatives, but ecologically they were both doing similar things — they were essentially at the bottom of the food chain, the major dinosaur in terms of the landscape, much more common than other dinosaurs, so it would make sense they would be affected first by any change in the environment."

The researchers note that just because some dinosaur groups might have been in decline before their end "does not automatically mean that dinosaurs were doomed to extinction," said researcher Mark Norell, chair of paleontology at the American Museum of Natural History. "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." 

Future research will focus on finding more dinosaurs of this age in other parts of the world. "That should help make the picture of the time right before the extinction clearer," Brusatte said.

The scientists detailed their findings online May 1 in the journal Nature Communications.

Follow LiveScience for the latest in science news and discoveries on Twitter @livescience and on Facebook.    

Copyright 2012 LiveScience, a TechMediaNetwork company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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