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Were dinosaurs undergoing long-term decline before mass extinction?

Were dinosaurs already undergoing a long-term decline before an asteroid hit at the end of the Cretaceous about 65.5 million years ago? A study led by Museum scientists gives a multifaceted answer. The findings, published in May, 2012, in the journal Nature Communications, suggest that in general, large-bodied, “bulk-feeding” herbivores were declining during the last 12 million years of the Cretaceous Period. But carnivorous dinosaurs and mid-sized herbivores were not. Created by American Museum of Natural History.

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Video transcript

Ever since the first dinosaurs were found about two centuries ago, people of always wondered what happened to these animals? Why did they go extinct? When did they go extinct? And we know the dinosaurs went extinct about sixty five million years ago, right at the end of the Cretaceous. We know there was a big asteroid that hit the planet at that time, we know there was massive volcanic eruptions going on at that time, but it's remained a bit of a mystery. Did one or both of those things cause dinosaurs to go extinct or were there other factors involved. We have the paper coming out in nature communications on the dinosaur extinction and how dinosaurs were changing during those ten to twelve million years right before their extinction. And this is a collaboration between myself and Mark Norell, my adviser here at the American Museum, and our colleagues Richard Butler and Albert Prieto-Márquez in Munich. So what we've done with this project is we've looked specifically at dinosaur anatomy there has been a lot of previous work on the dinosaur extinction over the last several decades, but what most people have done before is, they've looked at species counts. They've looked at dinosaur diversity in terms of how many species of dinosaurs there were and how that changed over time. What we do in this paper is that we take a completely different approach. So we're not as interested in the number of species as we are in the number of kinds. What we've done is we've tried to tease that signal out to look at the physical difference among different dinosaur species, how that represents itself as one comes up to the terminal Cretaceous event. Our results show really that dinosaurs were in a state of flux during the the final twelve million years before they went extinct; some groups of dinosaurs like the carnivorous dinosaurs and smaller species of plant eaters were pretty steady in the revolution during those twelve million years before the extinction, but other groups of dinosaurs specifically the very large plant-eating dinosaurs, things like the ceratopsians like triceratops and also the duck-billed dinosaurs, these animals seemed to have been undergoing a very long-term decline. We also found that different dinosaurs living in different parts of the world were changing in different ways. So, dinosaurs living in North America generally seem to have been undergoing a decline in biodiversity at least these large plant-eaters, whereas those living in Asia seem to have actually been increasing. I think it'll help us understand the extinction event a little better especially as the record becomes more densely sampled and we're able to understand some different animal and organismal groups and to see if they show some of the same patterns as these groups of non-avian dinosaurs do. We'll have a better picture of what was going on right before the big event.