Scientists believe a giant meteor collided with the Earth 65 million years ago, putting an end to the dinosaurs' reign. Ongoing volcanism and significant sea level changes on the planet at that time likely also contributed to the large reptiles' disappearances.

Though all the dinosaurs (except birds) were wiped out by these massive climate changes, a new study shows that some species were already in decline.

"People often think of dinosaurs as being monolithic — we say, 'The dinosaurs did this, and the dinosaurs did that,'" Richard Butler, one of the study's authors from Ludwig Maximilian University in Munich, told LiveScience. "But dinosaurs were hugely diverse."

Butler and his co-authors studied the varieties of body structures present within about 150 different species of dinosaurs. The more variability within a species, the more robust its population probably was.

The new research indicates that trends in biodiversity were not universal among all dinosaur groups before their extinction.

Large-bodied, heavy-feeding herbivores were already losing ground during the late Cretaceous period, before their mass die-off.

"Something was going on with large herbivores in the late Cretaceous, at least in North America. Maybe it was the fact that the local environments were in flux due to drastic sea level changes and mountain building at the time," Stephen L. Brusatte, a paleontologist at the American Museum of Natural History in New York, told Discovery News.

However, some scientists are skeptical of these results. While these massive herbivores were declining, several other species were not losing diversity, including carnivores and mid-sized herbivores.

"The decline in disparity during the final 12 million years might merely be 'evolutionary business as usual' and have little to do with the true final extinction," Paul Upchurch, a University College London paleobiologist, explained to Discovery News.

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Though the exact picture of the dinosaurs demise is still unclear. According to Brusatte, the new research undoubtedly shows "at the very least we can't envision the latest Cretaceous as a static, idyllic lost world that was suddenly exterminated by an asteroid impact."

The study was published online by Nature Communications on May 1.