Some dinosaurs may have been headed toward extinction long before a natural disaster suddenly ended their reign.
A new study out Tuesday claims some large-bodied herbivores, like the triceratops, were in slow decline before being totally wiped out 65 million years ago.
"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," said lead author Steve Brusatte, a Columbia University graduate student affiliated with the Museum of Natural History.
The researchers looked at the changes in biodiversity within different dinosaur groups over time to piece together pictures of the creatures' well-being.
An increase in one group's variability could indicate that it was evolving into more species, improving its chances of survival. Meanwhile, a decrease in variability could be the first warning signs of extinction in the long term.
The study found duck-billed hadrosaurs and horned and frilled ceratopsids may have experienced a decline in biodiversity over the last 12 million years of the Cretaceous period.
But complicating the findings, hadrosaurs only seemed to be declining in North America, while increasing in biodiversity in Asia.
Meanwhile, other groups of dinosaurs remained relatively stable or even slightly increased in biodiversity, including the heavily armored ankylosaurs, the fearsome T. rex, and enormous herbivores like the long-necked sauropods.
"Contrary to how things are often perceived, the Late Cretaceous wasn't a static 'lost world' that was violently interrupted by an asteroid impact," Brusatte said.
"Some dinosaurs were undergoing dramatic changes during this time, and the large herbivores seem to have been mired in a long-term decline, at least in North America."