Large, plant-eating dinosaurs were already in decline by the time a space rock smashed into Earth 65 million years ago and ended the reptiles' long reign, a study published on Tuesday says.
The findings by scientists in the United States and Germany do not dispute the mass extinction that so dramatically ended the Cretaceous era.
But they suggest the dinosaur kingdom, or at least some of its species, was not struck down in its prime as is often hypothesised.
"A lot of the time people think of the dinosaurs going extinct: 'oh, you know, an asteroid did it ... the dinosaurs were doing just fine, an asteroid came along and killed them all off'," Steve Brusatte, a palaeontologist at the American Museum of Natural History, told AFP.
"I think now we can say it was probably more complicated than that. You had some dinosaurs that were doing just fine, but you had others like these big plant eaters that were maybe in trouble.
"This was a world that was undergoing a lot of changes before the asteroid hit. It wasn't quite such a nice, easy story as we might like to think."
The study compared the skeletal structure of 150 different species of land-bound dinosaurs to see how they changed over time, the idea being to see if a species was up, down or stable in survival terms.
By this benchmark, the large herbivores -- specifically, horned and duckbilled dinosaurs -- were becoming less and less diverse during the last 12 million years of the Cretaceous.
The four-footed giants "were becoming more similar to each other, they were losing variability," said Brusatte. "Usually when you see these big decreases in the anatomy like this, that means that a group is in trouble."
Groups that show an increase in variety boost their chances of survival because they can fill new habitat niches or adapt to changing conditions, he explained.
But if big herbivores were on the skids towards the end of the Cretaceous, carnivorous dinosaurs and medium-sized herbivores were thriving, say the researchers.
"What we can say for certain now is when the asteroid hit and when these volcanoes began erupting, they didn't hit a world that was totally OK, they didn't hit a static world," said Brusatte.
"At the time, dinosaurs, at least some of them, were undoing major evolutionary changes and at least these plant eaters were declining."
The reason for their downward spiral is unclear but "was probably something ecological," he said. The findings are published in the journal Nature Communications.