NEW YORK — Dinosaurs were in decline long before an asteroid strike polished them off about 66 million years ago, a study says.

It’s the latest contribution to a long-running debate: Did the asteroid reverse the fortune of a thriving group of animals? Or were dinosaurs already struggling, and the disruptive effects of the asteroid pushed them over the edge to extinction? Or were the dinosaurs headed for oblivion anyway?

While some have argued that dinosaurs began petering out some 5 million or 10 million years before their final doom, the new paper suggests it started happening much earlier, maybe 50 million years before the asteroid catastrophe.

In terms of species, “they were going extinct faster than they could replace themselves,” said paleontologist Manabu Sakamoto of the University of Reading in England.

He led a team of British scientists who analyzed three large dinosaur family trees, looking for evidence of when extinctions began to outnumber the appearances of new species.

They found that started to happen about 50 million years before the asteroid for most groups of dinosaurs. Two other groups showed increases rather than declines; if their results are included, the overall time for the start of dinosaur decline shrinks to 24 million years before the final demise.

Declining groups include two-legged carnivores like T. rex and the immense, long-necked, four-legged plant eaters known as sauropods. Another familiar dinosaur, triceratops, belonged to a group that was on the rise.

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