Were Dinosaurs Undergoing Long-term Decline Before Mass Extinction?

Dinosaur Extinction
Four charts plotting the declines of the Ceratopsids, Coelurosaurs, Hadrosauroids, and Pachycephalosaurs, from 77 to 65 million years ago.

Despite years of intensive research about the extinction of non-avian dinosaurs about 65.5 million years ago, a fundamental question remains: were dinosaurs already undergoing a long-term decline before an asteroid hit at the end of the Cretaceous? A study led by scientists at the American Museum of Natural History gives a multifaceted answer.