For many years scientists thought that the Chicxulub meteorite impact wiped out the dinosaurs about 66 million years ago. But now a new study from a group of UK researchers found that most dinosaur populations were declining about 24 million years before the dramatic impact. The research points to a gradual decline scenario for many, but not all, dinosaur species and points out the Chicxulub event was not the cause of dinosaur extinction but the nail in the coffin for an already-weakened group of animals.
There are a number of reasons why the dinosaurs began dying off 90 million years ago. The planet was undergoing a major transformation, as the Earth's two supercontinents broke apart, and sea levels fluctuated wildly. At the same time, megavolcanoes were erupting in India's Deccan Traps, which geoscientist Gerta Keller has long argued spurred extinctions among dinosaurs in the seas and on land. All of these events, taken together, would have had the effect of shrinking the dinosaurs' once-massive tropical territories that stretched across the supercontinents Laurasia and Gondwana. As habitats shrank, so too did dinosaur populations. That left them extremely vulnerable to extinction when disaster struck and killed off huge numbers of dinosaurs. The smaller the population, the harder it is to bounce back when many individuals die.