A new study suggests that dinosaur populations were in decline before the asteroid impact 65 million years ago. Despite the multitude of popular stories to the contrary, the evidence that one impact caused the mass extinction of dinosaurs is slim. In fact the only evidence is correlation.
This new study looks at morphological variations within species of dinosaurs which is a first. The researchers noted that some herbivore species may have been in decline around 12 million years before the impact at the K-T boundary (i.e. end of the Cretaceous period).
Proponents of the Bolide Impact Hyposthesis have never provided a suitable answer as to why large reptiles like alligators and crocodiles passed across the K-T boundary unscathed. They have sometimes argued that these animals escaped to water but that does not explain why ocean dwelling ammonites also experienced extinction at the boundary. The facts will probably continue to demonstrate that extinctions are rarely, if ever, caused by a single event.