Some North American populations of herbivorous dinosaurs became extinct long before falling back to Earth asteroid (or comet). The meteorite crashed into the Earth more than 65.3 million years ago, was only one of many factors that triggered the death of the dinosaurs. By this time include a strong sea-level change and volcanic activity.
A scientist from Columbia University Stephen Bruzatte together with his colleagues first described the morphological diversity of these ancient creatures, that is, the variability of the structure of their bodies within specific groups. It is believed that the larger the number, the healthier population.
In previous studies, scientists have tried almost always evaluate the general population, based on the fossil record, but this method has a drawback - the uneven sampling: some geological formations preserve fossils better, some worse.
A team of scientists led by Bruzatte analyzed how different body sizes in the seven major groups of dinosaurs using databases with detailed descriptions of the skeletal structure of the representatives of about 150 species of dinosaurs. It turned out that the huge plant-eating dinosaurs (ceratopsids, hadrosaurs) became extinct long before the end of the Cretaceous period. But other species, for example, small herbivores Pachycephalosaurus and ankylosaurs, predators and Coelurosaurs tyrannosaurs remained quite stable.
Based on this, the researchers concluded that the herbivorous dinosaurs at that time were some extreme environmental pressures. Bruzatte attributes this to the change in sea level and mountain building. So after a couple of million years the problem of herbivores would have spread to other groups of dinosaurs, which would lead to their extinction even without intervention in the process of the meteorite.