This conclusion was reached by a new US-German research, shedding more light on one of the biggest enigmas in the history of the world, the extinction of the dinosaurs, a landmark event that paved the way for the prevalence of most mammals and eventually of human.
The new study shows that ultimately the extinction of the dinosaurs was not a single, brief event.
The researchers, led by paleontologist Stephen Brusatte from Columbia University and American Museum of Natural History, argue that large vegetarian dinosaurs like Triceratops were already going through a process of slow extinction during the last 12 million years of the Cretaceous period.
In contrast, meat-eating dinosaurs (including the terrifying tyrannosaurs) and some species of vegetarian dinosaurs did not enter in phase of gradual extinction, so the impact of the celestial body triggered their sudden destruction (except for the flying dinosaurs that survived after the cataclysmic event).
For the first time the new study focused on so-called “morphological diversity”, i.e. anatomical diversity of skeletal body-type observed within the individual groups of dinosaurs.
The previous studies were based almost exclusively on estimation of changes in the number of dinosaur species through time.
The basic biological rule is that if a species has a growing variety of body shapes, then this species has an evolutionary advantage for survival in changing ecosystems, while in the opposite case it is under threat of extinction.
Thus, the researchers estimated seven large groups of dinosaurs that totally included about 150 different species (incidentally, as pointed out by scientists at the end of the Cretaceous period there were hundreds of dinosaur species, among which there were significant differences in size, shape and nutrition).
Large vegetarian dinosaurs started declining slowly at least 12 million years before the asteroid fell on their heads. It is not sure whether any species of dinosaurs would have survived if the huge asteroid had not fallen to Earth.
According to Brusatte, “the world of dinosaurs already knew many changes before the asteroid hit the Earth. Their disappearance was not an easy and fast story, as most people think… What we can now say with confidence is that when the asteroid fell and volcanoes began to erupt, it did not hit a world where everything was going well“.